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TESS (Transiting Exoplanet Survey Satellite)

Overview    Spacecraft    Launch    Sensor Complement  References

TESS is a space telescope in NASA's Explorer program, designed to search for extrasolar planets using the transit method. The primary mission objective for TESS is to survey the brightest stars near the Earth for transiting exoplanets over a two-year period. The TESS project will use an array of wide-field cameras to perform an all-sky survey. It will scan nearby stars for exoplanets. 1) 2) 3)

In the first-ever spaceborne all-sky transit survey, TESS will identify planets ranging from Earth-sized to gas giants, orbiting a wide range of stellar types and orbital distances. The principal goal of the TESS mission is to detect small planets with bright host stars in the solar neighborhood, so that detailed characterizations of the planets and their atmospheres can be performed.

TESS will monitor the brightnesses of more than 200,000 stars during a two year mission, searching for temporary drops in brightness caused by planetary transits. Transits occur when a planet's orbit carries it directly in front of its parent star as viewed from Earth. TESS is expected to catalog more than 1,500 transiting exoplanet candidates, including a sample of ~500 Earth-sized and ‘Super Earth' planets, with radii less than twice that of the Earth. TESS will detect small rock-and-ice planets orbiting a diverse range of stellar types and covering a wide span of orbital periods, including rocky worlds in the habitable zones of their host stars.

The lead institution for TESS is MIT (Massachusetts Institute of Technology), with George Ricker as PI (Principal Investigator). The MIT/LL (Lincoln Laboratory) is responsible for the cameras, including the lens assemblies, detector assemblies, lens hoods, and camera mount. NASA/GSFC (Goddard Space Flight Center) provides project management, systems engineering, and safety and mission assurance. Orbital ATK (OA) builds and operates the spacecraft. The mission is operated from the OA Mission Operations Center.

The TESS Science Center, which analyzes the science data and organizes the co-investigators, collaborators, and working groups (with members from many institutions) is a partnership among MIT's Physics Department and Kavli Institute for Astrophysics and Space Research, the SAO (Smithsonian Astrophysical Observatory), and the NASA Ames Research Center. The raw and processed data are archived at the Mikulski Archive for Space Telescopes, at the Space Telescope Science Institute.

Some background: TESS is a NASA-based mission, selected in 2013 as an astrophysics mission in the Explorers Program. TESS has a long history, beginning as a small, privately funded mission in 2006. It started with financial backing from private companies, including Google, the Kavli Foundation, and donors at MIT. This all changed in 2008, when MIT proposed TESS as an official NASA astrophysics mission, re-structuring it as a SMEX (Small Explorer) Class Mission. After not being selected in this competitive process for NASA resources, TESS proposed again in 2010 as a NASA Explorer (EX) Class Mission. TESS is the first of this new classification of Explorer missions. In 2013, TESS was successful in the proposal process and NASA began the development of the project. MIT's Kavli Institute of Technology for Astrophysics (MKI) has remained as an original partner in the current TESS mission, joining NASA in the next search for new worlds. 4)

TESS stars will be 30-100 times brighter than those surveyed by the Kepler satellite; thus,TESS planets should be far easier to characterize with follow-up observations. These follow-up observations will provide refined measurements of the planet masses, sizes, densities, and atmospheric properties. 5)

TESS will provide prime targets for further, more detailed characterization with the James Webb Space Telescope (JWST), as well as other large ground-based and space-based telescopes of the future. TESS's legacy will be a catalog of the nearest and brightest stars hosting transiting exoplanets, which will comprise the most favorable targets for detailed investigations in the coming decades.

The Kepler project has provided ground-breaking new insights into the population of exoplanets in our galaxies; among the discoveries made using data from Kepler is the fact that the most common members of the exoplanet family are Earths and Super-Earths. However, the majority of exoplanets found by Kepler orbit faraway, faint stars. This, combined with the relatively small size of Earths and Super-Earths, means that there is currently a dearth of such planets that can be characterized with follow-up observations.

