GRAIL (Gravity Recovery and Interior Laboratory)
GRAIL is a lunar-orbiting mission of two co-orbiting minisatellites in NASA's Discovery Program with the objective to measure the moon's gravity field in unprecedented detail. The GRAIL lunar mission concept was selected in December 2007; it is based on the technologies introduced by the US-German GRACE (Gravity Recovery And Climate Experiment) mission that was launched on March 17, 2002. The Discovery class of missions is run through the DPO (Discovery Program Office) at NASA/MSFC (Marshall Space Flight Center). GRAIL is a NASA PI (Principal Investigator) mission lead by Maria Zuber of MIT (Massachusetts Institute of Technology), Cambridge, MA, USA. In addition to MIT, GRAIL’s science team includes NASA's Goddard Space Flight Center (GSFC), JPL, the Carnegie Institution of Washington, the University of Arizona, the University of Paris, and the SwRI (Southwest Research Institute). 1) 2) 3) 4) 5) 6) 7) 8) 9) 10)
Scientists will use the gravity field information from the two satellites to X-ray the moon from crust to core to reveal the moon's subsurface structures and, indirectly, its thermal history.
GRAIL has two primary objectives: to determine the structure of the lunar interior, from crust to core; and to advance understanding of the thermal evolution of the moon. These objectives will be accomplished by implementing the following lunar science investigations:
• Map the structure of the crust and lithosphere
• Understand the moon's asymmetric thermal evolution
• Determine the subsurface structure of impact basins and the origin of mascons [a mascon is a region of a planet’s or moon’s crust that contains an excess positive gravity anomaly, indicating the presence of additional mass in this area.]
• Ascertain the temporal evolution of crustal brecciation and magmatism
• Constrain deep interior structure from tides
• Place limits on the size of the possible inner core.
In addition, as a secondary objective, GRAIL observations will be used to extend knowledge gained on the internal structure and thermal evolution of the moon to other terrestrial planets.
GRAIL will be implemented with a science payload derived from GRACE and a spacecraft derived from the Lockheed Martin Experimental Small Satellite-11 (XSS-11), launched in 2005. The twin GRAIL spacecraft are being placed into a low-altitude (55 km), near-circular, polar lunar orbit to perform high-precision range-rate measurements between them using a Ka-band payload. The subsequent data analysis will to provide a lunar gravity grid of 30 km x 30 km with a high-accuracy (< 10 mGal) global gravity field.
GRAIL's total mission duration (270 days) includes a planned launch in September of 2011, followed by a low-energy trans-lunar cruise for the dual-spacecraft checkout and out-gassing, and a 90 day gravity mapping science phase. Initial science products will be available beginning 30 days after the start of the science phase and will be delivered to NASA's PDS (Planetary Data System) no later than 3 months after the end of the science phase.
Figure 1: GRAIL gravity measurement model of two lunar orbiting spacecraft (image credit: NASA, MIT)
GRAIL spacecraft naming contest:
In January 2012, the GRAIL twin spacecraft were renamed by NASA to Ebb and Flow after achieving lunar orbit. - A nationwide contest was started in October 2011 for US students from Kindergarten age to 12th grade. The winning suggestions came from the forth grade class of Nina DiMauro at the Emily Dickinson Elementary School in Bozeman, Montana. 11) 12) 13)
The students reasoned that Ebb and Flow would be good names for GRAIL-A and Grail-B, respectively, because the Moon's gravity is the reason for high tides and low tides on Earth. The jury was impressed that the students drew their inspiration by researching GRAIL and its goal of measuring gravity. Ebb and Flow truly capture the spirit and excitement of the mission.
The GRAIL project office, which provides the function of daily GRAIL project management and the instrument development organization, are both located at NASA/JPL in Pasadena, CA. The twin spacecraft (minisatellites) are being built by Lockheed Martin Space Systems Company (LMSSC) of Denver, CO. The lunar mission will use two identical spacecraft orbiting the moon in a low, polar orbit. 14)
The spacecraft are based on the flight-proven XSS-11 (Experimental Spacecraft System-11) technology demonstration satellite developed for the Air Force Research Laboratory (AFRL), also built by LMSSC and launched on April 11, 2005. The resulting design meets all the GRAIL mission and science requirements with ample technical margins that provide flexibility to solve problems that may arise during development and which meet or exceed design principles established by JPL. 15) 16) 17)
S/C structure: Each spacecraft bus is a rectangular composite structure shown in Figures 4 and 5. The structure consists of six panels and two solar array substrates to which each of the electronic boxes are mounted. The propulsion tank is mounted in the center of the spacecraft due to the tank size and contribution to CM (Center of Mass) motion. The rest of the spacecraft components and harness are nestled around the propulsion tank. The key requirement for the structures is to keep the CM of the spacecraft fixed to within 1 cm during science collection (Ref. 6).
The ACS (Attitude Control Subsystem) provides 3-axis stabilization. The main components of the ACS system are reaction wheels and thrusters for attitude control, a star tracker and inertial measurement unit for attitude determination and sun sensors for use in safe mode. The ACS algorithms and modes are largely based on past missions. The STR (Star Tracker) is main source of attitude determination for GRAIL during all mission phases. This 512 x 512 CCD device provides 3σ accuracy to within 22 arcsec allowing for highly precise attitude determination. The Honeywell Block-III MIMU (Miniature Inertial Measurement Unit) provides inertial measurements necessary when the STR is not available or is not usable. The sun sensor (Adcole) is only used during safe mode operations to help orient the spacecraft in a sun-pointed attitude. The sun sensor is located on the same plane as the solar panels and provides the angle to the sun to within ~1.5º.
The reaction wheels used on GRAIL are four RWAs (Reaction Wheel Assemblies) of Goodrich mounted in a pyramid configuration. These are used for maintaining correct orbiter orientation. There are eight coupled 0.8 N thrusters which provide control authority on all 3 spacecraft axis. These are used for all of the spacecraft TCMs (Trajectory Correction Maneuvers), to perform RWA desaturations, and to provide control during the LOI maneuver. The single 22 N main engine is aligned in the -X direction, providing thrust in the +X direction. The main engine is only used during the 30-60 minute long Lunar Orbit Insertion burn.
