The Transiting Exoplanet Survey Satellite

The Transiting Exoplanet Survey Satellite (TESS) will search for planets transiting bright and nearby stars. TESS has been selected by NASA for launch in 2017 as an Astrophysics Explorer mission. The spacecraft will be placed into a highly elliptical 13.7-day orbit around the Earth. During its two-year mission, TESS will employ four wide-field optical CCD cameras to monitor at least 200,000 main-sequence dwarf stars with I = 4-13 for temporary drops in brightness caused by planetary transits. Each star will be observed for an interval ranging from one month to one year, depending mainly on the star's ecliptic latitude. The longest observing intervals will be for stars near the ecliptic poles, which are the optimal locations for follow-up observations with the James Webb Space Telescope. Brightness measurements of preselected target stars will be recorded every 2 min, and full frame images will be recorded every 30 min. TESS stars will be 10-100 times brighter than those surveyed by the pioneering Kepler mission. This will make TESS planets easier to characterize with follow-up observations. TESS is expected to find more than a thousand planets smaller than Neptune, including dozens that are comparable in size to the Earth. Public data releases will occur every four months, inviting immediate community-wide efforts to study the new planets. The TESS legacy will be a catalog of the nearest and brightest stars hosting transiting planets, which will endure as highly favorable targets for detailed investigations.


Introduction
The study of exoplanets-planets outside our solar system-is one of the most exciting and rapidly advancing fields of science. Especially valuable are systems in which a planet's orbit carries it directly across the face of its host star. For such a "transiting" planet, it is possible to determine the planet's mass and radius, its orbital parameters, and its atmospheric properties. 1 Of particular interest are planets with sizes between those of the Earth and Neptune. Little is known about them, because there are no examples in the solar system. NASA's Kepler mission revolutionized exoplanetary science by revealing that such planets are abundant, 2,3 and seem to have a wide range of compositions 4 and interesting orbital configurations. 5 However, most of the Kepler stars are too faint for detailed follow-up observations.
The Transiting Exoplanet Survey Satellite, or TESS, will take the next logical step after Kepler by searching the nearest and brightest stars for transiting planets. The primary objective of TESS is to discover hundreds of transiting planets smaller than Neptune, with host stars bright enough for follow-up spectroscopy to measure planetary masses and atmospheric compositions. This objective will be achieved by conducting a 2-year all-sky survey, during which differential time-series photometry will be performed for hundreds of thousands of stars. The main value of TESS data will not be statistical completeness, but rather the relative ease of following up on discoveries with current and planned instruments.

History
The initial discussions leading to the TESS concept began at the Massachusetts Institute of Technology (MIT) and the Smithsonian Astrophysical Observatory (SAO) in late 2005. In early 2006, a proposal for a Mission of Opportunity was submitted to NASA to use the optical camera of the High Energy Transient Explorer 6 to perform a survey for transiting planets. This proposal was not selected.
Later in 2006 and early 2007, TESS was reformulated as a standalone small mission. Seed funding was obtained from private individuals, the Kavli Foundation, Google, MIT, the SAO, and the NASA Ames Research Center, but attempts to raise the funds required for a full mission were not successful.
In 2008, TESS was reconceived as a Small Explorer Mission (SMEX), a NASA program with a cost cap of $100m. The TESS SMEX proposal was funded for a phase A study during 2008 to 2009, but was not chosen to proceed into phase B.
During the next 2 years, the TESS concept was gradually refined, and a new proposal was submitted to the NASA Explorer program in 2011. The larger cost cap ($200m) enabled several key improvements, particularly the use of a high-Earth elliptical orbit, which provides a more stable platform for precise photometry than the low-Earth orbit that had been the basis of all the previous proposals. TESS was selected by NASA for a phase A study as an Explorer mission in Fall 2011, and proceeded to phase B in April 2013.

Design Considerations
This section describes some of the considerations that led to the current design of the TESS mission.

Sky Coverage
Although many transiting planets have been found, relatively few of them orbit stars bright enough to enable follow-up measurements of planet masses and atmospheres. Therefore, a prime objective of TESS is to monitor bright stars. Since the brightest stars are nearly evenly distributed over the entire sky, this desire led in the direction of an all-sky survey.

