Scatterings image

Artist’s conception of TESS observing an M dwarf star with orbiting planets.

The James Webb Space Telescope (JWST) may not lift off until 2019, but a compact multi-camera satellite that will seek targets for the huge space observatory will launch this spring. From its unusual elliptical Earth orbit, the U.S. spacecraft will survey most of the observable sky for variable light sources—especially stars that periodically dim and brighten due to the passage of a planet.

The Transiting Exoplanet Survey Satellite (TESS) will act as a “finder scope” for the more powerful JWST and the next generation of giant ground-based telescopes, said George Ricker of the Massachusetts Institute of Technology (USA), the principal investigator for the mission.  Ricker and his colleagues spoke at the American Astronomical Society winter meeting in Washington, D.C., this week.

The 300-kg spacecraft contains four wide-field CCD cameras, each of which will capture images of a swath of sky 24 degrees by 24 degrees in size. (For comparison, that’s roughly the size of the constellation Orion.) By repeatedly measuring the brightness of 200,000 of the nearest dwarf stars, scientists will be able to determine which stars are experiencing planetary transits—a slight dimming that occurs when a planet passes in front of the host star. It’s a tiny effect—less than 100 parts per million for an Earth-sized planet circling a star like the Sun, according to Ricker.

A photometric mission

The data from TESS can reveal the basic orbital parameters of extrasolar planets, but more detailed studies with JWST will still be needed to determine the masses, densities and potential habitability of these planets. That’s because the four TESS cameras were not designed to have a sharp focus across the entire field of view; the mission’s emphasis is on photometry, not imaging, Ricker said.

The biggest optical challenge in the TESS mission was tracking down the cause of unexpected focus shifts during the initial tests of the cameras in early 2017. Ricker said that the issue was traced to a reproducible crystallization effect in the silicone shock mounts for the lens elements. Subsequent tests showed that the focus becomes stable after the cameras have been at flight temperature for approximately one week.

An unusual orbit

After launch in March 2018, TESS will be nudged into a highly eccentric orbit that has never previously been used for artificial satellites. The path will take the satellite from just outside the Moon’s orbit to a perigee of 108,000 km, where it will transmit its accumulated data to Earth at 100 Mbits/s. The 2:1 resonance with the Moon—meaning that TESS will orbit Earth every 13.7 days, half the time of a lunar orbit—will give the spacecraft a nearly uninterrupted view of the solar neighborhood.

TESS is also a harbinger of the emphasis that astronomers are now placing on the search for Earthlike planets, especially as the U.S. astronomical community is gearing up for the next decadal survey by the U.S. National Academies. U.S. funding agencies tend to rely on the priorities set in these surveys when allocating money to big projects.

In preparation for the 2020 survey, NASA laboratories are floating several conceptual ideas for spaceborne observatories dedicated to extrasolar planets. The Habitable Exoplanet Imaging Mission (HabEx) would fly a 4-m-diameter telescope equipped with a coronagraph and “starshade,” each of which could block out the light from the central star of a distant planetary system. The Large Ultraviolet/Optical/Infrared Surveyor (LUVOIR) would, as its name implies, capture a wide range of wavelengths with imaging, spectroscopy and polarimetry tools. Of course, which missions will progress to “first light” is still an open question.