LRO (Lunar Reconnaissance Orbiter)
LRO (Lunar Reconnaissance Orbiter) + LCROSS
LRO is a NASA mission to the moon within the Lunar Precursor and Robotic Program (LPRP) in preparation for future manned missions to the moon and beyond (Mars). LRO is the first mission of NASA's `New Vision for Space Exploration', which President Bush announced on January 14, 2004, in sending more robot and human explorers beyond Earth orbit. The LRO requirements call for a mission life of one year in lunar orbit. The objectives of LRO are to: 1) 2) 3) 4) 5)
• Identify potential lunar resources
• Gather detailed maps of the lunar surface
• Collect data on the moon's radiation levels
• Study the moons polar regions for resources that could be used in future manned missions or robotic sample return missions
• Provide measurements to characterize future robotic explorers, human lunar landing sites and to derive measurements that can be used directly in support of future Lunar Human Exploration Systems.
The orbiter project is managed by NASA/GSFC while NASA/ARC manages the LRO payload. The CDR (Critical Design Review) of LRO was completed in Nov. 2006.
The spacecraft is being built and integrated at NASA/GSFC (inhouse development), Greenbelt, MD. The spacecraft architecture emphasizes modularity through the use of standard interfaces. LRO is a 3-axis stabilized, nadir pointed spacecraft designed to operate continuously during the primary mission.
The ACS (Attitude Control Subsystem) consists of the following components: 10 CSS (Coarse Sun Sensors), 4 RW (Reaction Wheels), 2 A-STR (Autonomous Star Trackers), and a RLG (Ring Laser Gyroscope) of Honeywell, referred to as MIMU (Miniature Inertial Measurement Unit). MIMU provides attitude rate information up to 18 º/s and attitude rate polarity from 18º/s up to 60 rpm. The reaction wheels are specifically designed to provide very quiet, smooth changes in pointing of the spacecraft. Once in observing mode, the reaction wheels keep the boresight of the instruments pointing continuously at the surface of the moon. The 2 A-STR, built by SELEX Galileo, provide a spacecraft attitude quaternion in the J2000 ECI (Earth Centered Inertial) reference frame.
The ACS hardware is controlled by ACS flight software (FSW) resident on the SBC (Single Board Computer). This software also includes some FDC (Failure Detection and Correction) algorithms used in safing. Part of the ACS FSW function is to provide commands to the SA (Solar Array) and the HGA (High Gain Antenna). Attitude and momentum control functions are performed in ACS control modes that process sensor data and generate appropriate actuator commands. 6) 7)
Figure 2: Photo of the MIMU device (image credit: NASA)
EPS (Electric Power Subsystem): The EPS is comprised of an articulated solar array (1 wing, 2-axis tracking), a Li-ion battery of ABSL (UK), and a DET system (21-35 V). Battery mass of 35 kg, and a capacity of 126 Ah.
The C&DH (Command and Data Handling) subsystem comprises a radiation hardened SBC (Single Board Computer) for flight software, telemetry and command handling functions, system clock, and interfaces to all instruments. Data storage is provided by four DSB (Data Storage Board) devices. The onboard system architecture uses the SpaceWire bus in support of high-speed interfaces (LROC, Mini-RF, HK, UART, and LAMP), while the MIL-STD-1553B low-speed bus is used for LEND, DLRE, CRaTER, LOLA, ACS (Attitude Control Subsystem), PSE (Power System Electronics), and the propulsion subsystem.
Table 1: Overview of LRO spacecraft parameters
A one year primary mission is planned in ~50 km polar orbit, possible extended mission in communication relay/south pole observing, low-maintenance orbit. 8)
Figure 3: Block diagram of the LRO spacecraft (image credit: NASA/GSFC)
The spacecraft payload includes seven instruments, two of which are connected to the Command and Data Handling (C&DH) unit developed by NASA Goddard Space Flight Center via the SpaceWire network, as shown in Figure 4. The Mini-RF instrument is connected to the SpaceWire network through the SpaceWire ASIC. of BAE Systems (Manassas, VA). Within the C&DH unit, the RAD750 flight computer communicates with the instruments and other boards via three interfaces: a four-port SpaceWire router, a 32 bit, 33 MHz PCI bus, and a redundant MIL-STD-1553 bus. 9) 10) 11)
The SpaceWire router is implemented in the SpaceWire ASIC that is in turn connected to the RAD750 microprocessor via the PCI bus and the second generation enhanced Power PCI bridge ASIC. Both the Ka-band and S-band communications boards include SpaceWire interfaces with routers, implemented in Actel FPGAs. The LROC instrument is connected directly to one of the processor board’s SpaceWire links, while the Mini-RF connects to the Housekeeping and Input/Output (HK/IO) board that also implements a SpaceWire router using an Actel FPGA and is then routed to the processor board across the SpaceWire bus via the FPGA that implements another SpaceWire router. The 400 Gb LRO mass memory is implemented in synchronous DRAM that is interfaced to the RAD750 computer via the PCI bus on a custom C&DH backplane.
The RAD750 SBC (Single Board Computer) is a Compact PCI 6U-220 card with two printed wiring boards (PWBs). The RAD750 microprocessor operates at 132 MHz with a 66 MHz bus to I/O and memory, both of which are accessed through the enhanced Power PCI bridge ASIC. A total of 36 MB of radiation hardened SRAM is available to the RAD750, along with 4 MB of EEPROM and 64 KB of Start-up ROM, all provided with additional bits for error correction code (ECC).
The SpaceWire ASIC, shown in Figure 3 is based on BAE Systems' reusable core architecture. Its primary function is to perform routing of data using the SpaceWire protocol via a router with four external links and two internal connections to the SoC (System-on-a-Chip) bus, the standard cross-bar switch connection medium of the reusable core architecture. The SpaceWire ASIC software is included in the RAD750 Board Support Package (BSP). The BSP is designed for operation with the VxWorks (versions 5.4, 5.5, and 6.2) RTOS (Real Time Operating System).
Two 16 kB blocks of on-chip scratchpad memory are provided, as well as a 32-bit RISC processor called the EMC (Embedded Microcontroller). The EMC performs housekeeping functions as well as providing support for the SpaceWire router. A PLL (Phase Locked Loop) is provided for the SpaceWire link interface, which is capable of 280 MHz operation.
Figure 5: Illustration of the SpaceWire ASIC (image credit: BAE Systems)
Software support for the CCSDS (Consultative Committee for Space Data Systems) File Delivery Protocol (CDFP) is split between the RAD750 CPU and the EMC within the SpaceWire ASIC. The function of the software executing within the SpaceWire ASIC is to assist in CDFP download to maximize downlink throughput by sending batches of CDFP data packets, known as Protocol Data Units (PDU), over the SpaceWire interface to the Ka-band communications link.
Figure 6: Design concept of the LRO spacecraft (image credit: NASA)
Figure 7: Instrument locations of deployed spacecraft (image credit: NASA)
Launch: The LRO and the companion LCROSS spacecraft were launched on June 18, 2009 on an Atlas V 401 launch vehicle from the Air Force Station at Cape Canaveral, FLA. LRO safely separated from LCROSS 45 minutes after launch. 12) 13)
LCROSS then was powered-up, and the mission operations team at NASA's Ames Research Center at Moffett Field, CA, performed system checks that confirmed the spacecraft is fully functional. LCROSS and its attached Centaur upper stage rocket separately impacted on the moon on Oct. 9, 2009, creating a pair of debris plumes that will be analyzed for the presence of water ice or water vapor, hydrocarbons and hydrated materials. The spacecraft and Centaur are tentatively targeted to impact the moon's south pole near the Cabeus region. The exact target crater will be identified 30 days before impact, after considering information collected by LRO, other spacecraft orbiting the moon, and observatories on Earth.
Orbit: Direct insertion orbit of LRO to the moon. 14)
• Minimum energy lunar transfer orbit (~ 4 days). The launch vehicle will inject LRO into a cis-lunar transfer orbit.
• Lunar orbit insertion sequence (4 maneuvers, 2-4 days, use of onboard propulsion system)
• Commissioning phase in lunar orbit: altitude of 30 km x 216 km, quasi-frozen orbit, up to 60 days
• Polar mapping phase for a duration of at least 1 year. The LRO orbit is nominally 50 km circular and polar, with a period of ~ 113 minutes. The orbital velocity is 1.6 km/s. LRO stays on near side of moon ~ 1 hour out of every two.
Figure 8: Illustration of lunar insertion orbit (image credit: NASA)
Viewing conditions of LRO from Earth for operations support:
• Communication/ranging (SLR) with the LRO spacecraft is possible during the near-side orbital phase of the moon
• Twice a month, LRO's orbit will be in full view of the Earth for roughly 2 days
• Twice a month, LRO will perform a momentum management maneuver while the ground has complete coverage
• Once a month, LRO will perform a station-keeping maneuver while the ground has complete coverage
• Twice a year, LRO's orbit will be in full view of the sun for roughly one month
• During the eclipse season, LRO will have a maximum lunar occultation of 48 minutes
• Twice a year, LRO will perform a 180º yaw maneuver
• Twice a year, the moon will pass through the Earth's shadow (lunar eclipse).
The S-band is used for TT&C (Telemetry, Tracking & Command) data. Proximity relay is planned to enable mission cross-support at S-band.
- Frequency: Transmit: 2271.2 MHz ±2.5 MHz; Receive: 2091.3967 MHz ±2.5 MHz
- Modulated RF (at the transponder output): 39.1±0.3 dBm (7.58 -8.71 W)
- Acquisition threshold: -121 dBm (receiver ON at all times)
- Modulation: BPSK
- Coherent downlink ranging generation
- Compatible with STDN and DSN ranging modes
- 1.7 MHz downlink subcarrier; 16 kHz uplink subcarrier
• Data rates in S-band: Uplink:4 kbps uplink capability; Downlink: 0.125, 2, 16, 32, 64, 128, 256 kbit/s (with/without ranging) and 1.093 Mbit/s (direct modulation)
• Control and status interface: UART (Universal Asynchronous Receive Transmit) serial port
• DC Power: ≤ 45 W (full mode): ≤ 10 W (receive only)
The Ka-band is used for the downlink of instrument data (40 W transmitter and high-gain antenna).
- Frequency: 25.65 GHz
- Bandwidth: 300 MHz (±150 MHz)
- Modulation: OQPSK
- DC power: ≤ 30 W
- I/Q channel data inputs: LVDS interface, I and Q staggered by half of a symbol bit
• Symbol rate inputs (after rate ½, K=7 Convolutional and R/S encoding done by C&DH):
- 228.7 Msps (Mega samples per second, normal operations)
- 114.3 Msps or 57.2 Msps (contingency operations)
Figure 9: Schematic view of the projected LRO mossion timelime (image credit: NASA) 16)
LRO mission status:
• December 2, 2019: The Chandrayaan-2 Vikram lander of ISRO (Indian Space Research Organization) was targeted for a highland smooth plain about 600 kilometers from the south pole; unfortunately ISRO lost contact with their lander shortly before the scheduled touchdown (Sept. 7 in India, Sept. 6 in the United States). Despite the loss, getting that close to the surface was an amazing achievement. The LROC (Lunar Reconnaissance Orbiter Camera) team released the first mosaic (acquired Sept. 17) of the site on Sept. 26 and many people have downloaded the mosaic to search for signs of Vikram. Shanmuga Subramanian contacted the LRO project with a positive identification of debris. After receiving this tip, the LROC team confirmed the identification by comparing before and after images. When the images for the first mosaic were acquired the impact point was poorly illuminated and thus not easily identifiable. Two subsequent image sequences were acquired on Oct. 14 and 15, and Nov. 11. The LROC team scoured the surrounding area in these new mosaics and found the impact site (70.8810°S, 22.7840°E, 834 m elevation) and associated debris field. The November mosaic had the best pixel scale (0.7 meter) and lighting conditions (72° incidence angle). 17)
Figure 10: This image shows the Vikram Lander impact point and associated debris field. Green dots indicate spacecraft debris (confirmed or likely). Blue dots locate disturbed soil, likely where small bits of the spacecraft churned up the regolith. "S" indicates debris identified by Shanmuga Subramanian. This portion of the Narrow Angle Camera mosaic was made from images M1328074531L/R and M1328081572L/R acquired on 11 November 2019 (image credit: NASA/Goddard/Arizona State University)
- The debris first located by Shanmuga is about 750 meters northwest of the main crash site and was a single bright pixel identification in that first mosaic (1.3 meter pixels, 84° incidence angle). The November mosaic shows best the impact crater, ray and extensive debris field. The three largest pieces of debris are each about 2 x 2 pixels and cast a one pixel shadow.
