Star Talk: Meet TESS, NASA’s next planet-hunting satellite


Finding life elsewhere is one of the holy grails of science.  Based on our understanding, the most suitable places for life are almost certainly to be on planets with atmospheres. Since 2009, a spacecraft named Kepler has been searching for planets around other stars.

Soon, the lifetime of this spacecraft will be over, but luckily its replacement will soon be going to the launch pad.

NASA’s Kepler spacecraft was sent into Earth orbit on March 7, 2009, and was the first instrument put into space that was designed and dedicated to hunt for planets around other stars. It monitored about 150,000 stars in a specific region of the summer constellation Cygnus, searching for any minute — and recurring — brightness changes in the light from a star.

These small, cyclical variations in a star’s light can be used to infer that one or more planets are passing between Earth and the star, blocking some of the starlight. In a sense, Kepler has been looking for regularly occurring partial stellar eclipses. Technically, that’s called a stellar transit.

Almost 4,000 planet observations by Kepler have been confirmed with followup studies using observatories on Earth.  Further, there are more than 2,000 Kepler findings waiting to be verified. These discoveries show that there are about 3,000 “solar systems” elsewhere, meaning that several of these systems contain multiple planets. We call planets around other stars extra-solar planets or exoplanets for short.

Kepler setbacks

The Kepler satellite, named after the 17th century astronomer and mathematician who first correctly described how the planets orbit the sun, began to experience problems during the summer of 2012. These issues were not with its astronomy instruments — such as its on-board telescope — but with the spacecraft hardware.

Just as with the Hubble Telescope, the Kepler satellite must be pointed very accurately at a celestial object and must maintain a “lock” on that object. Hubble and Kepler use devices called reaction wheels to reorient the spacecraft to aim its telescope.

 At minimum, three reaction wheels are needed to point the telescope correctly.  Kepler was sent into space with four reaction wheels, one for redundancy.  Several reaction wheels have failed on Hubble but, fortunately, when astronauts serviced the Space Telescope on shuttle missions, the malfunctioning wheels were replaced.

Hubble was the only man-made satellite of Earth that was designed – and placed into the appropriate orbit – to be serviced.  On July 14, 2012, one of the reaction wheels on Kepler failed, leaving no backup. A second failed the following May.

Although this additional failure left the Kepler spacecraft unable to target the Cygnus constellation, in 2013 NASA solicited proposals to continue Kepler’s goals using its two remaining reaction wheels and its rocket thrusters for pointing.

In May 2014, NASA announced a second-generation mission for Kepler called K2. The spacecraft can now observe two regions of the sky, the Leo-Virgo region and the Scopius-Centarius region.

The K2 mission has been approved through 2019, but at this time, the spacecraft’s on-board thruster fuel is expected to last only though 2018.

TESS up next

Kelper’s successor is called the Transiting Exoplanet Survey Satellite or TESS.  This spacecraft has been under development for many years and is scheduled to be launched as early as March 20 on a SpaceX Falcon 9 rocket.

Like the Kepler mission, TESS will use the transit method to find exoplanets. Beyond the abilities of the Kepler satellite, though, TESS will survey half a million stars across the entire sky — an area over 400 times greater than that studied by the Kepler mission. TESS is expected to discover thousands of planets orbiting in solar systems far beyond ours.

The primary mission of the TESS satellite will last two years. It will survey stars over a certain brightness that are similar to the sun as well as stars that are cooler.

Statistically, it is estimated that this mission will discover at least 3,000 more exoplanets, of those as much as 500 could be Earth-sized and some could be super-Earths, meaning planets that are up to twice the size of the Earth.

Nevertheless, the holy grail of exoplanet research is to find Earth-like planets, particularly, one where liquid water could exist – the habitable zone around a star.

March sky

Mercury and Venus will spend the first three weeks of March appearing near one another in the early evening twilight sky.  During the first week of March, however, it will be quite a challenge to find these two planets hovering low in the western sky very soon after sunset.

On the first few days of March, use binoculars and start looking directly west only 20 minutes after sunset.  Venus is bright but the competition from the relatively bright twilight sky at this time will be formidable.  If you find the two planets, watch dimmer Mercury shift its position alongside Venus over the first week of the month.

By midmonth, Mercury and Venus should be easily visible to the unaided eye about 30 minutes after sunset.  In fact, on March 18, a thin, waxing crescent moon will appear near Venus and Mercury.

In the south-southwestern sky an hour before the sun rises, Mars and Saturn appear to be closing in on one another.  By the end of the month, these two planets appear very close and will be rising together near the east-southeastern horizon around 2:30 a.m. Watch for the waning crescent moon between Mars and Saturn on March 10.

Jupiter is also crossing the night sky; currently, it is rising around midnight also from the east-southeast.  It’s against the stars of Libra, so it never gains very much altitude during the night. The waning gibbous moon appears near Jupiter on March 20.

Finally, we have a full moon on March 1 and then again on March 31. This is the second Blue Moon of the year, an occurrence that only happens every 19 years.

Richard Monda is an astronomer living in the Capital Region. Watch his “Eyes on the Sky” night sky videos at the Hudson Valley Community College YouTube page.