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Now Announcing TESS, NASA’s Next Planet-Hunter
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NASA announced on April 6 that the Transiting Exoplanet Survey Satellite (TESS) has been selected for development in its latest round of Explorer-class mission proposals. The satellite is scheduled to launch in 2017.

TESS was one of two future missions outlined by NASA in a press release on Saturday. The satellite will work on much the same principle as the Kepler satellite, which is still operational. Kepler is aimed towards a dense starfield in the constellation of Cygnus, where a photometer measures the brightness of approximately 150,000 stars every 30 minutes. When an exoplanet crosses in front of a star from Kepler’s point of view, the star’s brightness dims.

Measurements of the amount and duration of the dimming give us estimates of the size and orbit of the planet. Possible exoplanets are then verified by ground-based spectroscopic imaging, which reveals tiny wobbles in the star’s motion due to the tug of the exoplanet’s gravity. To date, that mission has discovered 841 confirmed exoplanets and nearly 2800 possible exoplanets.

In addition to its exoplanet data, Kepler has also helped push the boundaries of physical astronomy. The light curve data produced by Kepler have allowed astronomers to make groundbreaking advances in learning about the structures of stars, binary systems, and the process of planetary formation. In particular, the field of asteroseismology has been aided by Kepler’s data. Asteroseismology measures how stars pulse after flares occur on their surface. By studying how waves produced by those eruptions propagate, astronomers can gain insight into how the star is layered.

In recent months though, Kepler has shown signs of aging. Kepler uses a system of three reaction wheels to keep the imaging platform stabilized and pointing at its target starfield. In July 2012, one of the spacecraft’s three reaction wheels failed, forcing mission operators to switch to the spacecraft’s only backup wheel. In January 2013, another reaction wheel showed signs of giving out, causing a 10-day suspension in operations as operators tried to fix the problem. The problem was eventually solved, but when one of the remaining reaction wheels fails, the mission will come to an end.

So, how does TESS measure up to Kepler? Here’s some comparisons between the two spacecraft:

  • TESS will survey the entire sky through the use of four wide-field telescopes. This survey area is over 400 times larger than Kepler’s field of view.
  • TESS each of those four telescopes will have a CCD detector. Combined those cameras will have a total of 192 megapixels, compared to Kepler’s 95 megapixel CCD.
  • TESS will study approximately 2,000,000 stars, compared to Kepler’s 150,000.

One of the biggest drawbacks of Kepler is its orbit. After launch, it was placed in a heliocentric orbit to minimize the amount of fuel needed to keep its telescope pointed in the right direction. Had it been in Earth orbit, it would have been susceptible to slight changes in its orbit due to the influences of Earth’s lumpy gravitational field and the Moon. However, this choice of orbit means that the already distant spacecraft is slowly drifting further away from Earth.

As it drifts further away, downlinking data from Kepler requires the use of NASA’s Deep Space Network, which is extremely busy and only has a limited amount of bandwidth to use on any given mission. Kepler’s designers got around this problem by building in many of the data reduction processes into the spacecraft itself. Instead of sending back the raw images that the 95 megapixel CCD camera obtains, the computers on board Kepler only examine the pixels containing the stars of interest. The numerical information in each of those pixels are then stored for transmission back to Earth, while the rest of the image is dumped.

Even with the onboard data reduction, Kepler still produces about 12 gigabytes of data each month, which takes up significant time on the Deep Space Network. To circumvent this problem, TESS will remain in Earth-orbit, where other facilities can collect the data. “For TESS, we were able to devise a special new ‘Goldilocks’ orbit for the spacecraft — one which is not too close, and not too far, from both the Earth and the moon,”  said principal investigator George Ricker (MKI). Every two weeks, the satellite will swing by Earth just above the upper Van Allen belt to avoid radiation damage. The close passes allow TESS to downlink much larger amounts of data back to Earth than Kepler.

As an added bonus the orbit is stable over decades, meaning less fuel will be needed for station keeping. In turn, this allows for more mass to be devoted to scientific equipment, rather than fuel. The orbit also keeps the spacecraft operating within a narrow range of temperatures. Temperature has a large effect on the performance of CCD cameras, so minimizing the temperature variation will allow TESS to be a more sensitive camera.

Even with the added data capacity, TESS will still need to be selective in what stars it observes. Even though the satellite will be capable of measuring over two million stars multiple times each day, that is only a fraction of the number of stars that would be visible to TESS’s sensors. To narrow down the number of stars, TESS will be focusing on nearby G, K, and M-class stars, which are long-lived and relatively stable compared to their bigger, brighter relatives. These classes of stars are considered to have the highest probability of having habitable planets.

TESS’s planners are optimistic about the mission. Scientists are hoping that the new mission will be able to provide critical details such as the size, mass, and orbit of the exoplanets that it discovers. They also plan to use TESS’s findings in conjunction with the James Webb Space Telescope, which they hope will provide information about the exoplanets’ atmospheres, if present. “The selection of TESS has just accelerated our chances of finding life on another planet within the next decade,” Sara Seager (MIT) said.

By examining the nearest stars, the TESS team hopes to pick up on the presence of exoplanets around the most easily observable stars. The nearness of its targets also increase the chances that Earth-sized and smaller planets will be found. “The TESS legacy will be a catalog of the nearest and brightest main-sequence stars hosting transiting exoplanets, which will forever be the most favorable targets for detailed investigations,” said Ricker.

TESS is scheduled to launch in 2017. Development of the CCD cameras at the heart of the spacecraft are already in development, thanks to a seed grant given to MKI by Google in 2010. The budget for TESS (not including the launch system) is capped at $200 million, with operational costs not to exceed $55 million during the mission’s lifetime.

Image Credit: MIT Kavli Institute for Astrophysics & Space Research

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About The Author
Justin Cowart
Justin Cowart is a geologist interested in Earth and Solar System history. As a geologist, he spends hist time looking at the ground, but in his free time he looks to the skies as an amateur astronomer.

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