Figure 11: This before and after image ratio highlights changes to the surface; the impact point is near center of the image and stands out due the dark rays and bright outer halo. Note the dark streak and debris about 100 meters to the SSE of the impact point. Diagonal straight lines are uncorrected background artifacts (image credit: NASA/Goddard/Arizona State University)
Figure 12: Before and after images show the Vikram impact point. Changes to the surface are subtle and are more easily seen in the ratio image presented above (image credit: NASA/Goddard/Arizona State University)
• May 17, 2019: The Lunar and Planetary Institute (LPI), managed by Universities Space Research Association (USRA), has compiled and made available an atlas of the Moon's south pole. Given NASA's recent direction to implement Space Policy Directive-1 landing astronauts at the south pole by 2024, the LPI has compiled a series of maps, images, and illustrations designed to provide context and reference for those interested in exploring this area. 18) 19)
- The highlight of the new online atlas is a set of 14 topographic maps derived from Lunar Reconnaissance Orbiter (LRO) data. Dr. Julie D. Stopar, USRA staff scientist and director of the Regional Planetary Image Facility (RPIF) at the LPI, utilized these data to generate a series of south pole maps that can be used to visualize the terrain near the south pole.
- "There are many exciting places to explore on the Moon, but the south pole has long held promise for a sustainable human presence," says Dr. Stopar. "This collection can assist mission planners in this new era of south pole exploration."
- LRO has been in orbit collecting data since late June 2009—almost a decade. LRO is in a polar orbit, meaning that it passes near the poles multiple times each day, resulting in many opportunities to study the south pole over the entire mission. As a result, there is an abundance of topographic data and images already available from the poles, including several digital elevation models derived from LRO's Lunar Orbiter Laser Altimeter (LOLA) instrument. These data are freely available in NASA's Planetary Data System.
- The temperatures and illumination conditions at the lunar poles are dependent on local topography. At the poles, the Sun never rises much beyond a degree from the horizon, creating long shadows cast from topographically elevated areas. Over the course of a year, the Sun will appear to move around the lunar horizon, changing the angle and extent of the shadows. However, some areas near the poles, particularly those in low topographic areas are always in shadow, never receiving direct sunlight. These areas are permanently shadowed and very cold – so cold that volatiles like water-ice become trapped there. Water-ice trapped near the lunar poles is particularly of interest for future explorers, as it may serve as a ready source of breathable air, drinkable water, and spacecraft propellant. The new south pole maps can be used to identify and characterize topographically elevated (and illuminated) areas as well as permanently shadowed areas.
- Other content in the new atlas is drawn from the LPI's RPIF collection of lunar images and maps, and LPI's library of classroom illustrations. Links to additional data products derived from recent and ongoing planetary missions are also included.
Figure 13: Topographic map of the Moon's South Pole (polar-stereographic projection, scale: 1: 600,000), image credit: USRA/LPI (Lunar and Planetary Institute)
• May 13, 2019: Billions of years ago, Earth's Moon formed vast basins called "mare" (pronounced MAR-ay). Scientists have long assumed these basins were dead, still places where the last geologic activity occurred long before dinosaurs roamed Earth. 20)
- But a survey of more than 12,000 images reveals that at least one lunar mare has been cracking and shifting as much as other parts of the Moon - and may even be doing so today. The study adds to a growing understanding that the Moon is an actively changing world.
- Taken by NASA's Lunar Reconnaissance Orbiter Camera (LROC), the images reveal "wrinkle ridges" - curved hills and shallow trenches created by a lunar surface that is contracting as the Moon loses heat and shrinks. The features are described in a study published in Icarus on March 7, 2019, and led by Nathan Williams, a post-doctoral researcher at NASA's Jet Propulsion Laboratory in Pasadena, California.
- Previous research has found similar surface features in the Moon's highlands, but wrinkle ridges have never been seen in basins before now. For this study, Williams and his co-authors focused on a region near the Moon's north pole called Mare Frigoris, or the Cold Sea.
- The study estimates that some of the ridges emerged in the last billion years, while others may be no older than 40 million years old. That's relatively fresh in geologic terms; previous studies have estimated these basins all stopped contracting about 1.2 billion years ago.
- Both Earth and its Moon experience what's known as tectonics, processes that push up mountains, rip apart land masses and create quakes. On Earth, these processes occur constantly as the planet's mantle causes pieces of crust, called plates, to shift against one another. The Moon doesn't have tectonic plates; instead, its tectonic action occurs as the Moon slowly loses heat from when it was formed nearly 4.5 billion years ago. The heat loss causes its interior to shrink, crinkling the surface and creating distinctive features like those identified in the study.
- "The Moon is still quaking and shaking from its own internal processes," Williams said. "It's been losing heat over billions of years, shrinking and becoming denser."
- The effect is similar to a car tire in winter: As the temperature drops, air inside the tire contracts and creates a squishier surface.
Evidence of a Shrinking Moon
- The Moon's tectonic action is especially visible in Mare Frigoris. By poring over more than 12,000 images taken by LRO's camera, Williams and his co-authors identified thousands of tectonically created features.
- As the ground under Mare Frigoris shifts, it pushes up wrinkle ridges, which typically snake along the ground for several miles. The longest ones stretch about 250 miles (400 km) - greater than the distance between New York City and Washington, D.C. - and rise as much as 1,000 feet (333 m). Tectonic pushing and pulling of the lunar crust also sculpt curved hills called lobate scarps and shallow trenches known as graben.
- Geologists can date them by studying another common lunar feature: impact craters. The longer a surface is struck by meteors, the more debris gets flung up from the impacts and covers nearby terrain, altering the landscape in a process called "impact gardening."
- Craters collect more debris the longer they are around. The smaller they are, the less time they take to fill: Craters smaller than the size of a football field would typically fill to the brim in under a billion years. LROC's images revealed crisp tectonic features like the wrinkle ridges that formed after - and cut through - small, unfilled craters. That allowed Williams and his co-authors to deduce that the ridges emerged within the past billion years or so.
From Moonquakes to Marsquakes
- Studying seismic activity on the Moon isn't new. The Apollo astronauts brought several seismometers to the lunar surface, which recorded thousands of moonquakes between 1969 and 1977. The vast majority were quakes that occurred deep in the Moon's interior; a smaller number were determined to be of shallow depth, occurring in the lunar crust.
- A new paper in Nature Geoscience takes another look at these shallow moonquakes and establishes connections to some very young surface features called lobate thrust fault scarps. This opens the door to looking for similar connections with young wrinkle ridges described in the Icarus study. 21)
- Scientists - including Williams - now hope to glean similar science from Mars. NASA's InSight lander recently detected what is likely its first marsquake, along with several other seismic signals. The way a quake's seismic waves travel inside a planet can tell geologists about how rocky bodies are layered. That, in turn, can deepen our understanding of how Earth, its Moon and Mars first formed.
Figure 14: New surface features of the Moon have been discovered in a region called Mare Frigoris, outlined here in teal. This image is a mosaic composed of many images taken by NASA's Lunar Reconnaissance Orbiter (LRO), image credit: NASA
• March 8, 2019: Using the LAMP (Lyman Alpha Mapping Project) instrument of SwRI aboard NASA's LRO mission, scientists have observed water molecules moving around the dayside of the Moon. A paper published in Geophysical Research Letters describes how LAMP measurements of the sparse layer of molecules temporarily stuck to the surface helped characterize lunar hydration changes over the course of a day. 22) 23)
Figure 15: This LRO image of the moon shows areas of potential frost (image credit: NASA's Goddard Space Flight Center/Scientific Visualization Studio)
- Up until the last decade or so, scientists thought the Moon was arid, with any water existing mainly as pockets of ice in permanently shaded craters near the poles. More recently, scientists have identified surface water in sparse populations of molecules bound to the lunar soil, or regolith. The amount and locations vary based on the time of day. This water is more common at higher latitudes and tends to hop around as the surface heats up.
- “This is an important new result about lunar water, a hot topic as our nation’s space program returns to a focus on lunar exploration,” said SwRI’s Dr. Kurt Retherford, the principal investigator of the LRO LAMP instrument. “We recently converted the LAMP’s light collection mode to measure reflected signals on the lunar dayside with more precision, allowing us to track more accurately where the water is and how much is present.”
- Water molecules remain tightly bound to the regolith until surface temperatures peak near lunar noon. Then, molecules thermally desorb and can bounce to a nearby location that is cold enough for the molecule to stick or populate the Moon's extremely tenuous atmosphere, or "exosphere," until temperatures drop and the molecules return to the surface. SwRI's Dr. Michael Poston, now a research scientist on the LAMP team, had previously conducted extensive experiments with water and lunar samples collected by the Apollo missions. This research revealed the amount of energy needed to remove water molecules from lunar materials, helping scientists understand how water is bound to surface materials.
- "Lunar hydration is tricky to measure from orbit, due to the complex way that light reflects off of the lunar surface," Poston said. "Previous research reported quantities of hopping water molecules that were too large to explain with known physical processes. I'm excited about these latest results because the amount of water interpreted here is consistent with what lab measurements indicate is possible. More work is needed to fully account for the complexities of the lunar surface, but the present results show that work is definitely worth doing!"
- Scientists have hypothesized that hydrogen ions in the solar wind may be the source of most of the Moon's surface water. With that in mind, when the Moon passes behind the Earth and is shielded from the solar wind, the "water spigot" should essentially turn off. However, the water observed by LAMP does not decrease when the Moon is shielded by the Earth and the region influenced by its magnetic field, suggesting water builds up over time, rather than "raining" down directly from the solar wind.
- "These results aid in understanding the lunar water cycle and will ultimately help us learn about accessibility of water that can be used by humans in future missions to the Moon," said Amanda Hendrix, a senior scientist at the PSI (Planetary Science Institute) and lead author of the paper. "A source of water on the Moon could help make future crewed missions more sustainable and affordable. Lunar water can potentially be used by humans to make fuel or to use for radiation shielding or thermal management; if these materials do not need to be launched from Earth, that makes these future missions more affordable."
- The funding for this research came from NASA Goddard Space Flight Center's LRO program office, including an LRO LAMP subcontract between SwRI and PSI, and the team received additional support from a NASA Solar System Exploration Research Virtual Institute (SSERVI) cooperative agreement.
• January 17, 2019: By looking at the Moon, the most complete and accessible chronicle of the asteroid collisions that carved our young solar system, a group of scientists is challenging our understanding of a part of Earth’s history. 24)
- The number of asteroid impacts to the Moon and Earth increased by two to three times starting around 290 million years ago, researchers reported in a paper in the journal Science. 25)
- They could tell by creating the first comprehensive timeline of large craters on the Moon formed in the last billion years by using images and thermal data collected by NASA’s LRO (Lunar Reconnaissance Orbiter). When the scientists compared those to the timeline of Earth’s craters, they found the two bodies had recorded the same history of asteroid bombardment—one that contradicts theories about Earth’s impact rate.
- For decades, scientists have tried to understand the rate that asteroids hit the Earth by carefully studying impact craters on continents and by using radiometric dating of the rocks around them to determine the ages of the largest, and thus most intact, ones. The problem is that many experts assumed that early Earth craters have been worn away by wind, storms, and other geologic processes. This idea explained why Earth has fewer older craters than expected compared to other bodies in the solar system, but it made it difficult to find an accurate impact rate and to determine whether it had changed over time.
- A way to sidestep this problem is to examine the Moon. Earth and the Moon are hit in the same proportions over time. In general, because of its larger size and higher gravity, about twenty asteroids strike Earth for every one that strikes the Moon, though large impacts on either body are rare. But even though large lunar craters have experienced little erosion over billions of years, and thus offer scientists a valuable record, there was no way to determine their ages until the Lunar Reconnaissance Orbiter started circling the Moon a decade ago and studying its surface.
- “We’ve known since the Apollo exploration of the Moon 50 years ago that understanding the lunar surface is critical to revealing the history of the solar system,” said Noah Petro, an LRO project scientist based at NASA Goddard Space Flight Center in Greenbelt, Maryland. LRO, along with new commercial robotic landers under development with NASA, said Petro, will inform the development and deployment of future landers and other exploration systems needed for humans to return to the Moon's surface and to help prepare the agency to send astronauts to explore Mars. Achieving NASA’s exploration goals is dependent on the agency’s science efforts, which will contribute to the capabilities and knowledge that will enable America’s Moon to Mars exploration approach now and in the future.
- “LRO has proved an invaluable science tool," said Petro. "One thing its instruments have allowed us to do is peer back in time at the forces that shaped the Moon; as we can see with the asteroid impact revelation, this has led to groundbreaking discoveries that have changed our view of Earth.”
Figure 16: A 2014 LROC (Lunar Reconnaissance Orbiter Camera) image showing two similarly sized craters in Mare Tranquillitatis. Both are about 500 meters in diameter. One is littered with boulders and the other is not. This boulder discrepancy is likely due to age differences between the two craters. Image width is about 2 kilometers. North points up (image credit: NASA/GSFC/Arizona State University)
The Moon as Earth's Mirror
- LRO's thermal radiometer, called Diviner, has taught scientists how much heat is radiating off the Moon’s surface, a critical factor in determining crater ages. By looking at this radiated heat during the lunar night, scientists can calculate how much of the surface is covered by large, warm rocks, versus cooler, fine-grained regolith, also known as lunar soil.