TESS is designed to:

• Focus on Earth and Super-Earth size planets

• Cover 400 X larger sky area than Kepler

• Span stellar spectral types of F5 to M5

Transiting exoplanets allow the project to observe the following for those planets that transit nearby bright stars:

• Fundamental properties: mass, radius, orbit

• Dynamics: planet-planet interactions, mutual inclinations, moons, tides

• Atmospheric composition + structure: transmission spectrum, emission spectrum, albedo, phase function, clouds, winds.

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Figure 1: Left: Sizes and orbital periods of planets with host stars brighter than J = 10. Right: Currently known planets, including those from the Kepler and CoRoT missions as well as ground-based surveys. Figure on the right now including the simulated population of TESS exoplanet detections (image credit: NASA)

TESS will tile the sky with 26 observation sectors:

• At least 27 days staring at each 24° x 96° sector

• Brightest 100,000 stars at 1-minute cadence

• Full frame images with 30-minute cadence

• Map Northern hemisphere in first year

• Map Southern hemisphere in second year

• Sectors overlap at ecliptic poles for sensitivity to smaller and longer period planets in JWST CVZ (Continuous Viewing Zone).

 


 

Spacecraft:

The TESS mission is based on Orbital's LEOStar-2 platform, a flexible, high-performance spacecraft for space and Earth science, remote sensing and other applications. LEOStar-2 can accommodate various instrument interfaces, deliver up to 2 kW orbit average payload power, and support payloads up to 500 kg. Performance options include redundancy, propulsion capability, high data rate communications, and high-agility/high-accuracy pointing. TESS will be the eighth LEOStar-2 based spacecraft built for NASA. Previous missions include SORCE, GALEX, AIM, NuSTAR and the OCO-2 spacecraft.

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Figure 2: Illustration of Orbital ATK LEOStar-2 minisatellite (image credit: Orbital ATK)

The LEOStar-2 bus has a three-axis controlled, zero-momentum attitude control system, and two deployed solar array wings. The total observatory power draw is estimated to be 290 W, and the solar arrays are capable of producing 415 W. To achieve fine pointing, the spacecraft uses four reaction wheels and high-precision quaternions produced by the science cameras. The transmitter has a body-fixed high-gain antenna with a diameter of 0.7 m, a power of 2 W and a data rate of 100 Mbit/s. This is sufficient to downlink the science data during 4 hr intervals at each perigee.

Spacecraft bus

Heritage Orbital LEOStar-2 spacecraft bus

Launch mass

325 kg

Spacecraft size (deployed)

3.9 m x 1.2 m x 1.5 m

Redundancy

Selective

Solar arrays

400 W (EOL), Two wing solar array, fixed and articulating modes

Stabilization

3-axis zero momentum bias via 4 hydrazine thrusters, four wheel fine-pointing ACS (Attitude Control Subsystem)

Pointing accuracy

3.2 arcsec control, 2.7 arcsec knowledge

Propulsion subsystem

Mono-propellant propulsion subsystem

TCS (Thermal Control Subsystem)

Passive thermal control

Mission life

2 years

RF communications

Ka-band 100 Mbit/s science downlink

Table 1: Overview of spacecraft parameters 6)

DHU (Data Handling Unit): The DHU is a Space Micro Image Processing Computer (IPC-7000) which consists of six boards: an IPC (Image Processing Computer), which contains two Virtex-7 FPGAs (Field Programmable Gate Arrays) that serve as interfaces to the four cameras and perform high-speed data processing; a Proton 400 k single board computer, which is responsible for commanding, communicating with the spacecraft master avionics unit, and interfacing with the Ka-band transmitter; two 192 GB SSB (Solid-State Buffer) cards for mass data storage; an analog I/O power switch board to control instrument power; and a power supply board for the DHU.

