The Transiting Exoplanet Survey Satellite (TESS) is a space telescope for NASA's Explorer program, designed to search for exoplanets using the transit method in an area 400 times larger than that covered by the Kepler mission. It was launched on 18 April 2018, atop a Falcon 9 launch vehicle and was placed into a highly elliptical 13.70-day orbit around the Earth. The first light image from TESS was taken on 7 August 2018, and released publicly on 17 September 2018.

In the two-year primary mission, TESS was expected to detect about 1,250 transiting exoplanets orbiting the targeted stars, and an additional 13,000 orbiting stars not targeted but observed. After the end of the primary mission around 4 July 2020, scientists continued to search its data for more planets, while the extended missions acquire additional data. , TESS had identified 7,931 candidate exoplanets, of which 885 had been confirmed.

The primary mission objective for TESS was to survey the brightest stars near the Earth for transiting exoplanets over a two-year period. The TESS satellite uses an array of wide-field cameras to perform a survey of 85% of the sky. With TESS, it is possible to study the mass, size, density and orbit of a large cohort of small planets, including a sample of rocky planets in the habitable zones of their host stars. TESS provides prime targets for further characterization by the James Webb Space Telescope (JWST), as well as other large ground-based and space-based telescopes of the future. While previous sky surveys with ground-based telescopes have mainly detected giant exoplanets and the Kepler space telescope has mostly found planets around distant stars that are too faint for characterization, TESS finds many small planets around the nearest stars in the sky. TESS records the nearest and brightest main sequence stars hosting transiting exoplanets, which are the most favorable targets for detailed investigations. Detailed information about such planetary systems with hot Jupiters makes it possible to better understand the architecture of such systems.

The program is led by the Massachusetts Institute of Technology (MIT) with seed funding from Google. On 5 April 2013, it was announced that TESS, along with the Neutron Star Interior Composition Explorer (NICER), had been selected by NASA for launch. On 18 July 2019, after the first year of operation, the southern portion of the survey was completed, and the northern survey was started. The primary mission ended with the completion of the northern survey on 4 July 2020, which was followed by the first extended mission. The first extended mission concluded in September 2022 and the spacecraft entered its second extended mission which should last for another three years.

History The concept of TESS was first discussed in 2005 by the Massachusetts Institute of Technology (MIT) and the Smithsonian Astrophysical Observatory (SAO). The genesis of TESS was begun during 2006, when a design was developed from private funding by individuals, Google, and The Kavli Foundation. In 2008, MIT proposed that TESS become a full NASA mission and submitted it for the Small Explorer program at Goddard Space Flight Center, but it was not selected. It was resubmitted in 2010 as an Explorer program mission, and was approved in April 2013 as a Medium Explorer mission. TESS passed its critical design review (CDR) in 2015, allowing production of the satellite to begin. While Kepler had cost US$640 million at launch, TESS cost only US$200 million (plus US$87 million for launch). The mission will find exoplanets that periodically block part of the light from their host stars, events called transits. TESS will survey 200,000 of the brightest stars near the Sun to search for transiting exoplanets. TESS was launched on 18 April 2018, aboard a SpaceX Falcon 9 launch vehicle.

In July 2019, an Extended Mission 2020 to 2022 was approved. On 3 January 2020, NASA reported that TESS had discovered its first potentially habitable Earth-sized planet, TOI-700 d.

Mission overview TESS is designed to carry out the first spaceborne all-sky transiting exoplanet survey. It is equipped with four wide-angle telescopes and associated charge-coupled device (CCD) detectors. Science data are transmitted to Earth every two weeks. Full-frame images with an effective exposure time of two hours are transmitted as well, enabling scientists to search for unexpected transient phenomena, such as the optical counterparts to gamma-ray bursts. TESS also hosts a Guest Investigator program, allowing scientists from other organizations to use TESS for their own research. The resources allocated to Guest programs allow an additional 20,000 celestial bodies to be observed.

Orbital dynamics TESS uses a novel highly elliptical orbit around the Earth with an apogee approximately at the distance of the Moon and a perigee of . TESS orbits Earth twice during the time the Moon orbits once, a 2:1 resonance with the Moon. The orbit is expected to remain stable for a minimum of ten years.

In order to obtain unobstructed imagery of both the northern and southern hemispheres of the sky, TESS utilizes a 2:1 lunar resonant orbit called P/2, an orbit that has never been used before (although Interstellar Boundary Explorer (IBEX) uses a similar P/3 orbit). The highly elliptical orbit has a apogee, timed to be positioned approximately 90° away from the position of the Moon to minimize its destabilizing effect. This orbit should remain stable for decades and will keep TESS's cameras in a stable temperature range. The orbit is entirely outside the Van Allen belts to avoid radiation damage to TESS, and most of the orbit is spent far outside the belts. Every 13.70 days at its perigee of , TESS transmits the data it collected during its most recent orbit back to earth via its downlink. This transmission occurs over a period of approximately three hours.