- Large craters formed by asteroid impacts in the last billion years are covered by boulders and rocks, while older craters have few rocks, Diviner data showed. This happens because impacts excavate lunar boulders that are ground into soil over tens to hundreds of millions of years by a constant rain of tiny meteorites.
- Paper co-author Rebecca Ghent, a planetary scientist at University of Toronto and the Planetary Science Institute in Tucson, Arizona, calculated in 2014 the rate at which Moon rocks break down into soil. Her work thus revealed a relationship between an abundance of large rocks near a crater and the crater’s age. Using Ghent’s technique, the team assembled a list of ages of all lunar craters younger than about a billion years.
- “It was a painstaking task, at first, to look through all of these data and map the craters out without knowing whether we would get anywhere or not,” said Sara Mazrouei, the lead author of the Science paper who collected and analyzed all the data for this project while a Ph.D. student at the University of Toronto.
- The work paid off, returning several unexpected findings. First, the team discovered that the rate of large crater formation on the Moon has been two to three times higher over approximately the last 290 million years than it had been over the previous 700 million years. The reason for this jump in the impact rate is unknown. It might be related to large collisions taking place more than 300 million years ago in the main asteroid belt between the orbits of Mars and Jupiter, the researchers noted. Such events can create debris that can reach the inner solar system.
- The second surprise came from comparing the ages of large craters on the Moon to those on Earth. Their similar number and ages challenges the theory that Earth had lost so many craters through erosion that an impact rate could not be calculated.
- “The Earth has fewer older craters on its most stable regions not because of erosion, but because the impact rate was lower about 290 million years ago,” said William Bottke, an asteroid expert at the Southwest Research Institute in Boulder, Colorado and a co-author of the paper. “This meant the answer to Earth’s impact rate was staring everyone right in the face.”
Figure 17: By analyzing data on lunar craters provided by the Diviner instrument aboard the Lunar Reconnaissance Orbiter, scientists have made a fascinating discovery about the history of impacts on both the Earth and the Moon (video credit: Ernie Wright & David Ladd/NASA Goddard)
- Proving that fewer craters meant fewer impacts—rather than loss through erosion—posed a formidable challenge. Yet the scientists found strong supporting evidence for their findings through a collaboration with Thomas Gernon, an Earth scientist based at the University of Southampton in England who works on a terrestrial feature called kimberlite pipes.
- These underground pipes are long-extinct volcanoes that stretch, in a carrot shape, a couple of kilometers below the surface. Scientists know a lot about the ages and rate of erosion of kimberlite pipes because they are widely mined for diamonds. They also are located on some of the least eroded regions of Earth, the same places we find preserved impact craters.
- Gernon showed that kimberlite pipes formed since about 650 million years ago had not experienced much erosion, indicating that the large impact craters younger than this on stable terrains must also be intact. “So that's how we know those craters represent a near-complete record,” Ghent said.
- Ghent’s team, which also included Southwest Research Institute planetary astronomer Alex Parker, wasn’t the first to propose that the rate of asteroid strikes to Earth has fluctuated over the past billion years. But it was the first to show it statistically and to quantify the rate. Now the team’s technique can be used to study the surfaces of other planets to find out if they might also show more impacts.
- The team’s findings related to Earth, meanwhile, may have implications for the history of life, which is punctuated by extinction events and rapid evolution of new species. Though the forces driving these events are complicated and may include other geologic causes, such as large volcanic eruptions, combined with biological factors, the team points out that asteroid impacts have surely played a role in this ongoing saga. The question is whether the predicted change in asteroid impacts can be directly linked to events that occurred long ago on Earth.
- This research was funded in part by NASA’s Solar System Exploration Research Virtual Institute (SSERVI). Researchers at the Southwest Research Institute are part of 13 teams within SSERVI, based and managed at NASA’s Ames Research Center in California’s Silicon Valley. SSERVI is funded by the Science Mission Directorate and Human Exploration and Operations Mission Directorate at NASA Headquarters in Washington, DC.
Figure 18: The image shows a waning crescent Moon. Plotted on the night side is the LRO Diviner rock abundance map. The most prominent craters visible in the map are Tycho (85 million years old), Copernicus (797 million years old), and Aristarchus (164 million years old). The terminator passes through the Aristarchus plateau, dividing Aristarchus from its sister crater, Herodotus (image credit: Ernie Wright/NASA Goddard)
• December 10, 2018: The LRO spacecraft is operational and has about 20 kg of fuel left on board, Noah Petro, LRO project scientist, said at a meeting of the Lunar Exploration Analysis Group (LEAG) Nov. 15. “That may not seem like a lot, but we don’t go through much fuel on an annual basis,” he said, primarily to manage the spacecraft’s momentum and make minor orbit adjustments. 26)
- “All told, we have approximately seven years of fuel remaining,” he said. That could decrease, he said, if the spacecraft performs additional maneuvers, such as to phase its orbit to observe specific activities like lunar landings.
- Petro said that LRO will receive funding in fiscal year 2019 from the Commercial Lunar Payload Services (CLPS) program, where NASA will buy payload space on commercially developed landers. NASA announced on 29 November that it awarded contracts to nine companies working on such landers, although those companies will later have to compete for task orders to fly specific payloads, such as scientific instruments.
- NASA is offering LRO to assist those future commercial landers. “The LRO team is standing ready to help,” said Barbara Cohen, LRO associate project scientist, at the 29 November announcement. That can include identifying sites close to potential resources or have high scientific value while also being safe locations for spacecraft landings.
- That can include observations of the landing themselves. “We are working with some upcoming missions to try to pick landing dates that have favorable viewing geometries” that would allow LRO to observe the landings as they happen, she said. “We want to observe the plumes as the landers land and kick up dust and disturb the environment.”
- LRO is also supporting other lunar missions outside of the CLPS program. At the LEAG meeting, John Keller, deputy project scientist for LRO, noted that the mission is helping international missions, including imaging sites for proposed future missions by Europe, India, Japan and Russia.
- This includes two upcoming missions scheduled to attempt lunar landings next year. Keller said LRO is studying options of observing the landing of SpaceIL’s lander, developed by an Israeli team that competed in the now-defunct Google Lunar X Prize, and India’s Chandrayaan-2 lander, both targeting landings between March and May 2019.
- For future commercial missions, Keller said that LRO planned to be “proactive” and reach out to the individual companies. “We will go out and say, ‘Look, let us help you understand the LRO data set, how to use it and how it can it can help you be successful,’” he said. That could lead to discussions on how to further support those missions.
• April 11, 2018: Images from NASA's LRO are not only helping planners with future human missions to the moon, but they are also revealing new information about the moon's evolution and structure. 27)
- LRO has been circling the moon since 2009 and has made a range of discoveries at Earth's closest large celestial neighbor.
- The orbiter has found regions of possible ice in permanently shadowed regions of the moon, inside sheltered craters and caves. It provides elevation data and mineralogical mapping to help scientists better understand the age of craters, lava basins and other features on the moon. And it also acts as a scout for future human missions. That role came into focus late in 2017, when the Trump administration tasked NASA with heading back to the moon before journeying to Mars.
- Future landing missions could take advantage of mountain peaks or crater rims at the moon's north pole, the video's narrator explains during the lunar tour. LRO scientists have modeled the sunlight in these regions across centuries of time. By zooming in on the spots with consistent sun exposure, mission planners can put solar panels there to support future human missions.
Figure 19: Take a virtual tour of the Moon in all-new 4K resolution, thanks to data provided by NASA's Lunar Reconnaissance Orbiter spacecraft. As the visualization moves around the near side, far side, north and south poles, we highlight interesting features, sites, and information gathered on the lunar terrain (video credit: NASA's Goddard Space Flight Center/David Ladd & Ernie Wright)
• March 15, 2018: It might sound like something from a science fiction plot – astronauts traveling into deep space being bombarded by cosmic rays – but radiation exposure is science fact. As future missions look to travel back to the moon or even to Mars, new research from the University of New Hampshire's Space Science Center cautions that the exposure to radiation is much higher than previously thought and could have serious implications on both astronauts and satellite technology. *28)
- "The radiation dose rates from measurements obtained over the last four years exceeded trends from previous solar cycles by at least 30 percent, showing that the radiation environment is getting far more intense," said Nathan Schwadron, professor of physics and lead author of the study. "These particle radiation conditions present important environmental factors for space travel and space weather, and must be carefully studied and accounted for in the planning and design of future missions to the moon, Mars, asteroids and beyond."
- In their study, recently published in the journal Space Weather, the researchers found that large fluxes in GRCs (Galactic Cosmic Rays) are rising faster and are on path to exceed any other recorded time in the space age. They also point out that one of the most significant SEP (Solar Energetic Particle) events happened in September 2017 releasing large doses of radiation that could pose significant risk to both humans and satellites. Unshielded astronauts could experience acute effects like radiation sickness or more serious long-term health issues like cancer and organ damage, including to the heart, brain, and central nervous system. 29)
- In 2014, Schwadron and his team predicted around a 20 percent increase in radiation dose rates from one solar minimum to the next. Four years later, their newest research shows current conditions exceed their predictions by about 10 percent, showing the radiation environment is worsening even more than expected. ”We now know that the radiation environment of deep space that we could send human crews into at this point is quite different compared to that of previous crewed missions to the moon," says Schwadron.
- The authors used data from CRaTER on NASA's LRO (Lunar Reconnaissance Orbiter). Lunar observations (and other space-based observations) show that GCR radiation doses are rising faster than previously thought. Researchers point to the abnormally long period of the recent quieting of solar activity. In contrast, an active sun has frequent sunspots, which can intensify the sun's magnetic field. That magnetic field is then dragged out through the solar system by the solar wind and deflects galactic cosmic rays away from the solar system – and from any astronauts in transit.
- For most of the space age, the sun's activity ebbed and flowed like clockwork in 11-year cycles, with six- to eight-year lulls in activity, called solar minimum, followed by two- to three-year periods when the sun is more active. However, starting around 2006, scientists observed the longest solar minimum and weakest solar activity observed during the space age.
- Despite this overall reduction, the September 2017 solar eruptions produced episodes of significant Solar Particle Events and associated radiation caused by particle acceleration by successive, magnetically well-connected coronal mass ejections. The researchers conclude that the radiation environment continues to pose significant hazards associated both with historically large galactic cosmic ray fluxes and large but isolated SEP events, which still challenge space weather prediction capabilities.
• February 23, 2018: A new analysis of data from two lunar missions finds evidence that the Moon’s water is widely distributed across the surface and is not confined to a particular region or type of terrain. The water appears to be present day and night, though it’s not necessarily easily accessible. 30)
- The findings could help researchers understand the origin of the Moon’s water and how easy it would be to use as a resource. If the Moon has enough water, and if it’s reasonably convenient to access, future explorers might be able to use it as drinking water or to convert it into hydrogen and oxygen for rocket fuel or oxygen to breathe.
- “We find that it doesn’t matter what time of day or which latitude we look at, the signal indicating water always seems to be present,” said Joshua Bandfield, a senior research scientist with the Space Science Institute in Boulder, Colorado, and lead author of the new study published in Nature Geoscience. “The presence of water doesn’t appear to depend on the composition of the surface, and the water sticks around.” 31)
- Analyses of data from the M3 (Moon Mineralogy Mapper) spectrometer onboard the Chandrayaan-1 spacecraft have suggested that OH/H2O is recycled on diurnal timescales and persists only at high latitudes. However, the spatial distribution and temporal variability of the OH/H2O, as well as its source, remain uncertain.
- The results contradict some earlier studies, which had suggested that more water was detected at the Moon’s polar latitudes and that the strength of the water signal waxes and wanes according to the lunar day (29.5 Earth days). Taking these together, some researchers proposed that water molecules can “hop” across the lunar surface until they enter cold traps in the dark reaches of craters near the north and south poles. In planetary science, a cold trap is a region that’s so cold, the water vapor and other volatiles which come into contact with the surface will remain stable for an extended period of time, perhaps up to several billion years.
- The debates continue because of the subtleties of how the detection has been achieved so far. The main evidence has come from remote-sensing instruments that measured the strength of sunlight reflected off the lunar surface. When water is present, instruments like these pick up a spectral fingerprint at wavelengths near 3 micrometers, which lies beyond visible light and in the realm of infrared radiation.
- But the surface of the Moon also can get hot enough to “glow,” or emit its own light, in the infrared region of the spectrum. The challenge is to disentangle this mixture of reflected and emitted light. To tease the two apart, researchers need to have very accurate temperature information.
- Bandfield and colleagues came up with a new way to incorporate temperature information, creating a detailed model from measurements made by the Diviner instrument on NASA’s LRO (Lunar Reconnaissance Orbiter). The team applied this temperature model to data gathered earlier by the M3 (Moon Mineralogy Mapper), a visible and infrared spectrometer that NASA’s Jet Propulsion Laboratory in Pasadena, California, provided for India’s Chandrayaan-1 orbiter.
- The new finding of widespread and relatively immobile water suggests that it may be present primarily as OH, a more reactive relative of H2O that is made of one oxygen atom and one hydrogen atom. OH, also called hydroxyl, doesn’t stay on its own for long, preferring to attack molecules or attach itself chemically to them. Hydroxyl would therefore have to be extracted from minerals in order to be used.