Period Sensitivity
Ideally, the mission would detect planets with a broad range of orbital periods, with P min near the Roche limit of a few hours, and P max of a year or longer. However, the choice of P max determines the mission duration and thereby has a disproportionate influence on cost. Furthermore, transit surveys are strongly and inherently biased toward short periods. (As shown in Refs. 7 and 8, in an idealized imaging survey limited only by photon-counting noise, the effective number of stars that can be searched for transiting planets of period P varies as P −5∕3 .) Indeed, the period distribution of Kepler detections 2 reaches a maximum at ≈10 days, taking into account the period dependencies of both the transit detection efficiency and the occurrence of planets. Hence, even a value of P max as short as 10 days should yield many discoveries.
Choosing P max as short as 10 days rules out the detection of habitable-zone planets around Sun-like stars. However, it is possible to gain access to habitable-zone planets around M stars if at least a portion of the sky is observed with P max ≳ 40 days. (For an M0 star with T eff ¼ 3800 K, the range of orbital distances receiving a bolometric insolation within a factor of two of the Earth's insolation corresponds roughly to P ¼ 25-75 days.) And if there is a portion of the sky to be searched more extensively, it would be advantageous for that portion to coincide with the zones of longest continuous visibility with the James Webb Space Telescope (JWST), which will likely be the best telescope for follow-up studies of planetary atmospheres. Those zones are centered on the ecliptic poles. 9 Together these considerations led to the choices of P max ≈ 10 days in general, and P max ≳ 40 days for as large an area as possible surrounding the ecliptic poles.

Type of Stars Monitored
While it would be interesting to survey all types of stars, it is logical to concentrate on main-sequence dwarfs with spectral types F5 to M5. Evolved stars and early-type dwarfs are large, inhibiting the detection of small planets. Dwarfs with spectral types earlier than F5 also rotate rapidly, broadening their spectral lines and preventing precise radial-velocity (PRV) monitoring.
On the other side of the spectral sequence, M dwarfs are especially attractive targets. They are abundant: about threequarters of the stars in the solar neighborhood are M0-M5 dwarfs. They are relatively unexplored for transiting planets, because they constituted only a small minority of the Kepler target list. Furthermore, the transit signal of a small planet is easier to detect for an M dwarf than it would be for a larger star of the same apparent magnitude, facilitating both planet discovery and follow-up observations with JWST and other telescopes.
However, stars with spectral types later than M5 are rarer and optically faint. They could be observed advantageously at nearinfrared wavelengths, but this would greatly increase the mission's cost, complexity, and risk. Furthermore, planets transiting the very latest-type stars can be detected with ground-based instruments, as demonstrated by the MEarth survey. 10 For these reasons, the F5-M5 range of spectral types was considered to be most important for TESS.

Detector and Bandpass
The best astronomical detectors in the optical band are silicon charge-coupled devices (CCDs), which have excellent linearity and dynamic range for bright objects, and a long spaceflight heritage. A wide bandpass is preferred to reduce photon-counting noise. However, very wide bands present challenges in constructing a wide-field and well-focused optical system. Enhanced sensitivity to red wavelengths is desired because small planets are more easily detected around small stars, which are cool and red.
These considerations led to the choice of a 600 to 1000 nm bandpass. The red end represents the red limit of silicon CCD sensitivity, and the width of 400 nm was the largest practical choice for the optical design. This bandpass is centered on the traditional I C band but is much wider (see Fig. 1). It is comparable to the union of the R C , I C , and z bands.

Number of Stars Monitored
Kepler's transit detection rate 2 for "super-Earths" (1.25 − 2 R È ) with P < 10 days was about 0.2%. This represents the product of the occurrence rate (∼5%) and the geometric transit probability (∼5%). Therefore, TESS needs to monitor at least 500 stars for each super-Earth detection. The desire to find hundreds of such planets leads to the requirement to monitor ≳10 5 stars.
According to Galactic star count models, 11 the brightest 10 5 dwarf stars have a limiting apparent magnitude of I C ≈ 10. In fact, the TESS design employs a larger catalog (≳2 × 10 5 stars) selected according to transit detectability rather than strictly on apparent magnitude. This leads to a limiting magnitude of I C ≲ 10 − 13, depending on spectral type. The faint M dwarfs, for example, have such small sizes that they can still be usefully searched for small planets.

