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: 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.
• 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. 7)
Figure 4: 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 8)
Sensor complement: (Imager)
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 5: Illustration of the ASTERIA imager architecture (image credit: NASA, MIT)
Figure 6: 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) "JPL Deploys a CubeSat for Astronomy," NASA/JPL, 7 Dec. 2017, URL: https://www.jpl.nasa.gov/news/news.php?release=2017-314
8) "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).