NASA’s Transiting Exoplanet Survey Satellite (TESS) lifted off on a SpaceX rocket last night. TESS is a follow-up to the wildly successful Kepler mission that will hunt for transiting planets—that is, planets that pass in front of their stars, blocking some of the light. TESS is a bigger and better missing that is expected to discover five times as many planets (20,000 compared with Kepler’s 4,000) and around closer stars. Even the nearest stars to Earth will be observed by TESS (although only a small fraction of planets orbit at the right angle to be seen in transit).
I say “bigger”, but it’s actually more complicated than that. Kepler used one medium-size telescope (by spacecraft standards) to look at a single 10 x 10 degree patch of sky for years on end. TESS uses four small telescopes only 10 cm (4 inches) wide, each of which images an area 24 degrees wide, and which rotates to a new part of the sky every month. Over the course of a year, it will image an amazing 85% of the southern sky, followed by 85% of the northern sky the following year. Most parts of the sky will be observed for only one month, but areas near the celestial poles watched for the full year to look for longer-period planets.
This is a counterintuitive spacecraft. A 10 cm telescope is tiny. It’s the kind you can buy on Amazon for a couple hundred bucks. Compared with the 2.5 meter (8 foot) mirror of the Hubble Space Telescope, it seems like it would be useless. So how does it work? The difference between TESS and your average backyard telescope is—well, it’s in space, where there’s no air and no light pollution—but the main difference is having really, really good electronics.
This isn’t your cell phone camera that NASA’s using. It’s not even your fancy DSLR if you’re a photographer. True, it’s the same CCD technology, but NASA gets the best-quality chips that can read light levels extremely accurately—to one part in ten thousand—and that’s enough to spot an Earth-sized planet.
The other thing to remember about TESS is that it’s looking only at bright stars. For bright stars, having a very big telescope is actually a disadvantage because as faint as the stars are, a big telescope collects so much light that they’ll saturate the chip. There will be a big white splotch, and you won’t get good readings. This also sounds counterintuitive. Your cell phone camera can function in broad daylight, after all. But your cell phone camera is exposed for a hundredth of a second to take a photo in daylight. Maybe a whole second in low light, but TESS’s cameras are exposed for a full 2 minutes to take a single photo.
Looking at bright stars is especially important because we need bright stars to easily measure the masses of planets by radial velocity, which is what we need to tell planets that look like Earth, but bigger, apart from planets that look like Neptune, but smaller. So while it probably won’t turn up any potentially habitable planets, it will still give us a much better estimate of how many of them there are out there.
TESS has to go through a weird flight path including a gravity assist around the Moon to get to an orbit that is ideal for its operations, so observations will not start until June, but I’m eager to see what new discoveries it turns up starting this summer.