Aperture Size
Stars must be monitored with sufficient photometric precision to detect planetary transits. This leads to requirements on the collecting area and other properties of the cameras, although the derivation of those requirements is not straightforward because of the wide variation in the characteristics of the transit signals. As reviewed in Ref. 1, the transit depth varies with the radii of the star (R ⋆ ) and planet (R p ) (1) and the transit duration (T) depends on the stellar mean density (ρ ⋆ ) as well as the orbital period (P) and dimensionless transit impact parameter (b) In these equations, the fiducial case is a transiting planet with R p ¼ 2 R È in a 10-day circular orbit around a Sun-like star Monte Carlo simulations were performed 12 to establish the requirements for detecting a certain number of transiting planets, taking into account the likely distribution of stellar apparent magnitudes and radii; the occurrence of planets of various sizes and orbits; as well as noise due to sky background, instrumental readout, and other sources. The results of this study, summarized in Sec. 8, indicate that detecting hundreds of super-Earths requires ≈50 cm 2 of effective collecting area (i.e., the actual collecting area multiplied by all throughput factors including transmission losses and quantum efficiency). This corresponds to an effective aperture diameter of D ≈ 10 cm.

Time Sampling
Since the typical transit durations are measured in hours, the time sampling of the photometric observations should be substantially less than an hour. The timescale of the partial transit phases (ingress and egress) is ∼TðR p ∕R ⋆ Þ, which is typically several minutes for planets smaller than Neptune. Therefore, to avoid excessive smearing of the partial transit signals, a time sampling of a few minutes or shorter is preferred. For TESS, the brightness measurements of the preselected stars will be recorded every 2 min or shorter, which is consistent with these requirements and also enables asteroseismology (see Sec. 8.3).

Orbit
The ideal orbit for the TESS spacecraft would allow for continuous observations lasting for weeks, and would be stable to perturbations over the multiyear duration of the survey. It would offer a low-radiation environment, to avoid high trapped-particle fluxes and resulting degradation of the CCDs and flight electronics. It would also offer a stable thermal environment and minimal attitude disturbance torques, to provide a stable platform for precise photometry. The orbit would be achievable with a moderate ΔV, avoiding the need for a costly secondary propulsion unit. Furthermore, to facilitate data transfer, the spacecraft would be close to the Earth during at least a portion of the orbit. As explained in Sec. 6, these requirements were met by choosing an elliptical orbit around the Earth, with approximate perigee and apogee distances of 17 and 59 R È , and a period of 13.7 days.

Payload
The TESS payload consists of four identical cameras and a data handling unit (DHU). Each camera consists of a lens assembly with seven optical elements, and a detector assembly with four CCDs and their associated electronics. All four cameras are mounted onto a single plate. These components will now be discussed in turn.

Lens Assembly
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 (see Figs. 2 and 3 and Table 1). 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 (see Fig. 1). Each lens forms a 24 deg × 24 deg 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 field of view (FOV), and produce undersampled images similar to those of Kepler. 13 At nominal focus and flight temperature (−75°C), the 50% ensquared-energy half-width is 15 μm (1 pixel or 0.35 arc min) averaged over the FOV. Each lens is equipped with a lens hood, which reduces the effects of scattered light from the Earth and Moon.

Detector Assembly
The focal plane consists of four back-illuminated MIT/Lincoln Laboratory CCID-80 devices. Each CCID-80 consists of a 2048 × 2048 imaging array and a 2048 × 2048 frame-store region, with 15 × 15 μm pixels. The frame-store region allows rapid shutterless readout (≈4 ms). On each CCD there are four regions of 512 columns, each of which has a dedicated output register. The full well capacity is approximately 200,000 electrons. The four CCDs are abutted with a 2-mm gap, creating an effective 4096 × 4096 imaging array contained within a 62 mm × 62 mm square (see Fig. 3).
The CCDs have an operational temperature of −75°C to reduce the dark current to a negligible level. They are read out at 625 kHz and the read noise is <10 e − . The detector electronics are packaged onto two compact double-sided printed circuit boards, each 12 cm in diameter, which are located beneath the CCD focal plane and transmit digitized video data over a serial LVDS link to a DHU.

