ASTERIA (Arcsecond Space Telescope Enabling Research in Astrophysics)
ASTERIA is a technology demonstration and opportunistic science mission to advance the state of the art in CubeSat capabilities for astrophysical measurements. The goal of ASTERIA is to achieve arcsec-level line of sight pointing error and highly stable focal plane temperature control. These technologies will enable precision photometry, i.e. the careful measurement of stellar brightness over time. This in turn provides a way to study stellar activity, transiting exoplanets, and other astrophysical phenomena, both during the ASTERIA mission and in future CubeSat constellations. 1) 2) 3)
The ASTERIA project is a collaboration of MIT (Massachusetts Institute of Technology) with Sara Seager as PI (Principal Investigator) and NASA/JPL, funded through the Phaeton Program for training early career employees. JPL is responsible for overall project management, systems engineering, attitude determination and control, flight software, spacecraft implementation, integration and test, and mission operations. Flight hardware delivery is scheduled for March 2017, with launch and deployment targeted for Summer 2017.
The objectives of the project are to achieve arcsec-level line of sight pointing error, and highly stable focal plane temperature control. These technologies enable precision photometry—i.e. the measurement of the brightness of stars over time. Photometry is compelling because it provides a way to detect transiting exoplanets, and characterize their host stars.
• A CubeSat space telescope for demonstrating new technologies for studying nearby stars
• Stabilizes the position of a star image on the telescope detector
• Returns a star image to the same location on the telescope detector
• Holds the temperature of the telescope detector constant
• Downloads images of the star so that its brightness can be observed over time
• Serves as a pathfinder for a fleet of low-cost space telescopes observing multiple targets at once.
Background: The ASTERIA project, formerly called ExoplanetSat that was based on a 3U CubeSat mission, is motivated by the thought that no current mission has the capability to survey the nearest sun-like stars for an Earth analog. The best way to monitor the brightest sun-like stars (0 < V < 6) for long-duration transiting exoplanets is by a targeted star search; the brightest stars are too widely separated across the sky for a single telescope to continuously monitor. A fleet of dozens of identical ASTERIA spacecraft would be ideal to each monitor one star at a time, instead of a single space telescope surveying thousands of stars simultaneously. The detailed properties of each target star are well known in advance.
ASTERIA is a 6U CubeSat (~10 x 20 x 30 cm, 12 kg) that will operate in LEO (Low Earth Orbit). The payload consists of a lens and baffle assembly, a CMOS imager, and a two-axis piezoelectric positioning stage on which the focal plane is mounted. A set of commercial reaction wheels provides coarse attitude control. Fine pointing control is achieved by tracking a set of guide stars on the CMOS sensor and moving the piezoelectric stage to compensate for residual pointing errors. Precision thermal control is achieved by isolating the payload from the spacecraft bus, passively cooling the detector, and using trim heaters to perform small temperature corrections over the course of an observation.
The main spacecraft subsystem suppliers are Blue Canyon Technologies (Attitude Control Subsystem), Vulcan Wireless (Telecommunications Subsystem), MMA Design LLC (Solar Arrays), GomSpace (Power subsystem and Batteries), Spaceflight Industries (Flight computer), Ecliptic Enterprises (Focal Plane), Physik Instrumente (piezo stage), and Thermotive (thermal hardware). Morehead State University will provide spacecraft tracking, telemetry, and control services to the Mission Operations team at JPL. MIT will perform target selection and analysis of stellar photometry data from ASTERIA (Ref. 3).
Figure 1: Electrical Test Engineer Esha Murty (left) and Integration and Test Lead Cody Colley (right) prepare the ASTERIA spacecraft for mass properties measurements in April 2017 prior to spacecraft delivery (image credit: NASA/JPL)
The first objective of the ASTERIA mission is to demonstrate arcsec-level pointing control both during a single observation, and from one observation to the next. The goal is to maintain the target star image to within a fraction of a detector pixel over long durations. Image motion over the detector pixels can cause variations in the measured brightness, since both the between-pixel (interpixel) and within-pixel (intrapixel) response varies across the detector. By holding the star image to the same fraction a pixel throughout an observation, the technology demonstrated by ASTERIA minimizes instrument-derived photometric variation, and enables more sensitive photometric monitoring of real astrophysical variations in the star. 4)
Pointing control is achieved through a two-stage approach. A set of reaction wheels provides coarse three-axis control of the spacecraft body, holding an inertial attitude that points the payload to a target star. Within the payload, a two-axis piezoelectric stage provides an additional level of fine control by making small, rapid adjustments to the detector position to keep the target star stationary.
Figure 2: The ASTERIA ADCS architecture (image credit: NASA, MIT)
The second objective of ASTERIA is to demonstrate mK (milliKelvin)-level temperature stability of the imaging detector. The gain of each pixel is temperature sensitive, so tight thermal control reduces instrumental photometric variation that might otherwise be mistaken as an astrophysical signal. Unlike other space telescopes such as Kepler and Spitzer that reside in an Earth-trailing orbit, ASTERIA is subjected to day/night cycles that occur every 90 minutes in LEO. The lack of a stable thermal environment makes active temperature control all the more important.
