Ever since it debuted in 2009, NASA’s Kepler spacecraft has redefined our understanding of other star systems. But Kepler is in rough shape, and it won’t last forever. Now TESS, the Transiting Exoplanet Survey Satellite, is ticking along on schedule to launch in 2017. While the upcoming James Webb Space Telescope (JWST) is set to be a partial replacement for the Hubble telescope, the JWST will also be a sort of spiritual successor to Kepler, in that it will team up with TESS to hunt for exoplanets in the visible and IR bands.
Like Kepler, TESS will use the transit method, searching for exoplanets by watching hundreds of thousands of stars for the telltale dimming. But where Kepler must cast its eyes to a small patch of distant space, peering deeply but narrowly into the skies, TESS will make a shallower whole-sky survey of stars within a few hundred light years of Earth. Most of Kepler’s exoplanet discoveries came from one relatively small patch of the sky. But NASA officials say that TESS should be able to look at over 200,000 stars.
To make things easier for mission scientists, TESS breaks up its spherical viewfield into 26 “tiles” that overlap near its north and south poles. “The spacecraft’s powerful cameras will look continuously at each tile for just over 27 days, measuring visible light from the brightest targets every 2 minutes,” NASA officials said in a statement. Based on the characteristics of those little dips in brightness, TESS scientists will be able to tell how big the newly discovered exoplanets are, and how long they take to orbit their parent stars.
Left: The combined field of view of the four TESS cameras. Middle: Division of the celestial sphere into 26 observation sectors (13 per hemisphere). Right: Duration of observations on the celestial sphere. The dashed black circle enclosing the ecliptic pole shows the region which JWST will be able to observe at any time. Image and caption via GSFC/NASA
Now, TESS is an Explorer-class spacecraft, so it isn’t like Hubble or the other Great Observatories. A better comparison might be the Explorer-class Swift spacecraft, which is watching for gamma ray bursts. If it seems like two minutes isn’t very long to look at a star, it isn’t; TESS will take relatively brief glances, because it’s looking at a greater total area of the sky. But since the camera’s viewfields overlap at the poles, some places will be under almost constant observation. The idea is to follow up on TESS’s discoveries of potentially habitable worlds using the Observatory-class JWST, slated to launch in 2018 for really real this time.
Image via GSFC/NASA
That’s not all TESS has up its sleeve, though. While it will be a planet hunter first and foremost, scientists and other “guest investigators” will also have the opportunity to take time on TESS to observe black holes, supernovas, and a variety of other cosmic objects and phenomena.
One of the other major goals of TESS is to examine short-period objects and transient phenomena, like the visible light energy that accompanies a gamma ray burst. The spacecraft’s glancing gaze might pose difficulties for asteroseismologists, because while transits might take some days or weeks, starquakes can happen in seconds, so TESS’s sampling rate might bump into the Nyquist limit. But that kind of data loss is apparently a niche enough problem that project scientists were willing to make the compromise.
TESS also has unique orbital characteristics. It’s designed to survey both the northern and southern hemispheres and will use a new lunar orbit, dubbed P/2, to do so. This highly elliptical orbit will allow the spacecraft to survey its target range while simultaneously remaining balanced between the gravitational effects of the Moon and those of the Earth. NASA has published an article with more details on the spacecraft’s orbit and its characteristics, if you’re curious.
TESS’s never-before-used P/2 orbit. Image: GSFC/NASA
Between TESS and the JWST, our understanding of other solar systems should continue to advance for years to come, even as monuments like Hubble and Kepler are taken offline. The toolkit of astronomers continues to expand as technology gets thinner, lighter, stronger, and more durable. When TESS makes it to space aboard a Falcon 9 FT rocket, it will fill a gap in data collection and, no doubt, present new problems to be overcome. But if we can beat gimbal lock, jammed reaction wheels, and broken rover arms while still making do under an endless funding drought, believe it, we’ll get some good science out of TESS.
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