Data Handling Unit
The TESS DHU is a Space Micro Image Processing Computer (IPC-7000) which consists of six boards: an IPC, which contains two Virtex-7 field-programmable gate arrays (FPGAs) that serve as interfaces to the four cameras and perform high-speed data processing; a Proton 400K 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 solid-state buffer (SSB) cards for The CCDs produce a continuous stream of images with an exposure time of 2 s. These are received by the FPGAs on the IPC, and summed into consecutive groups of 60, giving an effective exposure time of 2 min. 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 postprocessing. A primary data product is a collection of subarrays (nominally 10 × 10 pixels) centered on preselected target stars. The Proton 400 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 (FFIs) are also stacked every 30 min and stored in the SSB. Data from the SSB are downlinked every 13.7 days at perigee.

Spacecraft
TESS uses the orbital LEOStar-2/750 bus equipped with a Kaband transmitter and a monopropellant (hydrazine) propulsion system. The 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 Mb s −1 . This is sufficient to downlink the science data during 4-h intervals at each perigee.
The cameras are bolted onto a common plate that is attached to the spacecraft, such that their FOV are lined up to form a rectangle measuring 24 deg × 96 deg on the sky (see Figs. 4 and 7). Four elliptical holes in the plate allow shimless alignment of the four cameras at the desired angles.

Orbit
The TESS orbit is elliptical, with nominal perigee and apogee of 17 R È and 59 R È , and a 13.7-day period in 2:1 resonance with the Moon's orbit. The TESS orbit is inclined from the ecliptic plane, thereby eliminating lengthy eclipses by the Fig. 3 (a) Two lens prototypes were constructed during phase A. One was subjected to thermal vacuum testing at the operational temperature; the other was subjected to vibration testing. (b) The detector assembly of one of the prototype lenses. The frame-store regions of the CCDs are covered.  Earth and Moon. The spacecraft orbit is operationally stable as a result of the Moon leading or lagging the spacecraft apogee by ≈90 deg, averaging out the lunar perturbations. 14 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 −75°C, with temperature variations < 0.1°C h −1 for 90% of the orbit, and <2°C h −1 throughout the entire orbit. This orbit can be reached efficiently using a small supplemental propulsion system (ΔV ≈ 3 km s −1 ) 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 Fig. 5), TESS is launched from Cape Canaveral into a parking orbit with an equatorial inclination of 28.5 deg. 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 40 deg. 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 to 12 years (driven by a Kozai-like mechanism 14 ). There are also short-term oscillations with a period of 6 months caused by solar perturbations (see Fig. 6). The orbit is stable on the timescale 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.

Star Selection
A catalog of preselected stars will be monitored at a cadence of 2 min or better. Ideally, the catalog will include ≳200;000 mainsequence FGKM stars that are sufficiently bright and small to enable the detection of transiting planets smaller than Neptune. This corresponds to approximately I C < 12 for FGK stars and I C < 13 for M dwarfs. 12 These stars are bright enough that it should be possible to assemble a list using currently available all-sky surveys. A draft catalog has been constructed using the 2MASS, 15 Tycho-2, 16 and UCAC4 17 catalogs, supplemented with smaller catalogs of nearby M dwarfs. For the M stars, infrared magnitudes allow for discrimination between dwarfs and giants. 18 G and K dwarfs are distinguished from giants through their relatively rapid reduced proper motion. 19 It may also be possible to use trigonometric parallaxes measured by the ongoing European "Gaia" mission, if they become available early enough for TESS mission planning. The TESS target catalog will be publicly released prior to launch.

Scanning Strategy
The four cameras act as a 1 × 4 array, providing a combined FOV of 24 deg × 96 deg or 2300 deg 2 (see Fig. 7). The north and south ecliptic hemispheres are each divided into 13 partially overlapping sectors of 24 deg × 96 deg, extending from an ecliptic latitude of 6 deg 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 antisolar. After two orbits, the FOV is shifted eastward in ecliptic longitude by about 27 deg, to observe the next sector. Observing an entire hemisphere takes 1 year, and the all-sky survey takes 2 years. (A video illustrating the TESS survey strategy, along with the pathway to the spacecraft orbit, can be seen at http://www .youtube.com/watch?v=mpViVEO-ymc.).    The overlap of the sectors is illustrated in Fig. 7. Approximately 30;000 deg 2 are observed for at least 27 days. Close to the ecliptic poles, approximately 2800 deg 2 are observed for more than 80 days. Surrounding the ecliptic poles, approximately 900 deg 2 are observed for more than 300 days.