Propulsion subsystem: It consists primarily of a main tank and a He-repressurization system. The main tank is a Ti tank with a 31 bar MEOP (Maximum Expected Operating Pressure) and a diaphragm to control fuel movement as the tank fuel load is reduced. The propulsion system utilizes a hydrazine blowdown approach to fire the thrusters and the main engine. The mission design calls for 103.5 kg of hydrazine. This will provide sufficient fuel to enter lunar orbit with the current spacecraft dry mass and provide adequate pressure to the thrusters for performance requirements.
The He-repressurization system is used after the LOI maneuver to raise the pressure in the main tank to an adequate pressure to complete the GRAIL mission. The He-repressurization system consists of a composite pressure vessel tank and a pyro activated latch valve. The mission design timeline has the repressurization system activating shortly after the LOI maneuver is completed.
The C&DH (Command & Data Handling) subsystem, the power management, and the lithium ion battery are also of XSS-11 and of MRO (Mars Reconnaissance Orbiter) heritage. The OBC is a BAE RAD750 SPC (Space Flight Computer) with a cPCI (Compact Peripheral Component Interface) backplane. The computer connects to the other boards in the C&DH system. The SFC and the LMSSC designed boards are providing together functionality for command, telemetry, instrument control and data collection, power switching and control as well as general house-keeping functions. The SFC provides all the resources necessary to run the LMSSC flight software.
EPS (Electric Power Subsystem): There are 2 non-articulated solar arrays of XSS-11 heritage that are deployed just after separation from the launch vehicle. The solar array is a dual wing direct energy transfer system designed to generate ~700 W of power. The solar arrays are sized for off-pointing during science mode. The spacecraft are designed to operate for up to a 45º off-point angle while remaining power positive. The single Li-ion battery has a capacity of 30 Ahr. The battery has been sized to keep a GRAIL orbiter alive for normal orbital situations. During lunar eclipses, which occur approximately every six months, the GRAIL spacecraft will not remain power positive if they are in a 55 km lunar orbit. In order to survive through a lunar eclipse the spacecraft would need to significantly reduce their power draw or increase their energy storage. The current mission design allows for all of the science to be collected within the current science window.
The science payload ranging antennas are in thermal enclosures and are mounted so that they are nominally on the line between the two spacecraft’s centers of mass. The other components of the payload instrument are on a single interior bus panel for easy integration and test.
Thermal subsystem: The thermal control of the GRAIL orbiters is a significant challenge for the spacecraft design due to the very difficult environment posed by a low (55 km) lunar orbit. During the course of the orbit each orbiter sees a direct solar flux change from 1323 W/m2 to 1414 W/m2 and a lunar albedo change from 0.06 (cold case) to 0.13 (hot case). Additionally, the key requirement is to keep the orbiters thermally as stable as possible to reduce errors induced by thermal changes.
The thermal subsystem has been designed to be a passively controlled system with minimal thermal variation over the course of an orbit. The two dominant thermal error sources are the geometric movement of the Ka-band horn relative to the spacecraft center of mass (due to change of spacecraft dimensions and/or spacecraft element dimensions with temperature variations), and change in the effective Ka-band horn phase center location relative to the Ka-band horn mounting location due to temperature variation of the horn. - The solution to the problem of creating a thermally stable spacecraft is to have a passively controlled system which employs a wide variety of radiators, blankets and heaters.
RF communications: The S-band telecommunications system for communication with the DSN uses heritage components (Themis and Genesis). The S-band transponder provides two-way Doppler, two-way ranging, and a 2 kbit/s uplink command rate for the spacecraft. The transponder provides at least 1 kbit/s of downlink for all mission phases. During most of the science collection period the downlink data rate will be 128 kbit/s.
Note: The gravity science data does not produce a large quantity of data, so the amount of science data collected can be downlinked with a data rate as low as 2 kbit/s for the entire mission duration. The excess downlink bandwidth above 2 kbit/s will be used for the E/PO imagery of the lunar surface.
The RF subsystem utilizes a pair of stacked S-band patch antennas mounted on opposite sides of the spacecraft (± X-axis). These 5 W antennas allow for simultaneous uplink and downlink. Similar antennas were used in a similar near earth application for the Genesis project.
Figure 2: Schematic view of GRAIL's multi channel communication system (image credit: NASA/JPL) 18)
Figure 3: Alternate view of the GRAIL communications concept (image credit: NASA)
Figure 4: Top view of the GRAIL spacecraft (image credit: NASA, LMSSC)
Figure 5: Bottom view of the GRAIL spacecraft (image credit: NASA, LMSSC)
The CDR (Critical Design Review) of the GRAIL payload was completed in August 2009. The system-level CDR was held November 9–13, 2009 at MIT in Cambridge, MA.
Table 1: Summary of some spacecraft parameters 19)
Figure 6: Photo of the twin GRAIL spacecraft prior to shipment to the launch site at Cape Canaveral (image credit: NASA/JPL)
Launch: The dual-spacecraft GRAIL mission was launched on Sept. 10, 2011 on a Delta-2 2920-10 vehicle from the Cape Canaveral Air Force Station, FLA. The launch provider was ULA (United Launch Alliance).
Note: Another American rocket era ended. The venerable Delta II rocket, steeped in history, flew what is almost certainly its final mission from Cape Canaveral. And it did so quite fittingly by blasting twin satellites to the moon for NASA on a unique path for a truly challenging mission to do “extraordinary science”. 23)
Figure 7: Photo of the twin GRAIL spacecraft in launch configuration (image credit: NASA)
Orbit and mission phases:
• The launch phase is short and involves the launch window and the initial spacecraft launch activities. There is a 21 day non-contiguous launch window available every few months. The reason for the unique launch window is two-fold. First, the design of the Trans-Lunar Cruise (TLC) mission phase allows for an extended launch window. Second, the GRAIL orbiters are not designed to survive through the December 2011 or June 2012 lunar eclipse if the spacecraft is in lunar orbit (Ref. 6).
• The TLC (Trans-Lunar Cruise) phase consists of a 4 month low-energy transfer via the Sun-Earth Lagrange point 1 (L1). Compared to a direct trajectory, this low-energy transfer was chosen to reduce the spacecraft fuel requirements (by ~130 m/s), to allow more time for spacecraft check-out and out-gassing, and to increase the number of days available in the launch period each month (Figure 8).