Science objectives

TESS's two-year all-sky survey would focus on nearby G-, K-, and M-type stars with apparent magnitudes brighter than magnitude 12. Approximately 500,000 stars were to be studied, including the 1,000 closest red dwarfs across the whole sky, an area 400 times larger than that covered by the Kepler mission. TESS was expected to find more than 3,000 transiting exoplanet candidates, including 500 Earth-sized planets and super-Earths. Of those discoveries, an estimated 20 were expected to be super-Earths located in the habitable zone around a star. The stated goal of the mission was to determine the masses of at least 50 Earth-sized planets (at most 4 times Earth radius). Most detected exoplanets are expected to be between 30 and 300 light-years away.

The survey was broken up into 26 observation sectors, each sector being 24° × 96°, with an overlap of sectors at the ecliptic poles to allow additional sensitivity toward smaller and longer-period exoplanets in that region of the celestial sphere. The spacecraft will spend two 13.70-day orbits observing each sector, mapping the southern hemisphere of sky in its first year of operation and the northern hemisphere in its second year. The cameras actually take images every 2 seconds, but all the raw images would represent much more data volume than can be stored or downlinked. To deal with this, cutouts around 15,000 selected stars (per orbit) will be coadded over a 2-minute period and saved on board for downlink, while full-frame images will also be coadded over a 30-minute period and saved for downlink. The actual data downlinks will occur every 13.70 days near perigee. This means that during the 2 years, TESS will continuously survey 85% of the sky for 27 days, with certain parts being surveyed across multiple runs. The survey methodology was designed such that the area that will be surveyed, essentially continuously, over an entire year (351 observation days) and makes up about 5% of the entire sky, will encompass the regions of sky (near the ecliptic poles) which will be observable at any time of year with the James Webb Space Telescope (JWST).

In October 2019, Breakthrough Listen started a collaboration with scientists from the TESS team to look for signs of advanced extraterrestrial life. Thousands of new planets found by TESS will be scanned for "technosignatures" by Breakthrough Listen partner facilities across the globe. Data from TESS monitoring of stars will also be searched for anomalies.

Asteroseismology The TESS team also plans to use a 30-minute observation cadence for full-frame images, which has been noted for imposing a hard Nyquist limit that can be problematic for asteroseismology of stars. Asteroseismology is the science that studies the internal structure of stars by the interpretation of their frequency spectra. Different oscillation modes penetrate to different depths inside the star. The Kepler and PLATO observatories are also intended for asteroseismology.

Extended missions During the 27 month First Extended Mission, July 2020 to Sept 2022, data collection was slightly changed: A new set of target stars will be selected The number of stars monitored at 2-minute cadence was increased from 15,000 to 20,000 per observing sector. Up to 1000 stars per sector will be monitored at a new fast 20-second cadence. The full-frame image cadence will be increased from every 30 minutes to every 10 minutes. The pointings and gaps in coverage will be slightly different during the extended mission. Regions near the ecliptic (omitted in the primary mission) will be covered.

During the 3 year Second Extended Mission, Sept 2022 to Nov 2025, the full-frame image cadence will be further increased from every 10 minutes to every 200 seconds, number of 2-minute cadence targets reduced to ~8000 per sector, and number of 20-second cadence targets increased to ~2000 per sector.

Launch

In December 2014, SpaceX was awarded the contract to launch TESS in August 2017, for a total contract value of US$87 million. The spacecraft was originally scheduled to launch on 20 March 2018, but this was pushed back by SpaceX to allow additional time to prepare the launch vehicle and meet NASA launch service requirements. A static fire of the Falcon 9 rocket was completed on 11 April 2018, at approximately 18:30 UTC. The launch was postponed again from 16 April 2018, and TESS was eventually launched on a SpaceX Falcon 9 launch vehicle from the SLC-40 launch site at Cape Canaveral Air Force Station (CCAFS) on 18 April 2018.

The Falcon 9 launch sequence included a 149-second burn by the first stage, followed by a 6-minute second stage burn. Meanwhile, the first-stage booster performed controlled-reentry maneuvers and successfully landed on the autonomous drone ship Of Course I Still Love You. An experimental water landing was performed for the fairing, as part of SpaceX's attempt to develop fairing reusability.

After coasting for 35 minutes, the second stage performed a final 54-second burn that placed TESS into a supersynchronous transfer orbit of at an inclination of 28.50°. The second stage released the payload, after which the stage itself was placed in a heliocentric orbit.

Spacecraft

In 2013, Orbital Sciences Corporation received a four-year, US$75 million contract to build TESS for NASA. TESS uses an Orbital Sciences LEOStar-2 satellite bus, capable of three-axis stabilization using four hydrazine thrusters plus four reaction wheels providing better than three arcsecond fine spacecraft pointing control. Power is provided by two single-axis solar arrays generating 400 watts. A Ka-band dish antenna provides a 100 Mbit/s science downlink.

Operational orbit