The CCDs (Charge Coupled Devices) produce a continuous stream of images with an exposure time of 2 seconds. These are received by the FPGAs on the IPC, and summed into consecutive groups of 60, giving an effective exposure time of 2 minutes. During science operations, the DHU performs real-time processing of data from the four cameras, converting CCD images into the data products required for ground post-processing. A primary data product is a collection of subarrays (nominally 10 x 10 pixels) centered on preselected target stars. The Proton400 k extracts these subarrays from each 2 min summed image, compresses them and stores them in the SSB prior to encapsulation as CCSDS packets for the Ka-band transmitter. Full frame images are also stacked every 30 minutes and stored in the SSB. Data from the SSB are downlinked every 13.7 days at perigee.

At perigee, science operations are interrupted for no more than 16 hours to point TESS 's antenna toward Earth, downlink data, and resume observing. This includes a nominal 4 hr period for Ka-band science data downlink using NASA's DSN (Deep Space Network). In addition, momentum unloading is occasionally needed due to the ~1.5 N m of angular momentum build-up induced by solar radiation pressure. For this purpose TESS uses its hydrazine thrusters.

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Figure 3: Left: Diagram illustrating the orientations of the four TESS cameras, lens hoods, and mounting platform. Right: Artist's conception of the TESS spacecraft and payload (image credit: Orbital ATK, TESS Team)

 

Launch: A launch of the TESS spacecraft is scheduled for 2017 at the Cape Canaveral Air Force Station in Florida. The launch provider is SpaceX using the Falcon-9 V1.1 launch vehicle. 7)

Orbit: HEO (Highly Elliptical Orbit) with a nominal perigee of 17 RE (Earth radii) equivalent to 108,000 km, and a nominal apogee of 59 RE or 373,000 km, inclination = 28.5º, period of 13.7 days in 2:1 resonance with the Moon's orbit.

The orbit remains above the Earth's radiation belts, leading to a relatively low-radiation environment with a mission total ionizing dose of <1 krad. The nearly constant thermal environment ensures that the CCDs will operate near -75ºC, with temperature variations <0.1ºC /hr for 90% of the orbit, and <2ºC/hr throughout the entire orbit (Ref. 3).

This orbit can be reached efficiently using a small supplemental propulsion system (ΔV ~3 km/s) augmented by a lunar gravity assist. The specific path to the orbit will depend on the launch date and launch vehicle. In a nominal scenario (illustrated in Figure 4), TESS is launched from Cape Canaveral into a parking orbit with an equatorial inclination of 28.5º. The apogee is raised to 400,000 km after two additional burns by the spacecraft hydrazine system, one at perigee of the first phasing orbit, and another at perigee of the second phasing orbit. An adjustment is made at third perigee, before a lunar flyby raises the ecliptic inclination to about 40º. A final period-adjust maneuver establishes the desired apogee and the 13.7 day period. The final orbit is achieved about 60 days after launch, and science operations begin soon afterward.

The orbital period and semimajor axis are relatively constant, with long-term exchanges of eccentricity and inclination over a period of order 8-12 years (driven by a Kozai-like mechanism) 8). There are also short-term oscillations with a period of six months caused by solar perturbations ( Figure 5). The orbit is stable on the time scale of decades, or more, and requires no propulsion for station-keeping. Table 2 lists a number of advantages of this type of orbit for TESS.

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Figure 4: Maneuvers and scenario for achieving the TESS mission orbit. PLEP (Post Lunar-Encounter Perigee) and PLEA (Post Lunar-Encounter Apogee), image credit: TESS Team

• Extended and unbroken observations: >300 hr orbit-1

• Thermal stability: <0.1ºC hr-1 (passive control)

• Earth/Moon stray light: ~106 times lower than in low-Earth orbit

• Low radiation levels: no South Atlantic anomaly or outer belt electrons

• Frequent launch windows: 20 days per lunation

• High data rates at perigee: ~100 Mbit s-1

• Excellent pointing stability: no drag or gravity gradient torques

• Simple operations: single 4 hr downlink & repoint every 2 weeks

• Long lifetime: several decades above GEO (>6.6 RE)