• Both spacecraft approach the Moon under the South Pole where they execute a 60 minute LOI (Lunar Orbit Insertion) maneuver to put them in an elliptical orbit with a period of just over 8 hours. The LOI for each of the spacecraft are separated by one day. Each LOI is simultaneously visible from the Goldstone and Canberra DSN complexes.
• After LOI, the two spacecraft start an approximately two month long period in which the two orbiters undergo orbit circularization and are positioned into formation to prepare for science instrument checkout. The first part of this period lasting approximately one month is called the OPR (Orbit Period Reduction) phase. The main activity during OPR is to perform a series of maneuvers to reduce the orbits to near circular with a mean 55 km lunar altitude and a period of ~ 115 minutes.
• After OPR, the spacecraft goes through a month long TSF (Transition to Science Formation) phase during which a series of maneuvers establish the proper formation and separation between the two spacecraft prior to the start of science collection. The final step during TSF is to perform the science instrument calibration and checkout. The co-orbiting spacecraft will have a separation distance which drifts between 175 km to 225 km.
• The 90 day Science Phase is divided into three 27.3 day nadir-pointed mapping cycles (lunar sidereal period). These cycles and the corresponding β-angles are shown in Figure 9. Two daily 8 hour DSN tracking passes acquire the science and "E/PO MoonKam" data.
• During the Science Phase the spacecraft will be in a near-polar, near-circular science orbit with a mean altitude of 55 km. The distance between the two spacecraft will fluctuate from 175 to 225 km. In between each mapping cycle, there is a 4 day period of maneuvers to reset the relative separation distance between the two spacecraft. The downlink data rate varies as the orbit geometry with respect to the Earth changes during a month. The major spacecraft impact during the Science Phase is the change of the β-angle of the solar array relative to the Sun. This has implications to both the power available to the spacecraft, the thermal flux from the Sun and the Lunar surface and the solar pressure among other factors. All of these have impacts on the quality of science data collection.
Figure 9: Illustration of the GRAIL mapping cycles (image credit: NASA/JPL)
• Following the Science Phase (or extended mission phase), a 5 day decommissioning period is planned, after which the spacecraft will impact the lunar surface in ~40 days.
Figure 10: An artist's view of the GRAIL mission in lunar orbit for gravity observations (image credit: NASA/JPL)
Figure 11: Overview of GRAIL mission phases (image credit: MIT)
Figure 12: GRAIL primary mission timeline, extending over nine months with seven distinct mission phases (image credit: JPL)
• Oct. 1, 2014: In a new study, based on data from NASA’s GRAIL (Gravity Recovery and Interior Laboratory) mission, scientists state to have solved a lunar mystery almost as old as the moon itself. 25) 26) 27)
Early theories suggested the craggy outline of a region of the moon’s surface known as Oceanus Procellarum, or the Ocean of Storms, was caused by an asteroid impact. If this theory had been correct, the basin it formed would be the largest asteroid impact basin on the moon. However, mission scientists studying GRAIL data believe they have found evidence the craggy outline of this rectangular region — roughly 2,600 km across — is actually the result of the formation of ancient rift valleys.
The nearside of the moon has been studied for centuries, and yet continues to offer up surprises for scientists with the right tools. The project team interprets the gravity anomalies discovered by GRAIL as part of the lunar magma plumbing system — the conduits that fed lava to the surface during ancient volcanic eruptions.
Figure 13: Earth's moon as observed in visible light (left), topography (center, where red is high and blue is low), and the GRAIL gravity gradients (right). The Procellarum region is a broad region of low topography covered in dark mare basalt. The gravity gradients reveal a giant rectangular pattern of structures surrounding the region (image credit: NASA, GSFC, JPL, Colorado School of Mines, MIT)
The surface of the moon’s nearside is dominated by a unique area called the Procellarum region, characterized by low elevations, unique composition, and numerous ancient volcanic plains. The rifts are buried beneath dark volcanic plains on the nearside of the moon and have been detected only in the gravity data provided by GRAIL. The lava-flooded rift valleys are unlike anything found anywhere else on the moon and may at one time have resembled rift zones on Earth, Mars and Venus.
Another theory arising from recent data analysis suggests this region formed as a result of churning deep in the interior of the moon that led to a high concentration of heat-producing radioactive elements in the crust and mantle of this region. Scientists studied the gradients in gravity data from GRAIL, which revealed a rectangular shape in resulting gravitational anomalies.
The rectangular pattern, with its angular corners and straight sides, contradicts the theory that Procellarum is an ancient impact basin, since such an impact would create a circular basin. Instead, the new research suggests processes beneath the moon’s surface dominated the evolution of this region. - Over time, the region would cool and contract, pulling away from its surroundings and creating fractures similar to the cracks that form in mud as it dries out, but on a much larger scale.
• Nov. 2013: Maps of crustal thickness derived from NASA’s GRAIL (Gravity Recovery and Interior Laboratory) mission revealed more large impact basins on the nearside hemisphere of the moon than on its farside. The enrichment in heat-producing elements and prolonged volcanic activity on the lunar nearside hemisphere indicate that the temperature of the nearside crust and upper mantle was hotter than that of the farside at the time of basin formation. Using the iSALE-2D hydrocode [impact-SALE is a multi-material, multi-rheology shock physics code based on the SALE (Simplified Arbitrary Lagrangian Eulerian) hydrocode] to model impact basin formation, the project found that impacts on the hotter nearside would have formed basins with up to twice the diameter of similar impacts on the cooler farside hemisphere. The size distribution of lunar impact basins is thus not representative of the earliest inner solar system impact bombardment. 28)
The research reveals that the familiar blotches that make up “the man in the moon”, from the vantage point of Earth, happened because the moon’s crust is thinner on the near side than the far side to our planet. 29)
Figure 14: The thickness of the moon’s crust as calculated by NASA’s GRAIL mission. The near side is on the left-hand side of the picture, and the far side on the right (image credit: NASA/JPL, S. Miljkovic)
• May 30, 2013: Analysis of NASA's GRAIL mission data has uncovered the origin of massive invisible regions that make the moon's gravity uneven, a phenomenon that affects the operations of lunar-orbiting spacecraft. 30)
GRAIL's twin spacecraft studied the internal structure and composition of the moon in unprecedented detail for nine months. They pinpointed the locations of large, dense regions called mass concentrations, or mascons, which are characterized by strong gravitational pull. Mascons lurk beneath the lunar surface and cannot be seen by normal optical cameras. GRAIL scientists found the mascons by combining the gravity data from GRAIL with sophisticated computer models of large asteroid impacts and known detail about the geologic evolution of the impact craters. The GRAIL data confirm that lunar mascons were generated when large asteroids or comets impacted the ancient moon, when its interior was much hotter than it is now.