- The research also suggests that any H2O present on the Moon isn’t loosely attached to the surface.
- “By putting some limits on how mobile the water or the OH on the surface is, we can help constrain how much water could reach the cold traps in the polar regions,” said Michael Poston of the SwRI (Southwest Research Institute) in San Antonio, Texas.
- Sorting out what happens on the Moon could also help researchers understand the sources of water and its long-term storage on other rocky bodies throughout the solar system.
- The researchers are still discussing what the findings tell them about the source of the Moon’s water. The results point toward OH and/or H2O being created by the solar wind hitting the lunar surface, though the team didn’t rule out that OH and/or H2O could come from the Moon itself, slowly released from deep inside minerals where it has been locked since the Moon was formed.
- “Some of these scientific problems are very, very difficult, and it’s only by drawing on multiple resources from different missions that are we able to hone in on an answer,” said LRO project scientist John Keller of NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
Figure 20: If the Moon has enough water, and if it's reasonably convenient to access, future explorers might be able to use it as a resource (image credit: NASA/GSFC)
• May 26, 2017: Something very strange happened to the camera aboard NASA’s LRO (Lunar Reconnaissance Orbiter) on October 13, 2014. LROC (Lunar Reconnaissance Orbiter Camera), which normally produces beautifully clear images of the lunar surface, produced an image that was wild and jittery. From the sudden and jagged pattern apparent in the image (Figure 21), the LROC team determined that the camera must have been hit by a tiny meteoroid, a small natural object in space. 32)
- LROC is a system of three cameras mounted on the LRO spacecraft. Two NACs (Narrow Angle Cameras) capture high resolution black and white images. The third WAC (Wide Angle Camera) captures moderate resolution images using filters to provide information about the properties and color of the lunar surface.
- The NAC works by building an image one line at a time. The first line is captured, then the orbit of the spacecraft moves the camera relative to the surface, and then the next line is captured, and so on, as thousands of lines are compiled into a full image.
- According to Mark Robinson, professor and principal investigator of LROC at ASU’s School of Earth and Space Exploration, the jittery appearance of the image captured is the result of a sudden and extreme cross-track oscillation of the camera. LROC researchers concluded that there must have been a brief violent movement of the left Narrow Angle Camera.
- There were no spacecraft events like solar panel movements or antenna tracking that might have caused spacecraft jitter during this period. “Even if there had been, the resulting jitter would have affected both cameras identically,” says Robinson. “The only logical explanation is that the NAC was hit by a meteoroid.”
How big was the meteoroid?
During LROC’s development, a detailed computer model was made to insure the NAC would not fail during the severe vibrations caused by the launch of the spacecraft. The computer model was tested before launch by attaching the NAC to a vibration table that simulated launch. The camera passed the test with flying colors, proving its stability.
Using this detailed computer model, the LROC team ran simulations to see if they could reproduce the distortions seen on the Oct. 13 image and determine the size of the meteoroid that hit the camera. They estimate the impacting meteoroid would have been about half the size of a pinhead (0.8 mm), assuming a velocity of about7 km/s and a density of an ordinary chondrite meteorite (2.7 gram/cm3).
“The meteoroid was traveling much faster than a speeding bullet,” says Robinson. “In this case, LROC did not dodge a speeding bullet, but rather survived a speeding bullet!”
How rare is it that the effects of an event like this were captured on camera? Very rare, according to Robinson. LROC typically only captures images during daylight and then only about 10 percent of the day, so for the camera to be hit by a meteor during the time that it was also capturing images is statistically unlikely. “LROC was struck and survived to keep exploring the moon,” says Robinson, “thanks to Malin Space Science Systems’ robust camera design.”
“Since the impact presented no technical problems for the health and safety of the instrument, the team is only now announcing this event as a fascinating example of how engineering data can be used, in ways not previously anticipated, to understand what is happening to the spacecraft over 380,000 km from the Earth," said John Keller, LRO project scientist from NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
Launched on June 18, 2009, LRO has collected a treasure trove of data with its seven powerful instruments, making an invaluable contribution to our knowledge about the moon.
“A meteoroid impact on the LROC NAC reminds us that LRO is constantly exposed to the hazards of space,” says Noah Petro, deputy project scientist from NASA Goddard. “And as we continue to explore the moon, it reminds us of the precious nature of the data being returned.”
Figure 21: The first wild back-and-forth line records the moment on October 13, 2014 when the left Narrow Angle Camera's radiator was struck by a meteoroid [image credit: NASA/GSFC, ASU (Arizona State University)]
• January 28, 2017: LRO is currently in its third extended mission, which lasts until October 2018, at which point the project will have proposed to extend the mission for two more years. Having launched in June 2009, LRO has orbited the Moon longer than any other operational mission. This extended baseline of measurements by its seven instruments allows for unprecedented measurements of the lunar surface and environment and their variability. 33)
• January 6, 2017: Powerful solar storms can charge up the soil in frigid, permanently shadowed regions near the lunar poles, and may possibly produce "sparks" that could vaporize and melt the soil, perhaps as much as meteoroid impacts, according to NASA-funded research. This alteration may become evident when analyzing future samples from these regions that could hold the key to understanding the history of the moon and solar system. 34)
- The moon has almost no atmosphere, so its surface is exposed to the harsh space environment. Impacts from small meteoroids constantly churn or "garden" the top layer of the dust and rock, called regolith, on the moon. "About 10 percent of this gardened layer has been melted or vaporized by meteoroid impacts," said Andrew Jordan of the University of New Hampshire, Durham. "We found that in the moon’s permanently shadowed regions, sparks from solar storms could melt or vaporize a similar percentage." Jordan is lead author of a paper on this research published online in Icarus August 31, 2016. 35)
Figure 22: This is a map showing the permanently shadowed regions (blue) that cover about three percent of the moon's south pole (image credit: NASA Goddard/LRO mission)
- Explosive solar activity, like flares and coronal mass ejections, blasts highly energetic, electrically charged particles into space. Earth's atmosphere shields us from most of this radiation, but on the moon, these particles — ions and electrons — slam directly into the surface. They accumulate in two layers beneath the surface; the bulky ions can't penetrate deeply because they are more likely to hit atoms in the regolith, so they form a layer closer to the surface while the tiny electrons slip through and form a deeper layer. The ions have positive charge while the electrons carry negative charge. Since opposite charges attract, normally these charges flow towards each other and balance out.
Figure 23: Illustration showing how solar energetic particles may cause dielectric breakdown in lunar regolith in a PSR (Permanently Shadowed Region). Tiny breakdown events could occur throughout the floor of the PSR (image credit: NASA/Andrew Jordan)
- In August 2014, Jordan's team published simulation results predicting that strong solar storms would cause the regolith in the moon's PSRs (Permanently Shadowed Regions) to accumulate charge in these two layers until explosively released, like a miniature lightning strike. The PSRs are so frigid that regolith becomes an extremely poor conductor of electricity. Therefore, during intense solar storms, the regolith is expected to dissipate the build-up of charge too slowly to avoid the destructive effects of a sudden electric discharge, called dielectric breakdown. The research estimates the extent that this process can alter the regolith.
- "This process isn't completely new to space science — electrostatic discharges can occur in any poorly conducting (dielectric) material exposed to intense space radiation, and is actually the leading cause of spacecraft anomalies," said Timothy Stubbs of NASA/GSFC, a co-author of the paper. The team's analysis was based on this experience. From spacecraft studies and analysis of samples from NASA's Apollo lunar missions, the researchers knew how often large solar storms occur. From previous lunar research, they estimated that the top millimeter of regolith would be buried by meteoroid impacts after about a million years, so it would be too deep to be subject to electric charging during solar storms. Then they estimated the energy that would be deposited over a million years by both meteoroid impacts and dielectric breakdown driven by solar storms, and found that each process releases enough energy to alter the regolith by a similar amount.
- "Lab experiments show that dielectric breakdown is an explosive process on a tiny scale," said Jordan. "During breakdown, channels could be melted and vaporized through the grains of soil. Some of the grains may even be blown apart by the tiny explosion. The PSRs are important locations on the moon, because they contain clues to the moon's history, such as the role that easily vaporized material like water has played. But to decipher that history, we need to know in what ways PSRs are not pristine; that is, how they have been weathered by the space environment, including solar storms and meteoroid impacts."
- The next step is to search for evidence of dielectric breakdown in PSRs and determine if it could happen in other areas on the moon. Observations from NASA's Lunar Reconnaissance Orbiter spacecraft indicate that the soil in PSRs is more porous or "fluffy" than other areas, which might be expected if breakdown was blasting apart some of the soil grains there. However, experiments, some already underway, are needed to confirm that breakdown is responsible for this. Also, the lunar night is long — about two weeks — so it can become cold enough for breakdown to occur in other areas on the moon, according to the team. There may even be "sparked" material in the Apollo samples, but the difficulty would be determining if this material was altered by breakdown or a meteoroid impact. The team is working with scientists at the Johns Hopkins University Applied Physics Laboratory on experiments to see how breakdown affects the regolith and to look for any tell-tale signatures that could distinguish it from the effects of meteoroid impacts.
• March 23, 2016: New NASA-funded research provides evidence that the spin axis of Earth’s moon shifted by about five degrees roughly three billion years ago. The evidence of this motion is recorded in the distribution of ancient lunar ice, evidence of delivery of water to the early solar system. 36) 37)
- “The same face of the moon has not always pointed towards Earth,” said Matthew Siegler of the Planetary Science Institute in Tucson, Arizona, lead author of a paper in today’s journal Nature. “As the axis moved, so did the face of the ‘man in the moon.’ He sort of turned his nose up at the Earth.” This interdisciplinary research was conducted across multiple institutions as part of NASA’s SSERVI (Solar System Exploration Research Virtual Institute) based at NASA/ARC (Ames Research Center) in Moffett Field, California.
- Water ice can exist on Earth’s moon in areas of permanent shadow. If ice on the moon is exposed to direct sunlight it evaporates into space. Authors of the Nature article show evidence that a shift of the lunar spin axis billions of years ago enabled sunlight to creep into areas that were once shadowed and likely previously contained ice. The researchers found that the ice that survived this shift effectively “paints” a path along which the axis moved. They matched the path with models predicting where the ice could remain stable and inferred the moon’s axis had moved by approximately five degrees. This is the first physical evidence that the moon underwent such a dramatic change in orientation and implies that much of the polar ice on the moon is billions of years old.
- “The new findings are a compelling view of the moon’s dynamic past,” said Dr. Yvonne Pendleton, director of SSERVI, which supports lunar and planetary science research to advance human exploration of the solar system through scientific discovery. “It is wonderful to see the results of several missions pointing to these insights.” - The authors analyzed data from several NASA missions, including Lunar Prospector, LRO (Lunar Reconnaissance Orbiter), LCROSS (Lunar Crater and Observation Sensing Satellite), and GRAIL (Gravity Recovery and Interior Laboratory), to build the case for a change in the moon’s orientation. Topography from the LOLA ( Lunar Orbiter Laser Altimeter) and thermal measurements from the Diviner lunar radiometer – both on LRO – are used to aid the interpretation of Lunar Prospector neutron data that support the polar wander hypothesis.
- Siegler noticed that the distribution of ice observed at each of the lunar poles appeared to be more related to each other than previously thought. Upon further investigation, Siegler – and co-author Richard Miller of the University of Alabama at Huntsville – discovered that ice concentrations were displaced from each pole by the same distance, but in exactly opposite directions, suggesting the spin axis in the past was tilted from what we see today. A change in the tilt means that some of the ice deposited long ago has since evaporated as it was exposed to sunlight, but those areas that remain in permanent shadow between the old orientation and the new one retain their ice, and thus indicate what happened.
- A planetary body can shift on its axis when there is a very large change in mass distribution. Co-author James Keane, of the University of Arizona in Tucson, modeled the way changes in the lunar interior would have affected the moon’s spin and tilt. In doing so, he found the Procellarum region on the lunar near-side was the only feature that could match the direction and amount of change in the axis indicated by the ice distributions near the poles. Furthermore, concentrations of radioactive material in the Procellarum region are sufficient to have heated a portion of the lunar mantle, causing a density change significant enough to reorient the moon.
- Some of this heated mantle material melted and came to the surface to form the visible dark patches that fill large lunar basins known as mare. It’s these mare patches that give the man in the moon his “face.”