Precision thermal control is achieved by isolating the payload from the spacecraft bus, and passively cooling the detector using a space-facing radiative surface. Thermal sensors and trim heaters located on the detector, act in a closed loop to perform small temperature corrections over the course of an observation, and maintain stability to the required precision.
Figure 3: Members of the ASTERIA team prepare the petite satellite for its journey to space (from left to right: Robert Bocchino, Amanda Donner, Cody Colley, Alessandra Babuscia, and Peter Di Pasquale), image credit: NASA/JPL-Caltech)
Figure 4: Artist's rendition of the ASTERIA nanosatellite in orbit (image credit: NASA/JPL)
Launch: ASTERIA was launched on Aug. 14, 2017 on the SpaceX CRS-12 Dragon flight to the ISS on a Falcon-9 v1.2 vehicle. The launch site was the Launch Complex 39A (LC-39A) at NASA's Kennedy Space Center, Florida. 5) 6)
Orbit: Near circular orbit with an altitude of~400 km, inclination = 51.6º.
Four small satellites inside the Dragon capsule were transferred to the space station for deployment later this year.
1) Kestrel Eye-2M is a pathfinder microsatellite (~50 kg) for a potential constellation of Earth-imaging spacecraft for the U.S. military.
2) ASTERIA (Arcsecond Space Telescope Enabling Research in Astrophysics), a 6U CubeSat (12 kg) of MIT and NASA/JPL. The objective is to test miniature telescope components that could be used in future small satellites to observe stars and search for exoplanets.
3) Dellingr, a NASA demonstration mission on a 6U CubeSat.
4) OSIRIS (Orbital Satellite for Investigating the Response of the Ionosphere to Stimulation and Space Weather) is a 3U Cubesat of PSU (Penn State University), University Park, PA, USA.
• April 12, 2018: The ASTERIA satellite, which was deployed into low-Earth orbit on 21 November 2017, is only slightly larger than a box of cereal, but it could be used to help astrophysicists study planets orbiting other stars. Mission managers at NASA's Jet Propulsion Laboratory in Pasadena, California, recently announced that ASTERIA has accomplished all of its primary mission objectives, demonstrating that the miniaturized technologies on board can operate in space as expected. This marks the success of one of the world's first astrophysics CubeSat missions, and shows that small, low-cost satellites could be used to assist in future studies of the universe beyond the solar system. 7)
- "ASTERIA is small but mighty," said Mission Manager Matthew W. Smith of JPL. "Packing the capabilities of a much larger spacecraft into a small footprint was a challenge, but in the end we demonstrated cutting-edge performance for a system this size."
- ASTERIA (Arcsecond Space Telescope Enabling Research in Astrophysics) with a mass of 10 kg allows researchers to monitor nearby stars for orbiting exoplanets that cause a brief drop in brightness as they block the starlight.
Figure 5: This gif shows a series of images from a single observation of a star by the ASTERIA spacecraft. In the first few images, the star appears to move as ASTERIA slews to and then locks onto the target star. Throughout the remainder of the frames, the spacecraft remains locked on the target star (image credit: NASA/JPL-Caltech)
Figure 6: This graph shows the precise position of a star as seen by ASTERIA. The colored tracks show the path of ASTERIA's pointing system as it attempts to hold steady on a target. The blue tracks show the spacecraft's pointing ability using coarse pointing control, but to reach the desired level of stability, ASTERIA engages what is known as two-stage fine pointing control, shown in orange. Arcseconds are a measure of angle on the sky, used to measure the apparent size of objects or and distances that objects travel.(The full moon is about 1800 arcseconds wide), image credit: NASA/JPL-Caltech)
Figure 7: A snapshot of the sky taken by ASTERIA, showing the spacecraft's full field of view; the constellation is visible in the bottom right. Images from the satellite are cropped when scientists want to monitor an individual star (image credit: NASA/JPL-Caltech)
- This approach to finding and studying exoplanets is called the transit method. NASA's Kepler Space Telescope has detected more than 2,300 confirmed planets using this method, more than any other planet-hunting observatory. The agency's next large-scale, space-based planet-hunting observatory, the Transiting Exoplanet Survey Satellite (TESS), is anticipated to discover thousands of exoplanets and scheduled to launch from Cape Canaveral Air Force Station in Florida on April 16.
- In the future, small satellites like ASTERIA could serve as a low-cost method to identify transiting exoplanets orbiting bright, Sun-like stars. These small satellites could be used to look for planetary transits when larger observatories are not available, and planets of interest could then be studied in more detail by other telescopes. Small satellites like ASTERIA could also be used to study certain star systems that are not within the field of view of larger observatories, and most significantly, focus on star systems that have planets with long orbits that require long observation campaigns.
