TESS Mission

The Transiting Exoplanet Survey Satellite (TESS) is an MIT-led NASA mission to spend two years discovering transiting exoplanets by an all-sky survey.

TESS has four identical, highly optimized, red-sensitive, wide-field cameras that together can monitor a 24 degree by 90 degree strip of the sky. By monitoring each strip for 27 days and nights, TESS will tile the southern hemisphere sky in the first year and the northern hemisphere sky in the second year. TESS is scheduled for launch no earlier than March 2018, aboard a SpaceX Falcon 9 rocket, and will go into a very eccentric, inclined orbit around the Earth. TESS will discover thousands of planets and is further specially designed to find a pool of small planets transiting small stars. TESS will deliver fifty rocky planets with measured masses for a lasting legacy. The TESS data has no proprietary time and the data segments will become public four months after observations.

Cameras

TESS is optimized for the detection of hundreds of Super Earths around nearby, bright stars. TESS has four identical cameras that will together survey the entire sky throughout its two-year prime mission.

The cameras are equipped with custom f/1.4 lenses, providing each camera with a wide (24×24 degree) field of view. The cameras have an effective aperture size of 10cm (about 4 inches) in diameter, which was determined by simulating the detectability of planets. High cadence is needed for the detection of exoplanets, so exposures of planet search targets and other stars of particular interest are obtained every 2 minutes with full-frame images (FFIs) of the entire field of view returned every 30 minutes, see full frame images.

TESS uses a red-optical bandpass covering the wavelength range from about 600 to 1000 nm. The lenses were optimized for this bandpass, with blue limit enforced by a coating on one of the optical elements. The limit at the red edge is determined by the quantum efficiency (QE) of the CCD detectors. The detectors are back-illuminated CCDs from MIT/Lincoln Lab, with 4096×4096 pixels fitted within 62×62 mm area. The imaging area consists of 2048×2048 pixels, with the remaining pixels used as a frame-store to allow rapid (about 4 ms), shutterless readout with read noise of less than 10 electrons per second. The CCDs operate at a temperature of around -75 deg C, which reduces dark current to a negligible level.

The CCDs read out continuously at 2-second intervals. The data are processed on the spacecraft by the data handling unit (DHU; for TESS, the DHU is a Space Micro Image Processing Computer). The DHU stacks the 2-second images in groups of 60 to produce the 2-minute or 30-minute cadence for observations. Postage stamps (at 2-minute cadence, nominally 10×10 pixels in size) and FFIs (at 30-minute cadence) are compressed and stored in two 192 GB solid-state buffer (SSB) cards. The data from the SSBs are returned to Earth when the spacecraft reaches perigee, every 13.7 days.

Many parts come together for TESS to operate. Image credit NASA GSFC

Here, two technicians install one of the flight cameras in a test chamber which will simulate the cold vacuum conditions of space where the camera will operate.

Orbit

To carry out an exhaustive two-year survey of extrasolar planets in both celestial hemispheres, TESS needed to occupy a very particular position in space, a highly stable place that maximized sky coverage and gave the observatory a mostly unobstructed view of the cosmos, all from a low-radiation, thermally benign environment.

That’s not an easy thing to design – but after extensive investigations by MIT, the Aerospace Corporporation, and Orbital ATK, the TESS team came across something that fits the bill: a never-before-used lunar-resonant orbit known as P/2 (for analysis details see http://adsabs.harvard.edu/abs/2013arXiv1306.5333G). TESS will launch from Cape Canaveral in the Spring of 2018. After launch, it will enter an initial 600km parking orbit around the Earth, before the solid rocket motor ignites and boost TESS into a phasing orbit with an apogee of 250,000km. Once in this orbit, the solid rocket motor falls away and TESS de-spins using its onboard thrusters, before opening up its solar panels for the first time. During the perigee points of this phasing orbit, TESS will perform a burn to increase its apogee to 400,000km. The spacecraft then departs from the phasing orbit, using a lunar gravity assist to begin the insertion it into its final orbit. TESS’ final orbit, which has a period of 13.7 days, puts it into a 2:1 resonance with the moon. This resonance means that perturbations from the moon’s gravity are roughly averaged to zero, because the moon is always leading or lagging TESS by 90 degrees at apogee. As a result, this highly elliptical high-Earth orbit (HEO) will remain stable for several decades. So once TESS reaches its final mission trajectory, few, if any, correctional maneuvers are required to keep it there. This is especially important for a mass-constrained spacecraft that can’t carry extra fuel to periodically power thrusters to maintain an orbit. For more details on TESS’ orbit, and how it came to be, see this article.

The orbit sequence for TESS: first, a parking orbit (green), followed by a phasing orbit (yellow) with a lunar gravity assist which puts TESS into its final orbit (red), which is in a 2:1 resonance with the Moon (orbit in white).