Figure 24: A cross-section through the Moon, highlighting the antipodal nature of lunar polar volatiles (in purple), and how they trace an ancient spin pole. The reorientation from that ancient spin pole (red arrow) to the present-day spin pole (blue arrow) was driven by the formation and evolution of the Procellarum—a region on the nearside of the Moon associated with a high abundance of radiogenic heat producing elements (green), high heat flow, and ancient volcanic activity (image credit: James Tuttle Keane, University of Arizona)
Figure 25: This polar hydrogen map of the moon’s northern and southern hemispheres identifies the location of the moon’s ancient and present day poles. In the image, the lighter areas show higher concentrations of hydrogen and the darker areas show lower concentrations (image credit: James Keane, University of Arizona; Richard Miller, University of Alabama at Huntsville)
• Dec. 18, 2015: LRO recently captured a unique view of Earth from the spacecraft's vantage point in orbit around the moon (Figure 26). "The image is simply stunning," said Noah Petro, Deputy Project Scientist for LRO at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "The image of the Earth evokes the famous 'Blue Marble' image taken by Astronaut Harrison Schmitt during Apollo 17, 43 years ago (1972), which also showed Africa prominently in the picture." 38) 39)
- LRO was launched on June 18, 2009, and has collected a treasure trove of data with its seven powerful instruments, making an invaluable contribution to our knowledge about the moon. LRO experiences 12 Earthrises every day; however the spacecraft is almost always busy imaging the lunar surface so only rarely does an opportunity arise such that its camera instrument can capture a view of Earth. Occasionally LRO points off into space to acquire observations of the extremely thin lunar atmosphere and perform instrument calibration measurements. During these movements sometimes Earth (and other planets) pass through the camera's field of view and dramatic images such as the one shown here are acquired.
- "From the Earth, the daily moonrise and moonset are always inspiring moments," said Mark Robinson of Arizona State University in Tempe, principal investigator for LROC. "However, lunar astronauts will see something very different: viewed from the lunar surface, the Earth never rises or sets. Since the moon is tidally locked, Earth is always in the same spot above the horizon, varying only a small amount with the slight wobble of the moon. The Earth may not move across the 'sky', but the view is not static. Future astronauts will see the continents rotate in and out of view and the ever-changing pattern of clouds will always catch one's eye, at least on the nearside. The Earth is never visible from the farside; imagine a sky with no Earth or moon - what will farside explorers think with no Earth overhead?"
- NASA's first Earthrise image was taken with the Lunar Orbiter 1 spacecraft in 1966. Perhaps NASA's most iconic Earthrise photo was taken by the crew of the Apollo 8 mission as the spacecraft entered lunar orbit on Christmas Eve Dec. 24, 1968. That evening, the astronauts — Commander Frank Borman, Command Module Pilot Jim Lovell, and Lunar Module Pilot William Anders — held a live broadcast from lunar orbit, in which they showed pictures of the Earth and moon as seen from their spacecraft. Said Lovell, "The vast loneliness is awe-inspiring and it makes you realize just what you have back there on Earth."
Figure 26: In this composite image we see Earth appear to rise over the lunar horizon from the viewpoint of the spacecraft, with the center of the Earth just off the coast of Liberia (at 4.04º North, 12.44º West). The large tan area in the upper right is the Sahara Desert, and just beyond is Saudi Arabia. The Atlantic and Pacific coasts of South America are visible to the left. On the moon, we get a glimpse of the crater Compton, which is located just beyond the eastern limb of the moon, on the lunar farside (image credit: NASA/GSFC, Arizona State University)
Legend to Figure 26: This image was composed from a series of images taken Oct. 12, 2015 when LRO was about 134 km above the moon's farside crater Compton. Capturing an image of the Earth and moon with LRO's LROC (Lunar Reconnaissance Orbiter Camera) instrument is a complicated task. First the spacecraft must be rolled to the side (in this case 67 º), then the spacecraft slews with the direction of travel to maximize the width of the lunar horizon in LROC's NAC (Narrow Angle Camera) image. All this takes place while LRO is traveling faster than 5760 km/hr (over 1,600 m/s) relative to the lunar surface below the spacecraft!
The high-resolution NAC of LROC takes black-and-white images, while the lower resolution WAC (Wide Angle Camera) takes color images, so you might wonder how we got a high-resolution picture of the Earth in color. Since the spacecraft, Earth, and moon are all in motion, we had to do some special processing to create an image that represents the view of the Earth and moon at one particular time. The final Earth image contains both WAC and NAC information. WAC provides the color, and the NAC provides the high-resolution detail.
• September 15, 2015: Earth's gravity has influenced the orientation of thousands of faults that form in the lunar surface as the moon shrinks, according to new results from NASA's LRO (Lunar Reconnaissance Orbiter) spacecraft (Figure 27). 40)
- In August, 2010, researchers using images from LRO's NAC (Narrow Angle Camera) reported the discovery of 14 cliffs known as "lobate scarps" on the moon's surface (Figure 28), in addition to about 70 previously known from the limited high-resolution Apollo Panoramic Camera photographs. Due largely to their random distribution across the surface, the science team concluded that the moon is shrinking.
- These small faults are typically less than10 km long and only tens of meters high. They are most likely formed by global contraction resulting from cooling of the moon's still hot interior. As the interior cools and portions of the liquid outer core solidify, the volume decreases; thus the moon shrinks and the solid crust buckles.
- Now, after more than six years in orbit, the LROC (Lunar Reconnaissance Orbiter Camera) has imaged nearly three-fourths of the lunar surface at high resolution, allowing the discovery of over 3,000 more of these features (Figure 29). These globally distributed faults have emerged as the most common tectonic landform on the moon. An analysis of the orientations of these small scarps yielded a surprising result: the faults created as the moon shrinks are being influenced by an unexpected source—gravitational tidal forces from Earth.
- Global contraction alone should generate an array of thrust faults with no particular pattern in the orientations of the faults, because the contracting forces have equal magnitude in all directions. "This is not what we found," says Smithsonian senior scientist Thomas Watters of the National Air and Space Museum in Washington. "There is a pattern in the orientations of the thousands of faults and it suggests something else is influencing their formation, something that's also acting on a global scale — 'massaging' and realigning them." Watters is lead author of the paper describing this research published in the October issue of the journal Geology.
- The other forces acting on the moon come not from its interior, but from Earth. These are tidal forces. When the tidal forces are superimposed on the global contraction, the combined stresses should cause predictable orientations of the fault scarps from region to region. "The agreement between the mapped fault orientations and the fault orientations predicted by the modeled tidal and contractional forces is pretty striking," says Watters.
- "The discovery of so many previously undetected tectonic features as the LROC high-resolution image coverage continues to grow is truly remarkable," said Mark Robinson of Arizona State University, coauthor and LROC principal investigator. "Early on in the mission we suspected that tidal forces played a role in the formation of tectonic features, but we did not have enough coverage to make any conclusive statements. Now that NAC images are available with appropriate lighting for more than half of the moon, structural patterns are starting to come into focus."
- The fault scarps are very young – so young that they are likely still actively forming today. The team's modeling shows that the peak stresses are reached when the moon is farthest from Earth in its orbit (at apogee). If the faults are still active, the occurrence of shallow moonquakes related to slip events on the faults may be most frequent when the moon is at apogee. This hypothesis can be tested with a long-lived lunar seismic network.
- "With LRO we've been able to study the moon globally in detail not yet possible with any other body in the solar system beyond Earth, and the LRO data set enables us to tease out subtle but important processes that would otherwise remain hidden," said John Keller, LRO Project Scientist at NASA's Goddard Space Flight Center, Greenbelt, Maryland.
- Launched on June 18, 2009, LRO has collected a treasure trove of data with its seven powerful instruments, making an invaluable contribution to our knowledge about the moon. LRO is managed by NASA's Goddard Space Flight Center in Greenbelt, Maryland, under the Discovery Program, managed by NASA's Marshall Space Flight Center in Huntsville for the Science Mission Directorate at NASA Headquarters in Washington, DC.
Figure 27: The gravitational forces the Moon and Sun exert are responsible for Earth’s rising and falling tides. Earth’s gravity also exerts forces on the Moon in the form of solid body tides that distort its shape. The Moon is slowly receding away from Earth and forces build as the Moon’s tidal distortion diminishes with distance and its rotation period slows with time. These tidal forces combined with the shrinking of the Moon from cooling of its interior have influenced the pattern of orientations in the network of young fault scarps (image credit: NASA/LRO/Arizona State University/Smithsonian Institution)
Figure 28: Thousands of young, lobate thrust fault scarps have been revealed in LROC images. Lobate scarps like the one shown here are like stair-steps in the landscape formed when crustal materials are pushed together, break and are thrust upward along a fault forming a cliff. Cooling of the still hot lunar interior is causing the Moon to shrink, but the pattern of orientations of the scarps indicate that tidal forces are contributing to the formation of the young faults (image credit: NASA/LRO/Arizona State University/Smithsonian Institution)
Figure 29: The map shows the locations of over 3,200 lobate thrust fault scarps (red lines) on the Moon. The black double arrows show the average orientations of the lobate scarps sampled in areas with dimensions of 40° longitude by 20° latitude and scaled by the total length of the fault scarps in the sampled areas. The pattern of the black double arrows (orientation vectors) indicates that the fault scarps do not have random orientations as would be expected if the forces that formed them were from global contraction alone. Mare basalt units are shown in tan (image credit: NASA/LRO/Arizona State University/Smithsonian Institution)
• On May 4, 2015, flight controllers at NASA/GSFC performed two station keeping burns to change LRO’s orbit. The new orbit allows LRO to pass within 20 km of the South Pole and 165 km over the North Pole. To optimize science return, team members made the decision to change the orbit after determining that the new orbit configuration poses no danger to the spacecraft. LRO can operate for many years at this orbit. 41)
- The new orbit enables exciting new science and will see improved measurements near the South Pole. Two of the instruments benefit significantly from the orbit change. The return signal from the LOLA (Lunar Orbiter Laser Altimeter) laser shots will become stronger, producing a better signal. LOLA will obtain better measurements of specific regions near the South Pole that have unique illumination conditions. The Diviner radiometer will be able to see smaller lunar features through the collection of higher resolution data.
- “The lunar poles are still places of mystery where the inside of some craters never see direct sunlight and the coldest temperatures in the solar system have been recorded,” said John Keller, LRO project scientist at NASA Goddard. “By lowering the orbit over the South Pole, we are essentially magnifying the sensitivity of the LRO instruments which will help us understand the mechanisms by which water or other volatiles might be trapped there.”
• March 2015: The Diviner Lunar Radiometer Experiment on board the LRO (Lunar Reconnaissance Orbiter) is currently mapping multispectral thermal emission from the lunar surface . These data provide key insights into the surface composition of the Moon, primarily from the three narrow channels centered near 8 µm. This is the location of the Christiansen Feature (CF), the position of which changes as a function of silicate composition. Diviner also maps the lunar surface at longer thermal wavelengths (23-400 µm) which have not generally been used in mineralogical analyses. 42)
- In previous work the team found that emissivity of the long wavelength channels varies as a function of solar incidence angle. A correction has been developed for the 8 µm channels that normalizes all Diviner daytime data to 0º incidence angles at the equator. Here the team describes the current correction for the thermal wavelengths that adopts a similar normalization method.
- The correction assumes that the data measured at an incidence angle of 0º are ideal and emissivity values measured under these conditions are close to true. Thus, we examined several relatively homogenous locations within the equatorial region of the Moon to determine the variation of emissivity with incidence angle (the equatorial zone has the most variation in incidence angle, Figure 30). The team plotted the spectra for each location, and compared the emissivity change with incidence angle for each channel. Because we assume that 0º incidence measurements are ideal, all emissivities were divided by the emissivity at 0º to develop a calibration curve for each channel (Figure 31). The team then applied the calibrations to their respective channel across the global data set to correct the emissivity values for higher incidence angle measurements.
Figure 30: The emissivity spectra measured at varying incidence angles from a spot within Mare Tranquilitatis. Note the decrease in spectral signal with increasing incidence angle (image credit: Diviner team)
- Results: The applied correction normalized the data and consistently increased the emissivity for the higher incidence angle measurements (Figure 32), bringing them closer to the emissivity values measured at 0º incidence. The spectra are still not exactly the same for the same location, but are more consistent than the uncorrected data. In some areas, we saw a bit of overcorrection, and the emissivity for channel 8 was always above 1.0.
Additionally, a global map of each of the thermal channels shows that the correction improves the consistency of the data set at different incidence angles. Figure 33 compares the global data set for several incidence angle bins with and without the correction for channel 6.
The current correction normalizes the data as a function of incidence angle, which increases the spectral signal, and allows for reliable interpretation of the emissivity data measured at varying incidence angles. This improves our ability to use data collected in the thermal wavelengths and allows the team to use this data to augment our interpretation of the lunar surface composition.