The origin of lunar mascons has been a mystery in planetary science since their discovery in 1968 by a team at NASA's Jet Propulsion Laboratory in Pasadena, CA. Researchers generally agree mascons resulted from ancient impacts billions of years ago. It was not clear until now how much of the unseen excess mass resulted from lava filling the crater or iron-rich mantle upwelling to the crust.
• The startling observations (Figures 15 and 16) come from data collected by NASA’s GRAIL mission. From March 7 to May 29, 2012, the mission’s twin spacecraft, Ebb and Flow, have been orbiting the moon (primary mission phase, three mapping cycles) and measuring its gravitational field. From the orbital measurements of the GRAIL twin spacecraft, planetary scientists have now stitched together a high-resolution map of the moon’s gravity — a force created by surface structures such as mountains and craters, as well as deeper structures below the surface. The resulting map reveals an interior gravitational field consistent with an incredibly fractured lunar crust. 31)
Figure 15: The gravity field of the moon as measured by NASA’s GRAIL mission (image credit: NASA, MIT)
Spacecraft-to-spacecraft tracking observations from the GRAIL mission have been used to construct a gravitational field of the Moon to spherical harmonic degree and order 420. The GRAIL field reveals features not previously resolved, including tectonic structures, volcanic landforms, basin rings, crater central peaks, and numerous simple craters. From degrees 80 through 300, over 98% of the gravitational signature is associated with topography, a result that reflects the preservation of crater relief in highly fractured crust. The remaining 2% represents fine details of subsurface structure not previously resolved. GRAIL elucidates the role of impact bombardment in homogenizing the distribution of shallow density anomalies on terrestrial planetary bodies. 32)
Figure 16: This illustration shows the variations in the lunar gravity field as measured by the GRAIL mission (image credit: NASA, MIT)
Legend to Figure 16: Red corresponds to mass excesses and blue corresponds to mass deficiencies.
Beneath its heavily pockmarked surface, the moon’s interior bears remnants of the very early solar system. Unlike Earth, where plate tectonics has essentially erased any trace of the planet’s earliest composition, the moon’s interior has remained relatively undisturbed over billions of years, preserving a record in its rocks of processes that occurred in the solar system’s earliest days.
• End of GRAIL mission on Dec. 17, 2012: On December 14, 2012, the twin probes, Ebb and Flow, comprising NASA's GRAIL mission, were commanded to descend into a lower orbit and target a mountain near the moon's north pole for impact. The formation-flying duo hit the lunar surface as planned on Dec. 17, 2012 on a mountain near the moon's north pole - bringing their successful prime and extended science missions to an end. The two probes were being sent purposely into the moon, because they no longer had enough altitude or fuel to continue any science operations.
NASA has named the site where twin agency spacecraft impacted the moon on Dec. 17, 2012 in honor of the late astronaut, Sally K. Ride, who was America's first woman in space and a member of the probes' mission team. 33) 34) 35)
The impact marked a successful end to the GRAIL mission, which in just a 90-day prime mission generated the highest-resolution gravity field map of any celestial body — including Earth — and determined the inner crust of the moon is nearly pulverized. Data from GRAIL’s extended mission and main science instruments are still being analyzed, and the findings will provide a better understanding of how Earth and other rocky planets in the solar system formed and evolved.
Figure 17: Artist's rendition of the GRAIL's spacecrafts' final orbit (image credit: NASA)
Along with GRAIL’s Science Mission, another project involving the two GRAIL twins began. MoonKAM, operated by Sally Ride Science, flew four cameras on each of the two vehicles to acquire images requested by students around the world. Nearly 120,000 images acquired by MoonKAM have been published to date. MoonKAM was used to engage middle school students into learning about the Moon by analyzing images they requested to be taken by GRAIL MoonKAM (Ref. 35).
• December 2012: The twin NASA probes orbiting Earth's moon have generated the highest resolution gravity field map of any celestial body. The new map, created by the GRAIL mission, is allowing scientists to learn about the moon's internal structure and composition in unprecedented detail. Data from the two spacecraft also will provide a better understanding of how Earth and other rocky planets in the solar system formed and evolved.36)
Figure 18: Closer look at the highland crust (image credit: NASA/JPL-Caltech/ IPGP)
Legend to Figure 18: This image, depicting the porosity of the lunar highland crust, was derived using bulk density data from NASA's GRAIL mission and independent grain density measurements from NASA's Apollo moon mission samples as well as orbital remote-sensing data. Red corresponds to higher than average porosities and blue corresponds to lower than average porosities. White denotes regions that contain mare basalts (thin lines) and that were not analyzed.
• Gravity models from the primary mapping phase of this mission have determined the gravity field of Earth’s natural satellite to unprecedented accuracy over both the near and farside hemispheres. From an average mapping altitude of about 55 km, a spherical harmonic model to degree and order 420 has been constructed, which corresponds to a spatial wavelength of about 25 km.37)
Analyses of these models, in combination with topography data from LOLA (Lunar Orbiter Laser Altimeter) onboard NASA’s LRO (Lunar Reconnaissance Orbiter), have led to several discoveries. The density of the upper portion of the lunar crust was found to be 2550 kg m-3, substantially less than generally assumed. Combined with independent estimates of crustal grain density, this bulk density implies average crustal porosities of about 12%. Lateral variations in crustal density were found to correlate with crustal composition. Lateral variations in porosity were found associated with the youngest large impact basins. From the GRAIL crustal densities, global crustal thickness maps were constructed and show that the average thickness is likely between 34 and 43 km, values up to 20 km thinner than previously inferred.
