Transiting Exoplanet Survey Satellite

History

The genesis of TESS began as early as 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 Explorers program mission, and was approved in 2013 as a Medium Explorer mission.

The transit technique has been a successful way of studying and discovering exoplanets, and when launched TESS will join a growing fleet of telescopes studying in this area.

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 will be transmitted to Earth every two weeks. Full-frame images with an effective exposure time of two hours will be transmitted as well, enabling scientists to search for unexpected, transient phenomena, such as the optical counterparts to gamma-ray bursts.

Orbital dynamics

In order to obtain unobstructed imagery of both the northern and southern hemispheres of the sky, TESS will utilize a 2:1 lunar resonant orbit called P/2, an orbit that has never been used before. The spacecraft's 373,000 km (232,000 mi) apogee is timed to keep the craft away from the Moon, which acts as a destabilizing agent. This highly elliptical orbit should remain stable for decades, and will keep TESS's cameras in a stable temperature range. The majority of the orbit is spent outside the Van Allen belts to avoid radiation damage to TESS. Every 13.7 days at its perigee of 108,000 km (67,000 mi), TESS will downlink the data it has collected during the orbit to Earth over a period of approximately three hours.

Scientific objectives

TESS's survey will focus on nearby G, K and M type stars with apparent magnitudes brighter than magnitude 12. Approximately 500,000 stars will be studied, including the 1,000 closest red dwarfs, across an area of sky 400 times larger than covered by the Kepler mission. TESS is expected to discover more than 3,000 transiting exoplanet candidates, including those which are Earth sized or larger. Of those discoveries, an estimated twenty could be super-Earths located in the habitable zone around a star.

The survey is broken up into 26 observation sectors, each sector being 24° × 96°, with an overlap of sectors at the ecliptic poles to allow for additional sensitivity toward smaller and longer-period exoplanets in that region of the celestial sphere. The spacecraft will spend two 13.7 day orbits observing each sector, mapping the northern hemisphere of sky in its first year of operation and the southern hemisphere in its second year. Each of TESS's cameras will take one-minute exposures focused on specifically targeted stars as part of its transit search, and 30-minute full-frame exposures to search for other transient events.

Spacecraft

In 2013 Orbital Sciences 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 arc-second fine spacecraft pointing control. Power is provided by two single-axis solar arrays generating 400 watts. A Ka-band dish antenna will provide a 100 Mbit/s science downlink.

Launch

TESS is planned for a launch in March 2018 on a SpaceX Falcon 9 Full Thrust rocket from the Cape Canaveral Air Force Station. The total value of the contract is US$87 million. SpaceX was awarded the launch in December 2014.

Science instrument

The sole instrument on TESS is a package of four wide-field-of-view CCD cameras. Each camera features a low-noise, low-power 16.8 megapixel CCD detector created by the MIT Lincoln Laboratory. Each has a 24° × 24° field of view, a 100 mm (4 in) effective pupil diameter, a lens assembly with seven optical elements, and a bandpass range of 600 to 1000 nm.

Stable light source for tests

One of the issues facing the development of this type of instrument is having an ultra-stable light source to test on. In 2015, a group at the University of Geneva made a breakthrough in the development of a stable light source. While this instrument was created to support ESA's CHEOPS exoplanet observatory, one was also ordered by the TESS program. Although both plan to look at bright nearby stars using the transit method, CHEOPS is focused on collecting more data on known exoplanets, including those found by TESS and other survey missions.

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. Kepler and PLATO observatories are also used for asteroseismology.