In a clean room inside a clean room at NASA’s Kennedy Space Center, a petite telescope is perched on a stand for a final series of checkouts prior to launch. The extra fastidiousness is because the observatory’s four cameras will fly without protective covers—one of several simplifying design decisions made to help ensure the Transiting Exoplanet Survey Satellite, or TESS, will meet its goal of measuring the masses of at least 50 small, rocky and potentially Earth-like worlds as part of the first all-sky, exoplanet survey.

TESS was proposed even before NASA’s planet-hunting Kepler space telescope, launched in 2009, demonstrated the viability of a space-based exoplanet survey. Both telescopes use the so-called “transit” method (as opposed to these techniques) to find planets, looking for worlds in silhouette as they pass in front of their parent stars relative to the telescopes’ lines of sight. Kepler not only established transits as the dominant planet-hunting technique, it also stunningly revealed our galaxy brims with planets, particularly worlds two to four times the size of Earth.

During its initial mission, Kepler scouted stars more than a thousand light-years away in a patch of sky within the constellations Cygnus, Lyra and Draco. So far, scientists have confirmed 2,341 exoplanets circling stars in Kepler’s original pool of some 170,000 targets. Another 4,496 candidate planets are pending, but many may never be confirmed because their stellar hosts are too dim to be easily observed by ground-based telescopes for necessary follow-up studies.

The TESS team took the opposite approach, starting with what ground observations would be needed to follow-up and confirm candidate transiting Earth-like planets, and then deciding on specific targets for the telescope. They selected about 200,000 stars for study during TESS’s two-year primary mission. Each of those target stars already has been plotted in detail by the European Space Agency’s ongoing Gaia space telescope, which is creating the best-yet all-sky catalogue of stellar positions and distances.

Most of TESS’s targets are within 300 light-years from Earth, significantly closer and up to a hundred times brighter than most of the stars studied by Kepler. “On TESS, we will be able to do ground-based follow-up on all of them. It will just be a matter of priorities, not abilities,” says project scientist Stephen Rinehart, with NASA’s Goddard Space Flight Center.

The transit technique pioneered by Kepler and planned for TESS reveals the size of a planet relative to its host star. If several transits can be observed, scientists also can determine how far from the star a planet orbits—information that can then be used to estimate its temperature and whether it could support liquid water on its surface, a key consideration for habitability.

But to assess a planet’s mass—which is needed to determine whether it is dense with metal and rock like Earth or instead composed of ice or gas—astronomers usually turn to ground-based telescopes. Often only a relatively modest observatory, it turns out, is needed to look for wobbles in a star’s spin caused by the slight but regular gravitational tugging of its orbiting planetary brood. The TESS project is enlisting dozens of astronomers and reserving time on several ground-based telescopes for such studies.

The Hunt Is On

The hunt for neighboring Earth-like planets begins about two months after TESS’s launch, which is currently slated for mid-April. That’s when the telescope should arrive in an unusual operational orbit that loops high around Earth, passing only as close as about 67,300 miles before zooming out to nearly the orbit of the moon some 234,000 miles away. When TESS is at the highest point of its orbit with respect to Earth, the moon will be 90 degrees to the left or right, serving as an orbital ballast that will keep the telescope gravitationally steady for decades without the use of steering thrusters.

This eccentric orbit allows TESS to spend most of its time in deep, dark space, with minimal interference from sunshine and the light reflecting off Earth and the moon. The spacecraft will swing around the planet every 13.7 days, orbiting exactly twice as fast as the moon. When TESS is closest to Earth it will suspend observations for 10 hours to transmit stored science data to one of NASA’s three Deep Space Network ground stations. Those transmissions will occur on high-rate, Ka-band frequencies—a first for the network that will pave the way for other data-intensive future space missions, including the James Webb Space Telescope.

TESS’s data will not only include measurements of each target star’s brightness taken every two minutes, but also a full-sky image taken every half-hour capturing more than 20 million stars and 10 million galaxies. “It’s just going to be this treasure trove of data. We expect that that archive will be mined for years,” says NASA astronomer Patricia Boyd, who leads Goddard’s TESS Guest Investigator Program.

The telescope is outfitted with four cameras positioned to cover a wedge of sky 24 degrees across and 96 degrees long, the equivalent of about 10,000 full moons. Shifting its field of view every two orbits, TESS will cover the sky’s entire southern hemisphere during the first year of operation and then flip to cover its northern hemisphere in the second year. In all, TESS will cover 90 percent of the sky, an area about 400 times larger than what Kepler observed.

Of key interest are the stars around the ecliptic poles, which will be included in each slice of the sky covered by TESS. These are the stars directly above and below the ecliptic plane, in which the planets move around the sun. Worlds in these regions will become primary targets for follow-up studies by the James Webb Space Telescope, which, among its many other tasks, will attempt to ferret out the atmospheric chemistry for some transiting exoplanets. Webb is slated to launch in 2019. “In that role TESS serves as a finder scope for Webb. We’re finding the particular star that actually potentially hosts an exoplanet around it,” says TESS lead scientist George Ricker, with Massachusetts Institute of Technology.

Ricker adds that “it shouldn’t be hard” to meet the TESS mission goal to measure the mass of 50 small planets. Simulations forecast that by the end of the initial three-year ground observation program, the team should have verified more like 500 small planets, not 50, he says.

Ultimately, TESS could contribute as many as 20,000 new planets to the exoplanet catalogue, most of which will be circling M-dwarf stars that are one quarter to one half the diameter of the sun and much dimmer and cooler. M-dwarfs, which comprise about 70 percent of stars in the Milky Way, are TESS’s primary targets.

“To me the most exciting thing about any new mission is the thing you don’t expect,” says Rinehart, the project scientist. “I really hope that somewhere along the line we find something bizarre, something that we can’t explain that requires us to really think hard about what it is we’re seeing. I think we will, but I have no idea what it will be.”