- The ASTERIA team has now demonstrated that the satellite's payload can point directly and steadily at a bright source for an extended period of time, a key requirement for performing the precision photometry necessary to study exoplanets via the transit method.
- Holding steady on a faraway star is difficult because there are many things that subtly push and pull on the satellite, such as Earth's atmosphere and magnetic field. ASTERIA's payload achieved a pointing stability of 0.5 arcseconds RMS, which refers to the degree to which the payload wobbles away from its intended target over a 20-minute observation period. The pointing stability was repeated over multiple orbits, with the stars positioned on the same pixels on each orbit.
- "That's like being able to hit a quarter with a laser pointer from about a mile away," said Christopher Pong, the attitude and pointing control engineer for ASTERIA at JPL. "The laser beam has to stay inside the edge of the quarter, and then the satellite has to be able to hit that exact same quarter — or star — over multiple orbits around the Earth. So what we've accomplished is both stability and repeatability."
- The payload also employed a control system to reduce "noise" in the data created by temperature fluctuations in the satellite, another major hurdle for an instrument attempting to carefully monitor stellar brightness. During observations, the temperature of the controlled section of the detector fluctuates by less than 0.02 Fahrenheit (0.01 Kelvin, or 0.01 degree Celsius).
- The 6U CubeSat, with a size of 10 x 20 x 30 cm, is about as long as a skateboard when its solar panels are deployed. COTS (Commercial-Off-The-Shelf) hardware is used whenever possible. "We're continuing to characterize CubeSat components that other missions are using or want to use," said Amanda Donner, mission assurance manager for ASTERIA at JPL.
• The ASTERIA 6U CubeSat was deployed from the International Space Station on November 21, 2017. It will test the use of CubeSats for astronomy research. 8)
Figure 8: ASTERIA deployed from the ISS on November 21 (UTC), 2017 (image credit: NASA)
- For the next few months, the ASTERIA technology demonstration will test whether a CubeSat can perform precise measurements of change in a star's light. This fluctuation is useful for a number of commercial and astrophysics applications, including the discovery and study of planets outside of our solar system, known as exoplanets.
Table 1: NanoRacks statement of the deployment of 5 CubeSats on 21 Nov. 2017 9)
Sensor complement: (Imager)
ASTERIA relies on precision photometry, a field that measures the flux, or intensity, of an object's light. To be useful to any scientist, a space telescope has to correct for internal sources of error while making these measurements.
The sensor suite includes an optics section, a dual-imager focal plane array realizing the two payload functions and a piezoelectric nano-positioning stage capable of moving the focal plane to zero-out the platform jitter.
Per the ExoPlanetSat baseline, the optics section was to use a f1.4/85 Zeiss lens with a 28.6º FOV (Field of View) and six elements, focusing an image 43 mm in diameter onto the focal plane. The focal plane array houses two active detector areas – one larger CMOS detector that fulfills the science function and a smaller CMOS sensor with smaller pixel sizes to act as a rapid-cadence star camera to provide attitude knowledge to the attitude control system. The science sensor operates at longer integration times and collects data on several pixel windows, one for the target star and several other bright stars for comparison plus dark areas of the sky for dark current correction.
Figure 9: Illustration of the ASTERIA imager architecture (image credit: NASA, MIT)
Figure 10: Photo of the ASTERIA imager (image credit: NASA/JPL)
1) "Arcsecond Space Telescope Enabling Research in Astrophysics (ASTERIA),"NASA/JPL, URL: http://www.jpl.nasa.gov/cubesat/missions/asteria.php
3) "Arcsecond Space Telescope Enabling Research in Astrophysics (ASTERIA)," NASA/JPL, URL: https://www.jpl.nasa.gov/cubesat/missions/asteria.php
4) "ASTERIA;" NASA News, Oct. 18, 2017, URL: https://www.nasa.gov/mission_pages/station/research/experiments/2513.html
5) "SpaceX CRS-12 Cargo Mission Launch," NASA, August 14, 2017, URL: https://www.nasa.gov/image-feature/spacex-crs-12-cargo-mission-launch
6) "CRS-12 Dragon Resupply Mission," SpaceX, URL: http://www.spacex.com/sites/spacex/files/crs12presskit.pdf
7) "Astrophysics CubeSat Demonstrates Big Potential in a Small Package," NASA/JPL, 12 April 2018, URL: https://www.jpl.nasa.gov/news/news.php?release=2018-072
8) "JPL Deploys a CubeSat for Astronomy," NASA/JPL, 7 Dec. 2017, URL: https://www.jpl.nasa.gov/news/news.php?release=2017-314
9) "NanoRacks Completes 13th CubeSat Deployment Mission from Space Station, First "Doublewide" Satellites," NanoRacks, 21 Nov. 2017, URL: http://nanoracks.com/13-cubesat-deployment-doublewide/
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 (firstname.lastname@example.org).