Figure 32: The emissivity spectra corrected with the calibration function. While there is still variability in the spectra with incidence, the signal is increased and there is more consistency to the data (image credit: Diviner team)
Figure 33: A comparison of the uncorrected and corrected global data set for channel 6 measured at 10-15º, 25-30º, and 40-45º incidence angles. The corrected data shows more consistency despite incidence angle (image credit: Diviner team)
• Feb. 4, 2015: The recent discovery of hydrogen-bearing molecules, possibly including water, on the moon has explorers excited because these deposits could be mined if they are sufficiently abundant, sparing the considerable expense of bringing water from Earth. Lunar water could be used for drinking or its components – hydrogen and oxygen – could be used to manufacture important products on the surface that future visitors to the moon will need, like rocket fuel and breathable air. 43)
- Recent observations by NASA's Lunar Reconnaissance Orbiter (LRO) spacecraft indicate these deposits may be slightly more abundant on crater slopes in the southern hemisphere that face the lunar South Pole. "There’s an average of about 23 parts-per-million-by-weight (ppmw) more hydrogen on the Pole-Facing Slopes (PFS) than on the Equator-Facing Slopes (EFS)," according to Timothy McClanahan of NASA/GSFC in Greenbelt, Maryland. - This is the first time a widespread geochemical difference in hydrogen abundance between PFS and EFS on the moon has been detected. It is equal to a 1% difference in the neutron signal detected by LRO's LEND (Lunar Exploration Neutron Detector) instrument.
- The hydrogen-bearing material is volatile (easily vaporized), and may be in the form of water molecules (two hydrogen atoms bound to an oxygen atom) or hydroxyl molecules (an oxygen bound to a hydrogen) that are loosely bound to the lunar surface. The cause of the discrepancy between PFS and EFS may be similar to how the Sun mobilizes or redistributes frozen water from warmer to colder places on the surface of the Earth, according to McClanahan.
- The team observed the greater hydrogen abundance on PFS in the topography of the moon's southern hemisphere, beginning at between 50 and 60º south latitude. Slopes closer to the South Pole show a larger hydrogen concentration difference. Also, hydrogen was detected in greater concentrations on the larger PFS, about 45 ppmw near the poles. Spatially broader slopes provide more detectable signals than smaller slopes. The result indicates that PFS have greater hydrogen concentrations than their surrounding regions. Also, the LEND measurements over the larger EFS don't contrast with their surrounding regions, which indicates EFS have hydrogen concentrations that are equal to their surroundings, according to McClanahan. The team thinks more hydrogen may be found on PFS in northern hemisphere craters as well, but they are still gathering and analyzing LEND data for this region.
Figure 34: LRO image of the moon's Hayn Crater, located just northeast of Mare Humboldtianum, dramatically illuminated by the low Sun casting long shadows across the crater floor (image credit: NASA/GSFC, Arizona State University)
• October 12, 2014: The LRO mission has provided researchers strong evidence the moon’s volcanic activity slowed gradually instead of stopping abruptly a billion years ago. Scores of distinctive rock deposits observed by LRO are estimated to be less than 100 million years old. This time period corresponds to Earth’s Cretaceous period, the heyday of dinosaurs. Some areas may be less than 50 million years old. 44) 45) 46)
The deposits are scattered across the moon’s dark volcanic plains and are characterized by a mixture of smooth, rounded, shallow mounds next to patches of rough, blocky terrain. Because of this combination of textures, the researchers refer to these unusual areas as irregular mare patches. The features are too small to be seen from Earth, averaging less than 500 m across in their largest dimension. One of the largest, a well-studied area called Ina, was imaged from lunar orbit by Apollo 15 astronauts.
Ina appeared to be a one-of-a-kind feature until researchers from Arizona State University in Tempe and Westfälische Wilhelms-Universität Münster in Germany, spotted many similar regions in high-resolution images taken by the two Narrow Angle Cameras that are part of the LROC (Lunar Reconnaissance Orbiter Camera). The team identified a total of 70 irregular mare patches on the near side of the moon.
Several earlier studies suggested that Ina was quite young and might have formed due to localized volcanic activity. However, in the absence of other similar features, Ina was not considered an indication of widespread volcanism. — The findings have major implications for how warm the moon’s interior is thought to be.
Figure 35: The feature called Maskelyne is one of many newly discovered young volcanic deposits on the Moon. Called irregular mare patches, these areas are thought to be remnants of small basaltic eruptions that occurred much later than the commonly accepted end of lunar volcanism, 1 to 1.5 billion years ago (NASA/GSFC, Arizona State University)
• On June 18, 2014, the LRO mission celebrated its 5th anniversary in space. After a four-day journey, the orbiter successfully entered lunar orbit on June 23, 2009. LRO has continued to shape our view of our nearest celestial neighbor. LRO data has shown us the tracks and equipment left behind from the Apollo astronauts, created the most precise map of the lunar surface, discovered the coldest known temperatures in the solar system, mapped the distribution of hydrogen and possibly water mixed in the lunar soil, identified craters and many other exciting science discoveries.
- In honor of the fifth anniversary, the LRO project kicked off the Moon as Art Campaign. The public was asked to select a favorite orbiter image of the moon for the cover of a special image collection. After two weeks of voting, the public has selected the image of Tycho central peak (Figure 36) as its favorite moon image. The stunningly beautiful Tycho central peak rests inside an impact crater and has a boulder over 100 m wide at its summit. It showcases a breathtaking view of the lunar landscape. 47)
Legend to Figure 36: On June 10, 2011, NASA's Lunar Reconnaissance Orbiter spacecraft pointed the LRO NAC (Narrow Angle Camera) to capture a dramatic sunrise view of Tycho crater. The Tycho crater's central peak complex, shown here, is about 15 km wide, left to right (southeast to northwest in this view). 48)
• May 29, 2014: A team od scientists combined observations from two NASA missions to check out the moon’s lopsided shape and how it changes under Earth’s sway – a response not seen from orbit before. The team drew on studies by NASA’s LRO (Lunar Reconnaissance Orbiter), which has been investigating the moon since 2009, and by NASA’s GRAIL (Gravity Recovery and Interior Laboratory) mission. Because orbiting spacecraft gathered the data, the scientists were able to take the entire moon into account, not just the side that can be observed from Earth. 49)
The lopsided shape of the moon is one result of its gravitational tug-of-war with Earth. The mutual pulling of the two bodies is powerful enough to stretch them both, so they wind up shaped a little like two eggs with their ends pointing toward one another. On Earth, the tension has an especially strong effect on the oceans, because water moves so freely, and is the driving force behind tides.
Earth’s distorting effect on the moon, called the lunar body tide, is more difficult to detect, because the moon is solid except for its small core. Even so, there is enough force to raise a bulge about 51 cm high on the near side of the moon and a similar one on the far side. The position of the bulge actually shifts a few cm over time. Although the same side of the moon constantly faces Earth, because of the tilt and shape of the moon’s orbit, the side facing Earth appears to wobble. From the moon’s viewpoint, Earth doesn’t sit motionless but moves around within a small patch of sky. The bulge responds to Earth’s movements like a dance partner, following wherever the lead goes.
A few studies of these subtle changes were conducted previously from Earth. But not until LRO and GRAIL did satellites provide enough resolution to see the lunar tide from orbit. To search for the tide’s signature, the scientists turned to data taken by LRO’s LOLA (Lunar Orbiter Laser Altimeter), which is mapping the height of features on the moon’s surface. The team chose spots that the spacecraft has passed over more than once, each time approaching along a different flight path. More than 350,000 locations were selected, covering areas on the near and far sides of the moon.
The researchers precisely matched measurements taken at the same spot and calculated whether the height had risen or fallen from one satellite pass to the next; a change indicated a shift in the location of the bulge. A crucial step in the process was to pinpoint exactly how far above the surface LRO was located for each measurement. To reconstruct the spacecraft’s orbit with sufficient accuracy, the researchers needed the detailed map of the moon’s gravity field provided by the GRAIL mission.
• March 2014: The LROC team assembled 10,581 LROC/NAC (Narrow Angle Camera) images, collected over 4 years, into a spectacular northern polar mosaic (Figure 37). A polar stereographic projection was used in order to limit mapping distortions when creating the 2-D map. In addition, the LROC team used improved ephemeris provide by the LOLA and GRAIL teams and an improved camera pointing model to enable accurate projection of each image in the mosaic to within 20 m. 50)
• January 2014: With precise timing, the camera aboard NASA's Lunar Reconnaissance Orbiter (LRO) was able to take a picture of NASA's LADEE (Lunar Atmosphere and Dust Environment Explorer) spacecraft as it orbited our nearest celestial neighbor. The LROC (Lunar Reconnaissance Orbiter Camera) operations team worked with their LADEE and LRO operations counterparts to make the imaging possible. LADEE is in an equatorial orbit (east-to-west) while LRO is in a polar orbit (south-to-north). The two spacecraft are occasionally very close and on Jan. 15, 2014, the two came within 9 km of each other. Since LROC is a pushbroom imager, it builds up an image one line at a time, thus catching a target as small and fast as LADEE is tricky! Both spacecraft are orbiting the Moon with velocities near 1600 m/s, so timing and pointing of LRO needs to be nearly perfect to capture LADEE in an LROC image. 51) 52)
Figure 38: LRO imaged LADEE as it passed ~9 km beneath it, on Jan. 15, 2014 (image credit: NASA/GSFC, Arizona State University)
• In November 2013, new findings of the CRaTER (Cosmic Ray Telescope for the Effects of Radiation) instrument project at UNH (University of New Hampshire) were published in the journal Space Weather, documenting the different effects and instrument responses with some of the best long-term measurements ever made of radiation in deep space. 53) 54)
Radiation in deep space comes from cosmic rays, from the solar wind and from solar energetic particles emanated during a solar storm. Particles from these sources rocket through space. Many can pass right through matter, such as our bodies. So-called ionizing radiation knocks electrons off of atoms within our bodies, creating highly reactive ions. Within Earth's protective atmosphere and magnetic field, we receive low doses of background radiation every day. The radiation hazards astronauts face are serious, yet manageable thanks to research endeavors such as the CRaTER instrument.
CRaTER measures realistic human radiation doses at the moon using a unique material called TEP (Tissue-Equivalent Plastic). Two pieces of this plastic, roughly 5 cm and 2.5 cm thick, respectively, are separated by silicon radiation detectors. The TEP-detector combo measures how much radiation may actually reach human organs, which may be less than the amount that reaches the spacecraft.
The LRO spacecraft launched as an exploration mission, a forerunner for humanity's return to the moon. But after completing its primary mission in 2010, LRO has become a powerful instrument for lunar and planetary science. CRaTER is an active participant in this scientific study, discovering a previously unmeasured source of hazardous radiation emanating from the moon itself.
This radiation comes from the partial reflection, also called an albedo, of galactic cosmic rays off the moon's surface. Galactic cosmic ray protons penetrate as much as a meter into the lunar surface, bombarding the material within and creating a spray of secondary radiation and a mix of high-energy particles that flies back out into space. This galactic cosmic ray albedo, which may interact differently with various chemical structures, could provide another method to remotely map the minerals present at the moon's surface.
According to the study, CRaTER directly measured the proton component of the moon's radiation albedo for the first time. The TEP radiation detector measures various components of radiation separately, which enables CRaTER to unfold the energy spectrum of the radiation albedo.
• Sept. 25, 2013: Repeat imaging of anthropogenic (human-made) targets on the Moon remains a Lunar Reconnaissance Orbiter Camera (LROC) priority as the LRO Extended Science Mission continues. These continuing observations of historic hardware and impact craters are not just interesting from a historical standpoint - each image adds to our knowledge of lunar science and engineering, particularly cartography, geology, and photometry. 55)
Figure 39: Luna 17, the Soviet Union spacecraft that carried the Lunokhod 1 rover to the surface. One can make out the rover tracks around the lander. LROC NAC image M175502049RE (image credit: NASA/GSFC, Arizona State University)
• On June 18, 2013, NASA's LRO (Lunar Reconnaissance Orbiter) was 4 years on orbit. Not only has LRO delivered all the information that is needed for future human and robotic explorers, but it has also revealed that the moon is a more complex and dynamic world than was expected. 56)
• January 2013: As part of the first demonstration of laser communication with a satellite orbiting the moon, scientists with NASA's LRO beamed an image of the Mona Lisa to the spacecraft from Earth. 57)
The image was transmitted in digital form from the NGSLR (Next Generation Satellite Laser Ranging) station at NASA/GSFC in Greenbelt, MD, to the LOLA (Lunar Orbiter Laser Altimeter) instrument on the spacecraft. By transmitting the image piggyback on laser pulses that are routinely sent to track LOLA's position, the team achieved simultaneous laser communication and tracking. This is the first time anyone has achieved one-way laser communication at planetary distances.
Precise timing was the key to transmitting the image. The Mona Lisa image was divided into an array of 152 x 200 pixels. Every pixel was converted into a shade of gray, represented by a number between zero and 4,095. Each pixel was transmitted by a laser pulse, with the pulse being fired in one of 4,096 possible time slots during a brief time window allotted for laser tracking. The complete image was transmitted at a data rate of about 300 bit/s (use of Reed-Solomon coding). LOLA reconstructed the image in the order the pixels were transmitted. The image was then sent back to Earth using radio waves.
This pathfinding achievement sets the stage for the LLCD (Lunar Laser Communications Demonstration), a high data rate laser-communication demonstrations that will be a central feature of NASA's next moon mission, the LADEE (Lunar Atmosphere and Dust Environment Explorer), a launch of LADEE is scheduled for the fall 2013.