Figure 19: Density of the lunar crust obtained from GRAIL primary mapping data (image credit: GRAIL science consortium)
Legend to Figure 19: The bulk density was calculated within circles 360 km in diameter. Regions in white were not analyzed, thin lines outline the maria, and solid circles correspond to prominent impact basins, whose diameters are taken as the region of crustal thinning. The largest farside basin is the South Pole-Aitken basin. Data are presented in two Lambert azimuthal equal-area projections centered over the near-(left) and farside (right) hemispheres, with each image covering 75% of the lunar surface.
During GRAIL’s XM (Extended Mission), the average altitude of the spacecraft was lowered to 23 km, allowing for the construction of gravity models with spatial resolutions considerably higher than obtained during the primary mission. The first extended mission gravity model extends up to spherical harmonic degree and order 660, corresponding to wavelengths of about 15 km, and models approaching degree and order 1000 will be attainable once all of the extended mission data have been processed.
Ultra-high-resolution gravity models will allow for substantial improvements in our understanding of how impact cratering has affected the lunar crust. These lessons will be directly applicable to other planetary bodies, on which impact craters are often in a less well-preserved state than those on the Moon.
With a 360 km resolution for the density estimates, it was possible to see the effects of large-scale compositional variations (such as those associated with the South Pole-Aitken basin) and the consequences of enhanced porosity surrounding the largest impact basins. Analyses using extended mission data will provide crustal density estimates with spatial scales appropriate for investigating simple and complex craters, impact melt pools interior to basins, and volcanic structures.
Depth dependence of porosity: The GRAIL bulk density and porosity estimates represent an average over the entire crust that depends upon the magnitude of surface relief in the study region and the density profile of the underlying crust. If the density of the crust were constant at all depths below the deepest topographic excursion, the density estimates in Figure 19 would represent an average over the depth scales of the surface topography, which is on the order of about 4 km.
In Figure 20, the global effective density of the lunar crust for each spherical harmonic degree is shown. The effective densities are most representative of the farside crust since this is where the highest-amplitude short-wavelength gravity signals are found. This figure shows that whereas the effective density is close to 2550 kg m-3 for degrees near 200, the effective density decreases to about 2400 kg m-3 at degrees near 420. This observation implies that the density of the crust decreases as one approaches the surface. From these observations, it will be possible to place constraints on how density and porosity vary as functions of depth below the lunar surface.
Figure 20: Effective density of the lunar crust as a function of spherical harmonic degree under the assumption that the surface topography is uncompensated (image credit: GRAIL science consortium, Ref. 37)
Legend to Figure 20: Lithospheric flexural signals are negligible beyond degree 150. For this preliminary extended mission gravity field, the spherical harmonic coefficients are valid globally to about degree and order 420.
• GRAIL’s XM (Extended Mission) initiated on August 30, 2012, was successfully completed on December 14, 2012. The XM provided an additional three months of gravity mapping at half the altitude (23 km) of the PM (55 km) and is providing higher-resolution gravity models that are being used to map the upper crust of the Moon in unprecedented detail.38)
On December 6, 2012, the average altitudes of the two GRAIL orbiters were lowered by another factor of two, to 11 km. This maneuver enabled a very high-resolution mapping campaign over the Orientale basin. Residuals with respect to GL0420A obtained during this period are shown in Figure 21. Residuals are large in magnitude in comparison with the PM field and indicate that substantial new gravity information has been obtained.
Figure 21: Residuals with respect to GL0420A obtained during the GRAIL endgame (image credit: MIT, NASA)
• Fall 2012: The timeline for the extended mission is shown in Figure 22. GRAIL’s extended mission will deliver geophysics at the scale of surface geology by flying in formation at an average altitude of 23 km over the surface of the Moon. The objective of extended mission is to determine the structure of lunar highland crust and maria, addressing impact, magmatic, tectonic and volatile processes that have shaped the near surface. A summary of GRAIL’s extended mission science investigations and measurement requirements is given in Table 2. 39)
The GRAIL XM (Extended Mission) was chosen to map the gravity at the even closer mean altitude of 25 km for three months beginning in September 2012. In general, three maneuvers were required each week from mid-August 2012 to December 2012 to keep the orbiters from impacting the Moon at this close range and maintain orbiter-to-orbiter separation close to 60 km. The last phase in the prime mission summary above, the Lunar Eclipse (LEC) Phase, replaced a decommissioning phase where the PM (Prime Mission) was set to end when the orbiters would eventually impact the Moon on June 4th, 2012. 40)
Figure 22: Timeline of GRAIL's extended mission (image credit: MIT, NASA)
GRAIL successfully completed its PM (Prime Mission) on schedule and under budget. The mission achieved NASA’s minimum mission success criteria for the primary mission in May 2012. GRAIL was successful in collecting its required data with a total science data volume at the end of prime mission of 637 MB or >99.99% of possible data. Following the primary mission, the two GRAIL spacecraft successfully transited the partial lunar eclipse of June 4, 2012 and the mission team is currently preparing for extended mission data gathering at very low altitude (23 km). Finally, GRAIL’s data set is already being used to facilitate current and future exploration of the Moon (Ref. 39).
Table 2: GRAIL’s extended mission science objectives
Figure 23: Measurement requirements for GRAIL’s XM (in blue) in comparison to the PM (in red and black), image credit: MIT, NASA (Ref. 38)
Table 3: The timeline of the nine-month GRAIL prime mission is divided into the following unique mission phases
• Extended science phase: The science phase of GRAIL's extended mission runs from August 30 to Dec. 14, 2012. Its goals are to take an even closer look at the moon's gravity field, deriving the gravitational influence of surface and subsurface features as small as simple craters, mountains and rilles. To achieve this unprecedented resolution, GRAIL mission planners are halving the operating altitude – flying at the lowest altitude that can be safely maintained. 41)
Figure 24: Heliocentric view of the extended mission of GRAIL (image credit: NASA/JPL)
The spacecraft systems and subsystems engineers thoroughly analyzed the extended mission design and concluded that there are no spacecraft limitations that would preclude its successful execution. There are three main areas where the team focused its analyses (Ref. 17):
1) Lunar eclipse survival: Using the worst-case eclipse phasing data provided by the GRAIL mission design team, the spacecraft team collectively developed a baseline spacecraft configuration to use prior to and during the lunar eclipse. The goal was to use mission phases, operating modes, and other parameter settings that had been previously checked out during the primary mission. The thermal team performed a detailed assessment of the component temperatures to ensure that they will remain within acceptable ranges and satisfy payload derived stability requirements. The team also used its thermal model to calculate a worst-case heater power during the lunar eclipse. Using these average power loads and the baseline spacecraft configuration assumptions, the power subsystem performed detailed battery analysis of the lunar eclipse. Results show that the worst-case depth-of-discharge will be well below the limit allowed by FP (Fault Protection). In fact, the eclipse depth-of-discharge will be only slightly higher than the LOI (Lunar Orbit Insertion) depth-of-discharge, the highest flight level to date.