Table 2: Characteristics of the TESS spacecraft orbit and comparisons to a low-Earth orbit

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Figure 5: Calculated time variations in the elements of the nominal TESS mission orbit. The units of each curve are specified in the legend; AOP (Argument of Perigee), GEO (Geosynchronous Earth Orbit), image credit: TESS Team

 


 

Sensor complement: (Four WFOV cameras)

The TESS payload consists of four identical cameras and a DHU (Data Handling Unit). Each camera consists of a lens assembly with seven optical elements, and a detector assembly with four CCDs (Charge Coupled Devices) and their associated electronics. All four cameras are mounted onto a single plate (Figure 6) that is attached to the spacecraft, such that their FOVs are lined up to form a rectangle measuring 24º x 96º on the sky. Four elliptical holes in the plate allow shimless alignment of the four cameras at the desired angles. 9) 10)

Each of the four cameras features:

- WFOV (Wide Field of View) of 24º x 24º

- 100 mm effective pupil diameter

- Lens assembly with 7 optical elements

- Athermal design

- 600nm - 1000 nm bandpass

- 16.8 Mpixel, low-noise, low-power, MIT/LL CCID-80 detector.

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Figure 6: Illustration of the four cameras mounted on a single plate (image credit: NASA, MIT)

 

Detector assembly: The focal plane consists of four back-illuminated MIT/LL CCID-80 devices. The CCID-80, developed for TESS, is a deep-depletion, frame-transfer CCD with a full frame store. The device has four outputs; each output is associated with an array of 512 (H) x 2048 (V) imaging pixels, for a total imaging area of 2048 (H) x 2048 (V). The die size is 32 (W) x 64 (H) mm for an area of 20.4 cm2. 11)

The imaging array, frame store, and serial registers all consist of conventional three-phase, 15 x 15 µm pixels. There is a three-phase charge injection register at the top of the array, and the serial register support bidirectional transfer. The pixel array employs a trough design feature to provide radiation mitigation for small charge packets. To enable the desired fast frame transfer time, the image array and frame store clocks are strapped with metal interconnect to reduce the RC delay from the clock lines. The output circuit is a single-stage MOSFET similar to others demonstrated at Lincoln Laboratory.

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Figure 7: The detector assembly of one of the prototype lenses. The light shield cover for the frame store regions is removed (image credit: MIT/LL)

Lincoln Laboratory supports several different styles of back-illumination processing. For TESS, a flow is used that involves: epoxy mounting the device wafer to a support wafer; wet chemical thinning the high resistivity float zone silicon to the 100 µm full depletion target; back-side passivation through an ion implantation, laser annealing sequence; deposition and patterning of antireflection and light shield coatings; and etches to provide access to the bond pads.

The true benefit of the 100 µm thick detector is shown in the Figure 8 spectral response curve. The project observed over 20% improvement in quantum efficiency at 1000 nm measurement wavelength over a 45 µm thick device. - A total of sixteen CCDs arranged in four mosaics will be needed.

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Figure 8: Measured quantum efficiency for the 100 µm thick CCID-80 device compared to a 45 µm thick reference MIT/LL CCD (image credit: MIT/LL)

Each of the four identical TESS lenses is an f=1:4 custom design consisting of seven optical elements, with an entrance pupil diameter of 10.5 cm (Figures 9 and 10). For ease of manufacture, all lens surfaces are spherical except for two mild aspheres. There are two separate aluminum lens barrels that are fastened and pinned together. All optical elements have antireflection coatings. The surface of one element also has a long-pass filter coating to enforce the cutoff at 600 nm. The red limit at 1000 nm is set by the quantum-efficiency curve of the CCDs (Figure 8).