Table 2: Summary of LRO science objectives and accomplishments as of the end of 2012 58)
• On Dec. 17, 2012, the LRO spacecraft witnessed the impact of the two GRAIL (Gravity Recovery and Interior Laboratory) spacecraft of NASA when they were intentionally crashed into a mountain near the moon's north pole. With just three weeks notice, the LRO team scrambled to get LRO in the right place at the right time to witness GRAIL's fiery finale. 59)
• August 2012: Scientists using the LAMP (Lyman Alpha Mapping Project) spectrometer on LRO, have made the first spectroscopic observations of the noble gas helium in the very rare atmosphere surrounding the moon. LAMP uses a novel method to peer into the perpetual darkness of the moon's so-called permanently shadowed regions. LAMP "sees" the lunar surface using the ultraviolet light from nearby space and stars, which bathes all bodies in space in a soft glow of ultraviolet light. -These remote-sensing observations complement in situ measurements taken in 1972 by the LACE (Lunar Atmosphere Composition Experiment) instrument deployed by Apollo 17. 60)
Although designed to map the lunar surface, the LAMP team expanded its science investigation to examine the far ultraviolet emissions visible in the tenuous atmosphere above the lunar surface, detecting helium over a campaign spanning more than 50 orbits. Because helium also resides in the interplanetary background, several techniques were applied to remove signal contributions from the background helium and determine the amount of helium native to the moon.
Table 3: Overview of mission phases: LRO flexible mission operations enabled new discoveries 61)
Figure 40: LRO orbit during various mission phase (image credit: NASA, Ref. 61)
• June 2012: According to data from NASA's LRO mission, ice may make up as much as 22% of the surface material in the Shackleton crater located near the Moon's south pole. The huge crater, named after the Antarctic explorer Ernest Shackleton, is ~ 4 km deep and 21.5 km in diameter. The small tilt of the lunar spin axis means Shackleton's interior is permanently dark and very cold. Researchers have long thought that ice might collect there. 62) 63) 64)
When a team of NASA and university scientists used LOLA (Lunar Orbiter Laser Altimeter) data to examine the floor of the Shackleton crater, they found it to be brighter than the floors of other nearby craters around the South Pole. This is consistent with the presence of small amounts of reflective ice preserved by cold and darkness.
Legend to Figure 41: The structure of the crater's interior was revealed by a digital elevation model constructed from over 5 million elevation measurements from LOLA.
The LRO team was able to map the crater’s elevations and brightness in extreme detail, thanks in part to LRO’s path: The spacecraft orbits the moon from pole to pole as the moon rotates underneath. With each orbit, LOLA maps a different slice of the moon, with each slice containing measurements of both poles. The upshot is that any terrain at the poles — Shackleton crater in particular — is densely recorded. Zuber and her colleagues took advantage of the spacecraft’s orbit to obtain more than 5 million measurements of the polar crater from more than 5,000 orbital tracks. 65) 66)
Legend to Figure 42: The contours of elevation are plotted every 5 m. The colors show relative elevation with purple lowest and yellow highest. The crater is 4.1 km deep. The spatial resolution of the topography is 10 m and the radial accuracy is < 1 m.
• The LRO spacecraft and its payload are operating nominally in 2012. The mission has been successfully transitioned from NASA's ESMD (Exploration Systems Mission Directorate), it will continue to perform science and measurements throughout 2012.
- New images acquired by the LRO spacecraft show that the Moon’s crust is pulling apart – at least in some small areas. The high-resolution images from LROC (Lunar Reconnaissance Orbiter Camera) provide evidence that the Moon has experienced relatively recent geologic activity (Figure 43). A team of researchers discovered small, narrow trenches typically only hundreds to a few thousand meters long and tens to hundreds of meters wide, indicating the lunar crust is being pulled apart at these locations. These linear troughs or valleys, known as “graben”, are formed when crust is stretched, breaks and drops down along two bounding faults. A handful of these graben systems have been found across the lunar surface. The team proposes that the geologic activity that created the graben occurred less than 50 million years ago (very recent compared to the Moon’s current age of over 4.5 billion years). 67) 68)
- On March 14, 2012, LRO was 1000 days in lunar orbit. 69)
Figure 43: Newly detected series of narrow linear troughs are known as graben, and they formed in highland materials on the lunar farside (image credit: NASA/GSFC, Arizona State University, Smithsonian Institution)
Legend to Figure 43: The graben are located on a topographic rise with several hundred meters of relief revealed in topography derived from LROC stereo images.
Legend to Figure 44: This new topographic map, from Arizona State University in Tempe, shows the surface shape and features over nearly the entire moon with a pixel scale close to 100 m. Called the Global Lunar DTM 100 m topographic model (GLD100), this map was created based on data acquired by LRO's WAC (Wide Angle Camera), which is part of the LROC imaging system.
Figure 45: Image of the Apollo 17 landing site with LROC-NAC taken at periapsis (image credit: NASA, ASU)
Legend: ALSEP (Apollo Lunar Surface Experiments Package), a portable scientific lab.
• In June 2011, NASA has declared full mission success for LRO after operating the spacecraft and its instruments for a one-year mission phase. Now that the final data from the instruments have been added to the agency's Planetary Data System, the mission has completed the full success requirements. The rich new portrait rendered by LRO's seven instruments is the result of more than 192 TB of data, images and maps, the equivalent of nearly 41,000 typical DVDs. 75)
- The LRO mission is ongoing with near continuous acquisition of science data.
- While studying the Hermite crater near the moon's north pole, LRO's Diviner Lunar Radiometer Experiment (DLRE) found the coldest spot in the solar system, with a temperature of 25 K.
• In May 2011, the following LR (Laser Ranging) results were presented at the 17th International Workshop on Laser Ranging, Bad Kötzting, Germany: (Ref. 113)
- One-way (uplink only) laser transponders have now been proven to work operationally (currently going on 2 years of operations)
- Two-way asynchronous transponders have been successfully demonstrated at planetary distances
- LRO-LR has been very successful thanks to support of ILRS
- LRO will be moved from 50 km circular mission orbit to reduced maintenance elliptical orbit late in 2011. LR is expected to continue at least through FY12.
• In April 2011, the PI of the Mini-RF instrument, Ben Bussey, is reporting that the anomaly of the instrument transmitter could not be fixed. Hence, the project was not able in collecting science data with the instrument anymore since the beginning of 2011.
- However, the rest of the Mini-RF is functioning nominally, and the project is investigating opportunities to conduct interesting science using Mini-RF in a “receive only” mode. One possibility is to conduct bistatic experiments, using the Arecibo Radio Telescope in Puerto Rico as the transmitter.
- Since entering lunar orbit in June 2009, Mini-RF has imaged over two thirds of the lunar surface, including more than 98% of both polar regions. 76)
• On March 15, 2011, the LRO team released the final set of data from the mission's exploration phase along with the first measurements from its new life as a science satellite. - Note: the science mission started after the exploration mission on Sept. 16, 2010 and is projected for two years. 77) 78)
- Among the latest products is a global map with a resolution of 100 m/pixel from the LROC (Lunar Reconnaissance Orbiter Camera). To enhance the topography of the moon, this map was made from images collected when the sun angle was low on the horizon. Armchair astronauts can zoom in to full resolution with any of the mosaics—quite a feat considering that each is 34,748 pixels by 34,748 pixels, or approximately 1.1 GByte.
- The complete data set contains the raw information and high-level products such as mosaic images and maps. The data set also includes more than 300,000 calibrated data records released by LROC. All of the final records from the exploration phase, which lasted from Sept. 15, 2009 through Sept. 15, 2010, are available through several of the Planetary Data System nodes and the LROC website.
• On Jan. 4, 2011, the Mini-RF instrument team for LRO found that the Mini-RF radar had suffered an anomaly and is not currently producing useful science data. Preliminary analysis indicates a possible fault in the Mini-RF radar transmitter. Mini-RF has suspended normal operations until analysis of the situation is completed. 79)
• In December 2010, NASA presented a most precise and complete map to date of the moon's complex, heavily cratered landscape. The digital elevation map was compiled with a dataset of LOLA (Lunar Orbiter Laser Altimeter). This dataset is being used to make digital elevation and terrain maps that will be a fundamental reference for future scientific and human exploration missions to the moon. While the current maps contain ~ 3 billion data points of LOLA so far, the project expects to continue these measurements for the next two years of the mission's science phase and beyond. 80)
The positional errors of image mosaics of the lunar far side, where direct spacecraft tracking (the most accurate) is unavailable, have been 1-10 km in the past. The LRO project is reducing these dimensions to a level of ≤ 30 m in the horizontal and to ≤ 1 m in the vertical plane. At the poles, where illumination rarely provides more than a glimpse of the topography below the crater peaks, the LRO project found systematic horizontal errors of hundreds of meters as well.
Figure 46: LOLA topographic map of the moon's southern hemisphere. The false colors indicate elevation: red areas are highest and blue lowest (image credit: NASA/GSFC)
• The first 2-year extended mission phase of LRO started in mid-September 2010 for additional lunar science measurements supported by NASA’s Science Mission Directorate (SMD).
• The one-year “exploration phase” of the LRO mission was completed on Sept. 16, 2010, meeting all objectives. It produced a comprehensive map of the lunar surface in unprecedented detail; searched for resources and safe landing sites for potential future missions to the moon; and measured lunar temperatures and radiation levels.
The mission is now turning its attention from exploration objectives to scientific research.
• In the summer months of 2010, the Mini-RF instrument of LRO is about half way through its first high-resolution polar-mapping campaign. It is imaging within 20º latitude of both poles using its S-zoom mode. Recently, Mini-RF imaged a potentially ice-rich crater near the north pole of the moon. Located at 84ºN, 157ºW, this permanently shadowed crater, about 8 km in diameter, lies on the floor of the larger, more degraded crater Rozhdestvensky (177 km in diameter). With no sunlight to warm the crater floor and walls, ice brought to the moon by comets or formed through interactions with the solar wind could potentially collect here. 81) 82)
The crater was first identified as a region of interest with Mini-SAR, a NASA instrument flown on the Chandrayaan-1 mission of ISRO in 2009, when it was seen to exhibit unusual radar properties consistent with the presence of ice. But with a Mini-RF resolution 10 times better than the radar (Mini-SAR) aboard the Chandrayaan-1 spacecraft, Mini-RF allows the project to see details of the crater’s interior. In particular, the CPR (Circular Polarization Ratio) measures the polarization characteristics of the radar echoes, which give clues to the nature of the surface materials. The inset image in Figure 47 shows a "same-sense" radar image of the crater (left) next to a colorized CPR image of the crater. Red pixels have CPR values greater than 1.2. The CPR values inside the crater are almost all greater than 1, whereas the CPR values outside the crater are generally low (much less than 1). Regions with CPR greater than 1 are relatively rare in nature, but are commonly seen in regions with thick deposits of ice (such as the Martian polar caps, or the icy Galilean satellites). They are also seen in rough, blocky ejecta around fresh, young craters, but in that occurrence, scientists also observe high CPR outside the crater rim. This feature has high CPR inside its rim, but low CPR outside. The Mini-RF team plans to examine data from the other LRO instruments, particularly temperature and topographic measurements, to better characterize the environment and setting of these unusual features near the poles of the moon.
Using data from the NASA Mini-SAR instrument on the Chandrayaan-1 spacecraft of ISRO, scientists have detected ice deposits near the moon's north pole. Mini-SAR found more than 40 small craters with water ice (Figure 48). The craters range in size from 2 - 15 km in diameter. Although the total amount of ice depends on its thickness in each crater, it is estimated there could be at least 600 million metric tons of water ice. 83)
• On June 23, 2010, LRO has been one full year in lunar orbit. In this timeframe of the mission, LRO has gathered more digital information than any previous planetary mission in history. To celebrate one year in orbit, NASA provided a list of 10 cool things already observed by LRO. Among these items - the Diviner instrument of LRO measured a temperature of -248º C (or 35 K) in the floor of the moon's Hermite Crater. This represents the coldest place measured anywhere in the solar system. 84)
Figure 49: The lunar far side topography observed by the LOLA instrument with the highest peaks of 6000 m (red) and the lowest areas of -6000 m (blue), image credit: NASA/GSFC
• The LRO spacecraft and its payload are operating nominally in 2010 in lunar orbit. LRO will have approximately 210 m/s of ΔV remaining after the 1-year nominal mapping mission is completed in mid-September of 2010. These reserves will be available for extended mission operations (Ref. 14).
• Rediscovery of the Russian Lunokhod-1 and -2 retroreflectors locations on the lunar surface (Luna 17 landed on the moon on Nov.17, 1970 releasing Lunokhod-1): Using LRO's mapping data, researchers at the UCSD (University of California San Diego) successfully pinpointed the location of a long lost light reflector on the lunar surface by bouncing laser signals from Earth to the Russian Lunokhod 1 retroreflector. The initial imaging of the two Russian rover locations, Lunokhod-1 and -2 were made in early 2010 by the LROC (Lunar Reconnaissance Orbiter Camera) team, led by Mark Robinson from Arizona State University in Tempe, AZ. 85) 86)
On April 22, 2010, Tom Murphy from UCSD and his team sent pulses of laser light from the 3.5 m telescope at the Apache Point Observatory in New Mexico, zeroing in on the target coordinates provided by the LROC images and altitudes provided by the LOLA (Lunar Orbiter Laser Altimeter). The new locations of Lunokhod-1 and -2 were quickly verified by the signal response from the retroreflectors.