2) Low altitude science operations: There have been no spacecraft anomalies during the primary mission that jeopardize the completion of the extended mission. The complexity of the spacecraft operations during the low-attitude science operations will be no greater than the TSF (Transition to Science Formation) phase, and therefore is within proven capabilities. All operational procedures and contingency plans are in place from the primary mission Science phase and are flight-proven. There are no changes to existing sequence designs, onboard command blocks, telemetry and command dictionaries, or ground software required for the extended mission.
3) Extending the mission by an additional six months poses no threats to any of the spacecraft components. This is because GRAIL’s 9-month primary mission duration was already well below known life-limitations. All components were compared against their flight limits and shown to have adequate margin to complete the extended mission. The lone exception was the reaction wheels where additional vendor life-testing using the qualification model was funded in March 2012 and is expected to demonstrate an increased flight limit of 3.6 billion revolutions.
• At the completion of the PM (Prime Mission) in late May 2012, periapsis-raising maneuvers circularized the spacecraft orbits at an altitude of ~84 km for the low-activity Low Beta Angle phase. For ten weeks subsequent to a lunar eclipse passage on June 4, 2012, the orientation of the orbit plane relative to the Sun did not allow for operation of the LGRS payloads while in orbiter-point configuration because of the Sun-Moon-Earth geometry. A second three-month XM (Extended Mission) science phase was initiated on August 30, 2012 (Ref. 38). A heliocentric view of the XM is shown in Figure 24.
• At the end of May 2012, the GRAIL mission has completed its prime mission earlier than expected, to study the moon from crust to core. Since March 8, the twin spacecraft have operated around the clock for 89 days. From an orbit that passes over the lunar poles, they have collected data covering the entire surface three times. The GRAIL team is now preparing for extended science operations starting Aug. 30 and continuing through Dec. 3, 2012.42) 43)
- Both spacecraft instruments will be powered off until Aug. 30. The spacecraft will have to endure a lunar eclipse on June 4. The eclipse and the associated sudden changes in temperature and the energy-sapping darkness that accompanies the phenomena were expected and do not concern engineers about the spacecraft's health.
- The extended mission goal is to take an even closer look at the moon's gravity field. To achieve this, GRAIL mission planners will halve their current operating altitude to the lowest altitude that can be safely maintained (23 km altitude during the extended mission).
- Along with mission science, GRAIL's MoonKAM education and public outreach program is also extended. To date over 70,000 student images of the moon have been obtained (Ref. 43).
LM (Lockheed Martin), the flight system manufacturer, is operating the orbiters from their control center in Denver, Colorado. The orbiters together have performed 28 propulsive maneuvers to reach and maintain the science phase configuration. Execution of these maneuvers, as well as the payload checkout and calibration activities, has gone smoothly due to extensive pre-launch operations planning and testing. The key to the operations success has been detailed timelines for product interchange between the operations teams and proven procedures from previous JPL/LM planetary missions (Ref. 42).
During the 82-day science phase, the Moon rotates three times underneath the GRAIL orbit, resulting in three mapping cycles of 27.3 days. From the start of Mapping Cycle 1 to the start of Mapping Cycle 2, the separation distance drifted from 75 km to 216 km. A small OTM (Orbit Trim Maneuver), the first and only prime mission maneuver using only the ACS thrusters, was executed near the end of Mapping Cycle 1. This OTM changed the separation drift rate to bring the orbiters closer together again to approximately 65 km at the end of Mapping Cycle 3 (end of the science phase). The data collected at the shorter separation distances enable determination of the higher-order gravity coefficients that describe shorter-wavelength and shallow structure, whereas that collected at the longer distances favors determination of the lower-order coefficients that describe longer-wavelength and deeper structure (Ref. 42).
• On March 14, 2012, MoonKAM operations started. Now students from around the world have the unique opportunity to schedule and capture video clips and images of the lunar surface using wide-angle and narrow-angle cameras, using the imagery to study lunar features such as craters, highlands and maria while also learning about past and future landing sites. 44)
• Solar flare on March 7, 2012: An X-class solar flare on March 7, the largest since 2003, raised the proton flux levels in the vicinity of the Earth (Figure ) and affected the orbiters in several areas:
- The USO (Ultra-stable Oscillator) on both orbiters saw frequency shifts
- The GRAIL-B GPA rebooted
- The GRAIL-B MoonKAM experienced an over-current condition on the 12 V power supply that powers the four camera heads
- The GRAIL-A MoonKAM had a communication interruption with the orbiter C&DH (Command and Data Handling) unit. The spacecraft fault protection turned off the GRAIL-B MoonKAM and power-cycled the GRAIL-A MoonKAM.
The GRAIL-A and GRAIL-B MoonKAMs were powered on again on March 13 and 14, respectively, but another 12 V power-supply over-current event on GRAIL-B on April 2, 2012 caused spacecraft fault protection to again power it off. Space weather was quiet at this time and apparently was not the cause of the over-current. After this event the USO heater #2 was turned on and the MoonKAM was left off until Mapping Cycle #2 was complete to ensure minimum thermal disturbance to science data collection. The GRAIL-B MoonKAM was turned on again on April 30.
Figure 25: Proton flux levels following solar flare of March 7 and times of orbiter events(image credit: NASA/JPL)
• On March 1, 2012 (a week ahead of schedule), the GRAIL mission started with the science collection phase aimed at precisely mapping the Moon’s gravity field, interior composition and evolution. The two spacecraft, Ebb and Flow, were in a near-polar and near-circular orbit with an average altitude of ~ 55 km and a period of ~ 115 minutes. Favorable power and thermal conditions permitted staying in continuous orbiter-point attitude on March 1 with a solar beta angle of 43º rather than the planned March 8 at 49º beta angle (Ref. 42). 45) 46)
In their operational phase (or science collection phase), the formation-flying spacecraft made detailed science measurements from lunar orbit with unparalleled precision by transmitting Ka-band radio signals between each other and Earth to help unlock the mysteries of the Moon’s deep interior.