Each lens forms a 24º x 24º unvignetted image on the four-CCD mosaic in its focal plane. The optical design was optimized to provide small image spots of a consistent size across the FOV (Field of View), and produce undersampled images similar to those of Kepler. At nominal focus and flight temperature (-75ºC), the 50% ensquared-energy half-width is 15 µm (one pixel or 0.35 arcmin) averaged over the FOV. Each lens is equipped with a lens hood, which reduces the effects of scattered light from the Earth and Moon (Ref. 3).

FOV (Field of View) of each lens

24º x 24º

Combined field of view

24º x 96º = 2300ºº (sq. deg.)

Entrance pupil diameter

10.5 cm

Focal ratio (f=#)

f/1.4

Wavelength range

600 - 1000 nm

Ensquared energy

50% within 15 x 15 µm (one pixel, or 0:35 x 0:35 arcmin)
90% within 60 x 60 µm (4 x 4 pixels, or 1:4 x 1:4 arcmin)

Table 3: Characteristics of the TESS lenses. Ensquared energy is the fraction of the total energy of the point-spread function that is within a square of the given dimensions centered on the peak.

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Figure 9: Diagram of the TESS lens assembly, CCD focal plane, and detector electronics (image credit: NASA, TESS Team)

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Figure 10: Left: Two lens prototypes that were constructed. Right: The detector assembly of one of the prototype lenses. The frame-store regions of the CCDs are covered (image credit: NASA, TESS Team)

 

Scanning strategy: The four cameras act as a 1 x 4 array, providing a combined FOV of 24º x 96º or 2300 square degrees (Figure 11). The north and south ecliptic hemispheres are each divided into 13 partially overlapping sectors of 24º x 96º, extending from an ecliptic latitude of 6º to the ecliptic pole. Each sector is observed continuously for two spacecraft orbits (27.4 days), with the boresight of the four-camera array pointed nearly anti-solar. After two orbits, the FOV is shifted eastward in ecliptic longitude by about 27º, to observe the next sector. Observing an entire hemisphere takes one year, and the all-sky survey takes two years.

The overlap of the sectors is illustrated in Figure 11. Approximately 30,000 square degrees are observed for at least 27 days. Close to the ecliptic poles, approximately 2800 square degrees are observed for more than 80 days. Surrounding the ecliptic poles, approximately 900 square degrees are observed for more than 300 days.

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Figure 11: Left: The instantaneous combined FOV of the four TESS cameras. Middle: Division of the celestial sphere into 26 observation sectors (13 per hemisphere). Right: Duration of observations on the celestial sphere, taking into account the overlap between sectors. The dashed black circle enclosing the ecliptic pole shows the region which JWST will be able to observe at any time (image credit: TESS Team)

 

Photometric performance: Figure 12 shows the anticipated photometric performance of the TESS cameras. The noise sources in this model are photon-counting noise from the star and the background (zodiacal light and faint unresolved stars), dark current (negligible), readout noise, and a term representing additional systematic errors that cannot be corrected by co-trending. The most important systematic error is expected to be due to random pointing variations ("spacecraft jitter"). Because of the non-uniform quantum efficiency of the CCD pixels, motion of the star image on the CCD will introduce changes in the measured brightness, as the weighting of the image PSF (Point Spread Function) changes, and as parts of the image PSF enter and exit the summed array of pixels.

The central pixel of a stellar image will saturate at approximately IC = 7:5. However, this does not represent the bright limit for precise photometry because the excess charge is spread across other CCD pixels and is conserved, until the excess charge reaches the boundary of the CCD. As long as the photometric aperture is large enough to encompass all of the charge, high photometric precision can still be obtained. The Kepler mission demonstrated that photon-noise{limited photometry can be obtained for stars 4 mag brighter than the single-pixel saturation limit. Since similar performance is expected for TESS, the bright limit is expected to be IC~4 or perhaps even brighter.