Figure 50: Illustration of the Lunokhod-1 retroreflector (image credit: NASA)
• At the end of 2009, LROC (Lunar Reconnaissance Orbiter Camera) has mapped in high resolution all the Apollo landing sites and 50 sites that were identified by NASA's Constellation Program to be representative of the wide range of terrains present on the moon (Figure 52). 87)
• The LRO mission played a major role in the support of the Oct. 9, 2009 LCROSS impact experiment. The role of LRO is to make detailed observations before, during, and after the LCROSS impact in a permanently shadowed crater near the lunar South Pole. The impact will occur within the highlands impact crater Cabeus on October 9, 2009. LRO has made detailed observations to support the LCROSS selection of this crater for the impact. The LRO team used laser altimetry data from both the LRO LOLA and JAXA Kaguya laser altimeters to determine regions of permanent shadows that would be the most likely regions to harbor frozen volatiles, if any are preserved in significant concentrations. The topography measurements also identified regions within the polar impact craters where the local slope is sufficiently small to maximize the transfer of kinetic energy from LCROSS impact into the lunar regolith target. LRO’s LEND neutron spectrometer provided maps of enhanced hydrogen concentrations that could indicate water ice embedded in the upper meter of the lunar regolith. LRO Diviner temperature measurements of the South Polar region were used to reveal the extremely low temperatures of cold traps that can potentially preserve volatiles in ice form. Mini-RF dual-frequency radar polarization imaging provided information that indicates the blockiness of the impact site and which can be used to further test for the presence of significant water ice on the basis of anomalous scattering behavior. These combinations of LRO measurements as well as other factors led the LCROSS team to choose the south polar crater Cabeus for its impact target. Before the impact occurs the Cabeus target area will be exhaustively observed by the LRO LROC, LOLA, and Mini-RF instruments in detail to characterize the pre-impact geology. 88)
• NASA has successfully completed its testing and calibration phase and entered its mapping orbit of the moon on Sept. 17, 2009. The spacecraft already has made significant progress toward creating the most detailed atlas of the moon's south pole to date. Scientists released preliminary images and data from LRO's seven instruments. 89) 90)
Figure 51: This image shows the daytime and nighttime lunar temperatures recorded by DIVINER (image credit: NASA/UCLA) 91)
• On Sept. 15, 2009 the LRO orbiter spacecraft was moved into a polar-inclination of 89.7º circular 50 km mean altitude orbit for a planned one-year duration to execute its baseline ESMD (Exploration Systems Mission Directorate) mission phase, the so-called “Exploration Mission”. The orbital period is typically 113 minutes. This polar orbit allows repetitive measurements at high latitudes, producing a dense net of observations. Since the Moon rotates once each sidereal month, successive groundtracks are separated by about 31 km at the equator. Observations accumulated during the one-year Exploration Mission result in complete global coverage.
LRO has already proved its keen eyes, imaging fine details of the Apollo landing sites in August with the LROC (Lunar Reconnaissance Orbiter Camera) imager. - During the nominal mission phase, the Maneuver Team designed maneuvers to allow for the successful viewing of the LCROSS impact on October 9, 2009. Maneuvering LRO for the LCROSS impact viewing included many iterations and re-plans to adapt to changing requirements for viewing the impact (Ref. 14).
Legend: Figure 52 of LROC (Lunar Reconnaissance Orbiter Camera) shows the spacecraft's first look at the Apollo 12 landing site (Apollo-12 was launched in Nov. 1969). The Intrepid lunar module descent stage, the experiment package ALSEP (Apollo Lunar Surface Experiment Package), and Surveyor 3 spacecraft are all visible. Astronaut footpaths are marked with unlabeled arrows. This image is 824 m in width. 92)
• During the commissioning phase, it was determined that LRO could perform coordinated observations with the ISRO (Indian Science Research Organization’) spacecraft Chandrayaan-1 (launched on October 22, 2008). As part of its instrument suite, Chandrayaan-1 carried the MiniSAR instrument – a synthetic aperture radar and sister instrument to LRO’s Mini-RF instrument. The goal for the coordinated observation was to perform a bistatic SAR experiment whereby Chandrayaan-1 would transmit from MiniSAR into a lunar South Pole crater and both Chandrayaan-1 and LRO would attempt to receive the return signal with their sister instruments. The experiment would attempt to find water ice in Erlanger Crater (Longitude: 29.16º, Latitude: -87.01º), one of the permanently shadowed craters at the lunar South Pole.
The experiment took place on August 20, 2009 with the corresponding instruments’ sensor footprints overlapping over the Erlanger crater for roughly 35 seconds. The close approach between the satellites was approximately 22.5 km, a majority of the difference being in the radial direction. After analyzing the encounter data, ISRO determined that, due to deteriorating spacecraft hardware, Chandrayaan-1 was not pointing at the Erlanger Crater during the experiment time. - A second attempt using a different crater was being investigated when communications were lost with Chandrayaan-1 on August 29, 2009 and the Chandrayaan-1 mission was terminated (Ref. 14).
• The first two successful SLR passes between a terrestrial ground station and a spacecraft orbiting the moon were obtained on July 1, and July 2, 2009 between the NGSLR station at Greenbelt, Maryland, USA, and the LRO (Lunar Reconnaissance Orbiter).
• On June 30, 2009 the LROC NAC and WAC cameras were activated. The cameras are working well and have returned first images of a region a few kilometers east of Hell E crater in the lunar highlands south of Mare Nubium. 93)
• On June 23, 2009, 4 1/2 days after launch, LRO has successfully entered orbit around the moon. During transit to the moon, engineers performed a mid-course correction to get the spacecraft in the proper position to reach its lunar destination. 94)
Sensor complement: (CRaTER, DLRE, LAMP, LOLA, LROC, LEND, Mini-RF)
The spacecraft payload consists of six instruments and one technology demonstration to perform investigations specifically targeted for preparing for future human exploration. The instruments are provided by various organizations in the United States, one is from Russia. 95)
Table 5: Summary of LRO instrument mass and power allocations
Table 6: Overview of the LRO instrument complement
CRaTER (Cosmic Ray Telescope for the Effects of Radiation):
CRaTER PI: Harlan E. Spence, UNH (University of New Hampshire), Durham, NH. The primary goal is to characterize the global lunar radiation environment and its biological impacts. The instrument consists of a single, integrated sensor and electronics box with simple electronic and mechanical interfaces to the spacecraft. The CRaTER sensor frontend design is based on standard stacked-detector, cosmic ray telescope systems. 96)
The objective of CRaTER is to measure LET (Linear Energy Transfer) spectra produced by incident galactic cosmic rays (GCRs) and solar energetic protons (SEPs). GCRs and SEPs with energies >10 MeV have sufficient energy to penetrate even moderate shielding. CRaTER is designed to return the following required data products:
• Measure and characterize that aspect of the deep space radiation environment, LET spectra of galactic and solar cosmic rays (particularly above 10 MeV), most critically important to the engineering and modeling communities to assure safe, long-term, human presence in space.
• Investigate the effects of shielding by measuring LET spectra behind different amounts and types of areal density, including tissue-equivalent plastic.
The CRaTER telescope consists of five ion-implanted silicon detectors (red areas in Figure 55), mounted on four detector boards (green areas), and separated by three pieces of tissue-equivalent plastic, hereinafter referred to as TEP (tan areas). All five of the silicon detectors are 2 cm in diameter. 97) 98) 99)
Table 7: Parameters of CRaTER
Figure 53: Detailed view of the CRaTER telescope (image credit: BU)
Figure 54: Illustration of detector location in CRaTER (image credit: University of Tennessee) 100)
Legend to Figure 54: The detectors (D1-D6) are made of silicon, the TEPs are composed of hydrogen, carbon, nitrogen, oxygen, fluorine, and calcium, in a tissue-equivalent mixture (A-150 plastic). The end caps are made of aluminum.
CRaTER is composed of three sets of detectors. The first set of detectors consists of thin silicon (140 µm thick) followed by a second, thicker detector (1000 µm thick). Thin detectors primarily detect particles with a high LET while thick detectors primarily detect low LET particles. Sandwiched between each of the three pairs of detectors is a slab of A-150 tissue-equivalent plastic (TEP). The first silicon detector pair D1 and D2 is on the zenith end, which faces away from the lunar surface out into deep space.
Then there is a 5.4 cm long section of TEP, followed by another detector pair D3 and D4, followed by 2.7 cm long section of TEP, and the final detector pair D5 and D6.
DLRE (Diviner Lunar Radiometer Experiment):
DLRE PI: D. Paige, UCLA. The overall objective is to measure the lunar surface thermal environment (temperatures) at scales that provide essential information for future surface operations and exploration (resolution 300 m). DLRE is a a multi-channel (9 channels) solar reflectance and infrared filter radiometer utilizing uncooled thermopile detector arrays. DLRE's spectral channels are distributed between two identical, boresighted telescopes, and an articulated elevation/azimuth mount allows the telescopes to view the lunar surface, space, and calibration targets. The IFOV response of each channel is defined by a linear, 21-element, thermopile detector array at the telescope focal plane, and its spectral response is defined by a focal plane bandpass filter.
The DLRE structure consists of an instrument optics bench assembly (OBA), an elevation/azimuth yoke, and an instrument mount. The OBA contains all of the instrument optical subassemblies, and is suspended from the yoke. Elevation and azimuth motors mounted on the yoke drive instrument articulation. The OBA is temperature controlled, and internal temperature gradients are minimized by design. Radiometric calibration is provided by views of blackbody and solar targets mounted on the yoke. The electronics subassemblies control signal processing, instrument operation and articulation, command processing, and data processing and are distributed between the OBA and the yoke. 101)
Figure 56: Illustration of the DLRE device (image credit: NASA)
The operation of DLRE is continuously in nadir pushbroom mapping mode using 21 detectors cross-track for each of its nine spectral channels. The FOV of each detector is 3.6 mrad cross track, yielding a resolution of 180 m on the lunar surface at an orbital altitude of 50 km. To facilitate spatial registration of DLRE's surface footprints in multiple spectral bands, and to reduce along-track smear, the integration period will be 0.128 seconds. The mapped data products will generally be at a resolution of ~500 m/pixel to increase the SNR (Signal-to-Noise Ratio), and to allow for anticipated errors in the reconstruction of the position and pointing of the LRO spacecraft.
Table 8: Spectral channel parameters of the DLRE instrument
Note: * is the intensity of reflected radiation from an isotropic reflector with broadband solar albedo of 0.1 in thermal equilibrium at the quoted temperature.
LAMP (Lyman-Alpha Mapping Project):
LAMP PI: A. Stern, SwRI (Southwest Research Institute), San Antonio, TX. The LAMP instrument is an imaging UV spectrometer. The objectives of LAMP are: 102)
• LAMP will be used to identify and localize exposed water frost in PSRs (Permanently Shadowed Regions)
• Provision of landform mapping (using Lyman-α albedos) in and around the PSRs of the lunar surface
• Demonstrate the feasibility of using starlight and UV sky-glow for future night time and PSR surface mission applications
• Assay the lunar atmosphere and its variability.
Viewing in the nadir direction from LRO, LAMP measures the signal reflected from the nightside lunar surface and PSR (Permanently Shadowed Regions) using Lyman-α skyglow and UV starlight as a light source. The LAMP data are taken entirely in pixel list (i.e., time tagged) mode, allowing mapping at a variety of resolutions. The reflectance data yield albedo maps of PSRs, the spectra of PSRs yield exposed water frost abundances, and the atmospheric spectra yield species abundances and variability.
Table 9: Summary of the LAMP instrument parameters
The LAMP instrument is of ALICE heritage flown on the Rosetta mission of ESA and the New Horizon mission of NASA. LAMP is comprised of a telescope and Rowland-circle spectrograph. LAMP has a single 40×40 mm2 entrance aperture that feeds light to the telescope section of the instrument. Entering light is collected and focused by an f/3 off-axis paraboloidal (OAP) primary mirror at the back end of the telescope section onto the instrument's entrance slit. After passing through the entrance slit, the light falls onto a toroidal holographic diffraction grating, which disperses the light onto a double-delay line (DDL) microchannel plate (MCP) detector. The 2D pixel format detector (1024 x 32) is coated by a CsI solar-blind photocathode and has a cylindrically curved MCP stack that matches the Rowland-circle. LAMP is controlled by an Intel 8052 compatible microcontroller, and utilizes lightweight, compact, surface mount electronics to support the science detector, as well as the instrument support and interface electronics.
Figure 57: Schematic view of the LAMP instrument (image credit: SwRI)
Figure 58: The LAMP design as seen from above (left) and below (right), image credit: SwRI