• Feb. 2012: At the completion of the OPR phase, both orbiters were in low, near-circular polar orbits with GRAIL-B lapping GRAIL-A every three days. Now the flight team’s task was to bring the orbiters to their final science orbits,6 with GRAIL-B leading GRAIL-A. Five TSMs (Transition-to-Science Maneuvers) were required to place the orbiters into the science formation.
- On Feb. 7, 2012, TSM-A1 adjusted GRAIL-A’s orbit eccentricity and argument of periapsis so that no eccentricity correction maneuvers would be required in the science phase. Then on Feb. 13, 2012, when GRAIL-B was 30 minutes ahead of GRAIL-A, TSM-B1 placed GRAIL-B into the same orbit as GRAIL-A. The two orbiters were now “in synch” but too far apart for science data collection.
- The next three maneuvers brought the orbiters to within 75 km to start the science phase: TSM-A2 on Feb. 20 , 2012 put GRAIL-A in the science orbit (period=113.6 minutes) and started a drift to reduce the separation distance. TSM-B2 on Feb. 24 2012 slowed the drift rate. TSM-B3 on Feb. 29, 2012 stopped the drift when the orbiters were 75 km apart but also imparted a slow positive drift rate targeted for a separation distance of 225 km on March 30, at the end of the first mapping cycle.
- On March 1, 2012 the orbiters were commanded from “sun-point” to “orbiter-point” attitude mode where the solar panels are closely aligned with the orbit plane (Ref. 42).
A typical ground track on March 2, 2012 and the corresponding inter-orbiter range residuals along that track as measured by the Ka-band dual one-way tracking is shown in Figure 26. The bottom portion of the figure shows the topography along the track and the orbit altitude. The figure clearly shows that the gravity measurement is more sensitive at lower orbit altitudes. The altitude varies between 24 and 86 km on this orbit.
Figure 26: Inter-orbiter Ka-band range residuals from GRAIL match the ground track topography from the LOLA instrument on the Lunar Reconnaissance Orbiter, and are more sensitive at lower altitudes (image credit: NASA/JPL)
Figure 27: Imagery of the south pole of the far side of the moon acquired with MoonKAM on the Ebb spacecraft (image credit: NASA/JPL)
• On January 17, 2012, the two spacecraft, GRAIL-A and GRAIL-B, were renamed to Ebb and Flow, respectively. The NASA jury announced the names selected from a nationwide student contest for the twin spacecraft. Nearly 900 classrooms with more than 11,000 students from 45 states, Puerto Rico and the District of Columbia participated in the contest (Ref.11).
- The winning names were submitted by the forth grade class of Nina DiMauro at the Emily Dickinson Elementary School in Bozeman, Montana. 48)
The winning students wrote: "We have been studying the Solar System and learning about the Sun, Planets, and the Moon. We think Ebb and Flow (or Flood) are good names for Grail-A and Grail-B because the Moon's gravity is the reason we have high tides and low tides. We thought it would be good to have names that represent something very important about the moon and what it causes to happen on Earth. Grail-A and Grail-B are on a journey just like the Moon is on a journey around Earth."
The jury was really impressed that the students drew their inspiration by researching GRAIL and its goal of measuring gravity. Ebb and Flow truly capture the spirit and excitement of our mission.
• The OPR (Orbital Period Reduction) phase started on January 2, 2012. Each of the spacecraft have conducted three period reduction maneuvers to get into lower orbits. GRAIL-A made its first orbit adjustment burns on January 6 and 7. GRAIL-B followed one week later and also made three successful engine firings.
• The GRAIL-A spacecraft reached lunar orbit on Dec. 31, 2011. The spacecraft is in an elliptical orbit of 90 km x 8,363 km around the moon that takes approximately 11.5 hours to complete. 49)
This low-energy, long-duration trajectory has given mission planners and controllers more time to assess the spacecraft's health. The path also allowed a vital component of the spacecraft's single science instrument, the USO (Ultra Stable Oscillator), to be continuously powered for several months. That allowed it to reach a stable operating temperature long before science measurements from lunar orbit are to begin.
Figure 28: Artist's concept of GRAIL-B performing its lunar orbit insertion burn (image credit: NASA/JPL)
• On Oct. 5, 2011, GRAIL-B successfully executed its first flight path correction maneuver. The rocket burn helped refine the spacecraft's trajectory as it travels from Earth to the moon and provides separation between itself and its mirror twin, GRAIL-A. The first burn for GRAIL-A occurred on Sept. 30. 2011. These burns are designed to begin distancing GRAIL-A and GRAIL-B's arrival times at the moon by approximately one day and to insert them onto the desired lunar approach paths. 51)
Sensor complement: (LGRS, E/PO)
The GRAIL mission has two primary science objectives on each of the GRAIL orbiters to determine the structure of the lunar interior, from crust to core and to improve the understanding of the thermal evolution of the Moon. The GRAIL mission has one secondary science objective which is to extend the knowledge gained on the internal structure and thermal evolution of the Moon to other terrestrial planets (Ref. 6).
The Moon offers a unique opportunity for understanding the evolution of all terrestrial bodies as it is easy to get to and has been studied extensively. Several samples have been returned from the lunar surface which allow for a strong base understanding of the lunar composition. These samples very well represent the early history of the solar system due to the lack of tectonic activity on the Moon. This allows for the Moon to be used as a planetary time capsule to peer back into the distant past to near when the Moon formed to allow for reconstructing the early history of the solar system.
The GRAIL mission will accomplish these goals by performing global, regional and local high-resolution (30 km x 30 km), high-accuracy (< 10 mgal) gravity field measurements with twin, low-altitude (55 km) polar-orbiting spacecraft using a Ka-band ranging instrument.
Note: 1 gal is the measure of acceleration (used in particular in gravity measurements) and is defined in SI units as. 1 gal = 10-2 m s-2 = 1 cm s-2 ; 1 mgal = 10-5 m s-2 [named after Galileo Galileo (1564-1642), Italian mathematician, astronomer and physicist].
