Rick Robinson

Jun 19th 2019

The TESS Observatory Is Making History


The first science results sent back by NASA’s TESS observatory mission, more officially the Transiting Exoplanet Survey Satellite, are incredible. Right out of the gate, TESS discovered several exoplanets — extraterrestrial worlds orbiting stars light-years beyond the sun. The telescope also began detailed examination of hundreds of other stars that show possible hints of orbiting planets.

Above and beyond its assigned tasks, TESS has caught the early turn-ons of dozens of supernovae, or exploding stars. These discoveries were possible because the design team solved technical challenges that didn’t exist when planning for the mission a decade ago. What were those challenges, and how did they overcome them?

Needed: A Placid Place in Space

As TESS project manager Robert Lockwood recounted, the Transiting Exoplanet Survey Satellite mission was originally expected to do its work in what space mission planners call LEO, or low Earth orbit, where the Hubble Telescope orbits, along with a host of other research satellites and telescopes.

LEO is the easiest corner of space for us to reach, and it lifts a satellite above Earth’s hazy, damp, turbulent and light-polluted atmosphere. For most astronomical research, this is enough to make all the difference.

But, according to Lockwood, during a 2010 conference, George Ricker, of MIT, principal investigator for the upcoming TESS mission, got some unwelcome news. LEO would not suffice for the extremely delicate measurements of starlight that TESS needed to detect the shadows of planets many light-years away.

Spacecraft in low Earth orbit pass in and out of Earth’s own shadow every 90 minutes or so, producing temperature changes that can distort precision instrument readings. To do its job, the TESS observatory needed to be in a really placid region of space, not constantly passing in and out of Earth’s shadow.

Searching for Stability

Moving beyond LEO posed its own challenges, observed Lockwood. Spacecraft in high orbits tend to wander, as they are more exposed to the gravity of other planets, radiation forces and other phenomena that can gradually change orbits.

If a spacecraft’s limited supply of maneuvering propellant is regularly expended to adjust its orbit (what mission planners call “station-keeping”), the science mission is doomed when the supply runs out.

TESS’s project team found its solution in an orbit never previously used for a space mission, called a 2:1 lunar resonance orbit. This is an elongated orbit that goes around the Earth twice for each lunar orbit (thus, twice a month). It is a tricky orbit that took months for TESS to reach — but it is so placid and stable that no further station-keeping will be needed for the next hundred years, at minimum.

The Art of Pinpoint Aiming

Even this ultra-stable orbit is not enough to give TESS the pointing precision it needs, according to Lockwood. To keep its aim precise, TESS uses a set of “momentum wheels” to absorb any stray angular momentum. At occasional intervals, when the wheels have built up so much angular momentum that they have reached their maximum spin rate, a few grams of propellant must be burned by the attitude thrusters to “unload” the wheels and slow them down.

How stable is stable? Lockwood explained that TESS can hold its aiming “drift” to no more than 0.02 arc seconds in an hour — one part in a million, or about the diameter of a quarter as seen from 160 miles away. Ricker’s description of this aiming precision is short and to the point: “Exquisite.”

But in space, every new technology solution often means another technology challenge, and the TESS mission is no exception. The lunar 2:1 lunar resonance orbit keeps TESS out of Earth’s troublesome shadow, but also means that TESS only dips relatively close to Earth for a few hours twice a month. The rest of the time it is roughly midway between Earth and the moon — a long way to send its results back to Earth-based scientists.

Don’t Forget to Call Home

One solution would have been to send along a high-power radio transmitter, like the transmitters used on missions to the outer planets, Lockwood said. But those are heavy and expensive, so the TESS team came up with a better solution. TESS downloads its data to Earth via the same deep-space network used for interplanetary missions, but its transmissions use the ultra-short Ka (“kay-ay”) radio band. This is capable of downloading 109 megabits of data per second, compared to 2-4 megabits per second for the more conventional S-band download technology.

Thanks to the Ka-band downlink, TESS is able to send back two weeks’ worth of data in a few hours, each time it whips past Earth. In fact, it is able to send back all the data its cameras capture, not just the specific data needed for the exoplanet-hunting mission. As Lockwood put it, “a lot of non-level one science is available” — which is why TESS can discover supernova explosions on the side, without interfering with its primary work.