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Figure 12: Top: Expected 1σ photometric precision as a function of stellar apparent magnitude in the IC band. Contributions are from photon-counting noise from the target star and background (zodiacal light and unresolved stars), detector read noise (10 e-), and an assumed 60 ppm of incorrigible noise on hourly timescales. Bottom: The number of pixels in the photometric aperture that optimizes the signal-to-noise ratio (image credit: TESS Team)

 


1) "TESS - Transiting Exoplanet Survey Satellite," NASA, URL: http://tess.gsfc.nasa.gov/

2) "TESS - Transiting Exoplanet Survey Satellite, Discovering new Earths and Super-Earths in the solar neighborhood," NASA Facts, URL: http://tess.gsfc.nasa.gov/documents/TESS_FactSheet_Oct2014.pdf

3) George R. Ricker, Joshua N. Winn, Roland Vanderspek, David W. Latham, Gaspar A. Bakos, Jacob L. Bean, Zachory K. Berta-Thompson, Timothy M. Brown, Lars Buchhave, Nathaniel R. Butler, R. Paul Butler, William J. Chaplin, David Charbonneau, Jørgen Christensen-Dalsgaard, Mark Clampin, Drake Deming, John Doty, Nathan De Lee, Courtney Dressing, E. W. Dunham, Michael Endl, Francois Fressin, Jian Ge, Thomas Henning, Matthew J. Holman, Andrew W. Howard, Shigeru Ida, Jon M. Jenkins, Garrett Jernigan, John Asher Johnson, Lisa Kaltenegger, Nobuyuki Kawai, Hans Kjeldsen, Gregory Laughlin, Alan M. Levine, Douglas Lin, Jack J. Lissauer, Phillip MacQueen, Geoffrey Marcy, P. R. McCullough, Timothy D. Morton, Norio Narita, Martin Paegert, Enric Palle, Francesco Pepe, Joshua Pepper, Andreas Quirrenbach, S. A. Rinehart, Dimitar Sasselov, Bun'ei Sato, Sara Seager, Alessandro Sozzetti, Keivan G. Stassun, Peter Sullivan, Andrew Szentgyorgyi, Guillermo Torres, Stephane Udry, Joel Villasenor, "The Transiting Exoplanet Survey Satellite," Oct. 28, 2014, URL: http://arxiv.org/pdf/1406.0151.pdf

4) "Mission History," NASA, URL: http://tess.gsfc.nasa.gov/mission_history.html

5) http://tess.gsfc.nasa.gov/whytess.html

6) "TESS Fact Sheet," Orbital ATK , URL: https://www.orbitalatk.com/space-systems/science-national-security-satellites/science-environment-satellites/docs/FS011_13_OA_3862%20TESS.pdf

7) Matt Williams, "Exoplanet-Hunting TESS Satellite to be Launched by SpaceX," Universe Today, Jan. 6, 2015, URL: http://www.universetoday.com/117845/exoplanet-hunting-tess-satellite-to-be-launched-by-spacex/

8) Joseph W. Gangestad, Gregory A. Henning, Randy R. Persinger, George R. Ricker, "A High Earth, Lunar Resonant Orbit for Lower Cost Space Science Missions," Advances in the Astronautical Sciences, Vol. 150, 2014, URL: http://arxiv.org/pdf/1306.5333v3.pdf

9) "TESS Science Instrument," NASA, URL: http://tess.gsfc.nasa.gov/instrument.html

10) "TESS - Transiting Exoplanet Survey Satellite," MIT, 2014, URL: http://www.stsci.edu/~pmcc/TESS/TESS_AAS_2014_factsheet.pdf

11) V. Suntharalingam, J. S. Ciampi, M. J. Cooper, R. D. Lambert, D. M. O'Mara, I. Prigozhin, D. J. Young, K. Warner, B. E. Burke, "Scientific, Back-Illuminated CCD Development for the Transiting Exoplanet Survey Satellite," Image Sensors World, Vaals, The Netherlands, June 8-11, 2015 , URL: http://www.imagesensors.org/Past%20Workshops/2015%20Workshop/2015-20Papers/Sessions/Session_10/10-01_Suntharalingam_IISW2015.pdf
 


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 (herb.kramer@gmx.net).

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