Data Downlink and Housekeeping Operations
At perigee, science operations are interrupted for no more than 16 h to point TESS's antenna toward Earth, downlink data, and resume observing. This includes a nominal 4-h period for Ka-band science data downlink using NASA's 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.

Ground-Based Data Analysis and Follow-Up
The TESS data will be processed with a data reduction pipeline based on software that was developed for the Kepler mission. 20 This includes pixel-level calibration, background subtraction, aperture photometry, identification and removal of systematic errors, and the search for transits with a wavelet-domain matched filter.
Once the data are processed and transits are identified, selected stars will be characterized with ground-based imaging and spectroscopy. These observations are used to establish reliable stellar parameters, confirm the existence of planets, and establish the sizes and masses of the planets. Observations will be performed with committed time on the Las Cumbres Observatory Global Telescope Network and the MEarth observatory. In addition, the TESS science team members have access to numerous other facilities (e.g., Keck, Magellan, Subaru, HARPS, HARPS-North, Automated Planet Finder) through the usual telescope time allocation processes at their home institutions. The TESS team includes a large group of collaborators for follow-up observations and welcomes additional participation. Figure 8 shows the anticipated photometric performance of the TESS cameras. The noise sources in this model are photoncounting 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 cotrending. The most important systematic error is expected to be due to random pointing variations ("spacecraft jitter"). Because of the nonuniform 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 changes, and as parts of the image PSF enter and exit the summed array of pixels.

Photometric Performance
The central pixel of a stellar image will saturate at approximately I C ¼ 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. 21 Since similar performance is expected for TESS, the bright limit is expected to be I C ≈ 4 or perhaps even brighter.

Transit Detections
Monte Carlo simulations are used to verify that the science objectives can be met and to anticipate the properties of the detected planetary systems. 12 These simulations are based on a model of the local neighborhood of main-sequence FGKM stars. 11 Simulated stars are populated randomly with planets, and "observed" by TESS. Those for which transits are observed with a sufficiently high signal-to-noise ratio are counted as detections. In addition, the simulated star catalog is populated with eclipsing binaries that may be blended with brighter stars to produce transit-like signals; the detections of these "false positives" are also recorded.
Among the features of the current simulations are: (1) a realistic distribution of stars and eclipsing binaries based on local census data; (2) probability distributions for planetary occurrence and orbital properties taken from the Kepler results; 3 (3) variation in stellar surface density and zodiacal light with position on the celestial sphere; (4) variation in the duration of TESS observations depending on ecliptic coordinates. Figure 9 illustrates some of the results. TESS is expected to find thousands of planets smaller than Neptune, including hundreds of super-Earths (1.25 − 2 R È ) and tens of planets comparable in size to Earth. These will be accompanied by a comparable number of false positives (as has been the case for the Kepler mission), a majority of which are background eclipsing binaries. Some false positives will be identifiable using TESS data alone, based on the detection of secondary eclipses, ellipsoidal flux modulation, or transit-specific image motion. In other cases, ground-based observations will be required to check for composite spectra, large radial-velocity variations, color-dependent transit depths, and resolved companions that are indicative of false positives.

Asteroseismology
Observing photometric variations due to stellar oscillations ("asteroseismology") provides sensitive diagnostics of the stellar mass, radius, and internal dynamics. Based on the Kepler experience with mode amplitudes as a function of stellar parameters, 22 TESS can be expected to detect p-mode oscillations on about 6000 stars brighter than V ¼ 7.5, including (a) the majority of all stars brighter than V stars, and (d) virtually all the giant stars. Stars that are not suitable for planet searching but are appropriate for asteroseismology (such as giants) can be added to the TESS input catalog at minimal cost.
The key features enabling TESS's advances in this area are all-sky coverage and relatively fine time sampling. Compared to similar stars studied by CoRoT and Kepler, the TESS stars will be more numerous, brighter, and nearer to the Earth. TESS stars will have accurately known parallaxes, will be much more amenable to interferometric and radial-velocity studies, and are themselves among the likely targets for future, imaging-based missions to study exoplanets.