Data from past missions have been successfully used to obtain lunar gravity maps. However, these data sets are poor for the backside of the Moon as they rely on DSN (Deep Space Network) coverage of a single spacecraft. GRAIL will provide much better near side and vastly improved far side fidelity of the lunar gravity field.
Table 4: Lunar science objectives: from crust to core
LGRS (Lunar Gravity Ranging System):
Each spacecraft features a Ka-band LGRS of GRACE heritage to measure the intersatellite range rate. The objective is ultra-precise satellite-to-satellite tracking (SST) in low-low orbit. Variations in the gravity field cause the range between the two satellites to vary. The relative range is measured by LGRS utilizing a dual-one-way ranging measurement to precisely measure the relative motion between the two orbiters.
The LGRS consists of an USO (Ultra-Stable Oscillator), MWA (Microwave Assembly), a TTA (Time-Transfer Assembly), and the GPA (Gravity Recovery Processor Assembly). The USO provides a steady reference signal that is used by all of the instrument subsystems. Within the LGRS, the USO provides the reference frequency for the MWA and the TTA. The MWA converts the USO reference signal to the Ka-band frequency, which is transmitted to the other orbiter. The function of the TTA is to provide a two-way time-transfer link between the spacecraft to both synchronize and measure the clock offset between the two LGRS clocks. The TTA generates an S-band signal from the USO reference frequency and sends a GPS-like ranging code to the other spacecraft. The GPA combines all the inputs received from the MWA and TTA to produce the radiometric data that is downlinked to the ground. In addition to acquiring the inter-spacecraft measurements, the LGRS also provides a one-way signal to the ground based on the USO, and is transmitted via the X-band RSB (Radio Science Beacon). The steady-state drift of the USO is measured via the one-way Doppler data provided by the RSB. The elements of the instrument work together to get µ-level precision relative range differences of the two orbiters. The instrument block diagram and interactions is shown in Figure 29.
Figure 29: Block diagram of the LGRS instrument (image credit: NASA/JPL)
The key measurement is the LOS (Line Of Sight) range-rate made with an accuracy of 4.5 µm/s over a 5-second sample interval. These data are collected along with DSN tracking data over a period of 27.3 days, providing global coverage of the Moon. The entire set of mapping cycle data is then processed to recover the global gravity map.
Table 5: Science requirements and system performance, 90-day mission CBE (Current Best Estimate) performance
E/PO (Education/Public Outreach):
GRAIL is NASA's first planetary mission carrying instruments fully dedicated to education and public outreach. The Lunar Ops E/PO payload is a set of cameras which will image the lunar surface. These will be operated by middle school children to provide an early exposure to the challenges and processes used in spacecraft operations.
MoonKAM (Moon Knowledge Acquired by Middle school students):
MoonKAM is GRAIL's signature education and public outreach program led by Sally Ride, America's first woman in space, and her team at Sally Ride Science in collaboration with undergraduate students at the University of California, San Diego. The goal is to engage middle schools across the country in the GRAIL mission and lunar exploration. Tens of thousands of fifth- to eighth-grade students will select target areas on the lunar surface and send requests to the GRAIL MoonKAM Mission Operations Center (MOC). Photos of the target areas will be sent back by the GRAIL satellites and made available in the Images section of this Web site. Students will use the images to study lunar features such as craters, highlands, and maria while also learning about future landing sites. 52)
While the two GRAIL spacecraft perform their gravitational experiment, they will also serve as eyes on the Moon for Earth's students. The GRAIL MoonKAM will allow classrooms to request pictures of the lunar surface from cameras on the twin satellites and, similar to EarthKAM (on ISS), students will be able to analyze the images when they are posted to the GRAIL MoonKAM website.
The MoonKAM camera was provided by Ecliptic Enterprises Corporation, Pasadena, CA; it is of RocketCamTM DVS (Digital Video System) family heritage. The MoonKAM system contains a digital video controller and four camera heads. Each spacecraft, Ebb and Flow, carries one MoonKAM system. This system can be used to take images or video of the lunar surface with a frame rate up to 30 frames per second. 53) 54)
Figure 30: Components from a RocketCam digital video system by Ecliptic Enterprises with a digital video controller (image credit: NASA/Sally Ride Science)
Legend to Figure 30: The left photo is a MoonKAM camera head; the right photo shows the four camera heads and a digital video controller.
The GRAIL project consists of the components shown in Figure 31: the FS (Flight System), the GS (Ground System), which includes the MS (Mission System) and SDS (Science Data System), and the LS (Launch System). The Mission System is composed of MDN (Mission Design and Navigation), and the MOS (Mission Operations System), which includes the DSN (Deep Space Network) and the GDS (Ground Data System). The SDS is responsible for the processing, distribution, and archival of GRAIL science data products.
Within the Mission System, the MDN is responsible for the formulation of the interplanetary trajectory and orbital design during the development phases of the mission. The MOS is composed of the people, processes and procedures, ground hardware and software, and facilities required to operate the GRAIL FS. The MOS provides support for FS testing during the ATLO (Assembly, Test and Launch Operations). 55) 56)
Figure 31: GRAIL project architecture (image credit: NASA/JPL)
Figure 32: Operational view of MOS, showing key relationships and responsible organizations (image credit: NASA/JPL)
MOS (Mission Operations System): The MOS is distributed between JPL and Lockheed Martin (LM). JPL is responsible for overall mission management and provides many of the operations teams needed to conduct operations. LM provides the primary Mission Control Center, and is responsible for spacecraft and real-time operations of the two orbiters, as well as two high-fidelity flight system simulators.
The SDS (Science Data System) operates from JPL, performing Level-1 data processing, and the Level-0 and Level-1 data archiving in the Planetary Data System (PDS), in cooperation with the GRAIL Science Team led by the GRAIL Principal Investigator (PI) at the Massachusetts Institute of Technology (MIT).
MoonKAM Operations, led by SRS (Sally Ride Science) located in San Diego, CA, is responsible for day-to-day MoonKAM E/PO operations, interfacing directly with the MOS payload operations team. The color-coding in Figure 32 indicates which operational functions each organization contributes. An operations function is defined as a group of related activities that when combined with other operations functions, supports the overall accomplishment of mission operations.
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (email@example.com).