Full Frame Images
In addition to monitoring 200,000 preselected stars with a 2 min cadence, TESS will return a nearly continuous series of FFIs with an effective exposure time of 30 min. As Kepler has shown, a 30 min cadence still allows for the efficient discovery of transiting planets. The FFIs will expand the transit search to any star in the FOV that is sufficiently bright, regardless of whether it was selected ahead of time. This will reduce the impact of any imperfections in the target star catalog, and allow the search sample to extend beyond the FGKM dwarfs that are the focus of the mission. In addition, the transit candidates that are identified in the FFIs during the baseline TESS mission could be selected for shorter-cadence observations during an extended mission.
The FFIs will also enable a wide variety of other scientific investigations, such as the detection and monitoring or nearby supernovae and gamma-ray burst afterglows, bright AGN outbursts, near-Earth asteroids, eclipsing and close binaries, novae, flaring stars, and other variable stars. Each 30 min FFI will provide precise (≈5 mmag) photometry for approximately 10 6 bright galaxies and stars (I C < 14 − 15) within a 2300 deg 2 field. Over 2 years, the TESS all-sky survey will provide such data for approximately 20 million bright objects, many of which will exhibit short-term variability. Thus, the TESS FFIs will provide broad-ranging fundamental data for timedomain astronomy. In particular, TESS will complement the Large Synoptic Survey Telescope, which is limited to observing objects fainter than 16th magnitude. 23

Partners and their Roles
The lead institution for TESS is MIT, which hosts the principal investigator, Dr. George Ricker. The MIT Lincoln Laboratory is responsible for the cameras, including the lens assemblies, detector assemblies, lens hoods, and camera mount. NASA's Goddard Space Flight Center provides project management, systems engineering, and safety and mission assurance. Orbital Sciences Corporation (OSC) builds and operates the spacecraft. The mission is operated from the OSC Mission Operations Center.
The TESS Science Center, which analyzes the science data and organizes the coinvestigators, 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, and the NASA Ames Research Center. The raw and processed data are archived at the Mikulski Archive for Space Telescopes (MAST), at the Space Telescope Science Institute.

Discussion
Kepler has revealed the vast numbers of small planets that exist in the Galaxy. TESS will find them around stars in the local neighborhood, which will be easier to study with current and planned instruments, including the JWST. The TESS launch is currently scheduled for late 2017, and the all-sky survey for 2018 to 2019. An extended mission of several years is a realistic possibility, and is compatible with the supply of consumable materials and the stability of the spacecraft orbit. An extended mission would increase the number of planet detections, especially those with longer orbital periods, including planets in the habitable zones of their host stars.
A broad community effort will be needed to harvest all of the scientific potential of the survey. TESS will serve as the "People's Telescope," with a public release of the target catalog prior to launch, the first data release 6 months after launch, and subsequent data releases every 4 months, to stimulate community-wide effort and optimize target selection for subsequent studies. Light curves, full-frame images, and catalogs of "objects of interest" will be available on the MAST. The TESS team will also establish an electronic clearinghouse to exchange information and coordinate the follow-up effort.
One of the most scientifically productive-and challengingtypes of follow-up observations is PRV monitoring. PRV monitoring is used to confirm the planetary nature of TESS detections, provide constraints on planetary densities and possible interior compositions, and inform the interpretation of planetary atmosphere studies with JWST and other instruments. PRV monitoring is likely to be the rate-limiting step in realizing the full benefits of TESS for exoplanetary science. Even though TESS stars are relatively bright and favorable for study, the expected number of new and interesting PRV targets will greatly exceed the time budgets of currently available observatories. Many groups are aiming to rectify this situation by building new PRV instruments. Among the new instruments being planned with TESS targets in mind are ESPRESSO, 24 NRES, 25 IRD, 26 CARMENES, 27 HPF, 28 HRS, 29 SPIRou, 30 MAROON-X, 31 CODEX, 32 and G-CLEF. 33 The TESS legacy will be a catalog of the nearest and brightest stars hosting transiting planets, which will likely be the most favorable targets for detailed investigations in the decades or even centuries that follow.