posted November 25, 2013
Another Earth. What an enrapturing prospect. The idea that somewhere in our universe a water-filled planet sits close enough to a star to produce life, and potentially, humanlike creatures of its own — capable of thought, communication, and invention — is a concept so wondrous and remote that it has been relegated mostly to the fantasies of Hollywood. In fact, a film called “Another Earth” was released in 2011, and its liberal alteration of scientific fact only helped to confirm the public’s perception that such things merely exist in the realm of human imagination. Well, that notion may soon change, as science begins to catch up with previously imagined concepts of what might exist beyond the familiar confines of our solar system.
The Transiting Exoplanet Survey Satellite (TESS) is a space telescope that has been designed with the specific mission of locating and observing the most promising Earthlike planets in our celestial sky. The concept for TESS was awarded funding by NASA’s Explorer program — a space exploration program aimed at providing flights for lower-cost projects — in 2012 after years of design and development. Aerospace was involved in many phases of the project, and TESS is currently in the early stages of construction with a planned launch scheduled for 2017.
TESS, in many respects, can be viewed as the follow-up to NASA’s Kepler space observatory, which was launched in 2009. Kepler’s mission, though plagued by technical issues, has been incredibly successful, discovering thousands of exoplanets, hundreds of which are Earth and super-Earth-size (10 times the mass of Earth and below). Scientists have used this data to make staggering calculations about the potential number of Earth-like planets in the Milky Way Galaxy and the universe. By examining the number of Kepler-discovered Earth-like planets within the “habitable zone” — the region around a star where the formation of liquid water on planets with sufficient atmospheric pressures is possible — scientists predict that there could be as many as two billion Earth or super-Earth analogues in the Milky Way Galaxy alone. As a result, the notion that life may exist on other planets doesn’t appear nearly as remote as it did only a handful of years ago.
And TESS is going to take things a step further. Instead of indiscriminately cataloguing planets in a fixed portion of the sky — as Kepler has effectively done — TESS will search for the best Earthlike planets by surveying the entire sky. The TESS all-sky survey will search for planets orbiting stars that are brighter and closer to Earth, thus making them ripe for atmospheric categorization and other observational analyses. Both Kepler and TESS utilize a transiting method to locate potential planets. This means that the telescopes focus upon a star and wait for a potential planet to orbit in front of the star. When the potential planet passes in front of the star, it causes a brief dip in the visual brightness of the star. By measuring the dip in light, Kepler and TESS can detect the size of a planet, thus categorizing it as a potential Earth, super-Earth, or Uranus-sized planet.
All of the Earthlike planets that Kepler has discovered surround stars that are too dim and distant to provide the opportunity for scientists to observe the planets in great detail. TESS hopes to discover more promising Earthlike planets by focusing upon bright M through K stars. The smaller, redder M dwarfs are abundant and the star’s smaller size makes it easier to detect a smaller planet with the transiting method. The goal is to find as many of these planets as possible and to eventually study them with the proposed James Webb Space Telescope (scheduled for launch in 2018) and other high-powered telescopes of the future. The planets that TESS locates could very well become the focus of research, analysis, and even communications efforts for many years to come.
In the fall of 2007, Aerospace’s Randy Persinger and Dr. David Bearden traveled to the Ames Research Center to evaluate a selection of NASA proposals. In their evaluation, Persinger and Bearden noted a number of engineering and design inefficiencies with M.I.T.’s TESS proposal. As a result of their analysis, NASA asked Persinger to assist the project’s principal investigator, M.I.T.’s George Ricker, to deliver a redesigned and more efficient version of TESS. Over the course of two months, Persinger and a number of Aerospace employees worked tirelessly with M.I.T. to turn the proposal around. By stripping out an on-board tracking system and lowering the satellite’s data rate, the team was able to deliver a strong proposal that was subsequently selected for a six-month, in-depth study, known as Phase A.
Though streamlined and more efficient, TESS was not selected in the first round for the Operational Small Explorers Mission in January of 2009. After a few months of inactivity on the TESS front, Persinger and the Aerospace team were called back into action in 2010 as TESS was up for potential selection as a NASA Explorer mission. For this proposal, the Aerospace team realized that a change of orbit was necessary to provide continuous light curves similar to Kepler. “M.I.T., George Ricker, Orbital Sciences, and I got together and we concluded that going to High Earth Orbit, by using a lunar gravity assist with the moon was the solution,” says Persinger. “This P/2 HEO, in 2 to 1 resonance with the moon, is stable and gives us continuous viewing capability and it also affords us the opportunity — as we come around to the perigee — to conveniently download the data.”
On a basic level, the P/2 HEO allows TESS to escape the challenging thermal and attitude control environment of a lower orbit, providing stability for the satellite as it carefully observes stars and planets. At the beginning of the mission, TESS will start in a low Earth orbit and then, utilizing its solid rocket motor, will boost its apogee up to the altitude of the moon. After a few months, the apogee and the moon will coincide. By carefully targeting where it flies by the moon, TESS will enter into an inclined, highly eccentric orbit with very high perigee and apogee. After the moon fly-by, TESS will return to perigee and perform a small burn to change the orbit period. When the burn concludes, TESS will be in two-to-one resonance with the moon; that is, for every orbit of the moon, TESS will do exactly two orbits. As a result, the moon will alternate a quarter-orbit ahead or behind the satellite when it’s at apogee, which cancels out the perturbations of the moon’s gravity — making for a very stable orbit.
NASA accepted the new proposal in 2011 for another Phase A study, even though the P/2 HEO had never been flown before. As a result, Persinger was concerned about how the orbit’s credibility would hold up during the rigorous NASA evaluation that was to follow. Persinger called upon Aerospace’s Joe Gangestead and Greg Henning to further examine the orbit and address problematic components in its design.
“The task for Greg and I was to study the TESS orbit in-depth and to really understand not only that it works, but why it works” says Gangestead. Over the course of several months, Gangestead, Henning, and Persinger dove into the fundamental properties of the P/2 HEO orbit and developed an air-tight understanding of its potential reaction to a dynamic selection of variables. “Not only did we have an orbit that worked, but by the time we were done, we had a lot of insight into why it worked,” says Gangestead. TESS was selected for flight this past spring and Aerospace’s in-depth analysis was critical in solidifying the team’s successful proposal. Currently in its construction phase, TESS will continue to receive Aerospace support into the future. “[George Ricker] wants me and other key Aerospace experts to continue for the life of the program,” says Persinger. “So over the next four years we will switch from delivering proposal-creation advice to actual project management, systems engineering, and payload systems engineering advice.”
Gangestead views TESS as an essential catalyst for future scientific discoveries. “Everyone wants to know: ‘are there Earth-like planets?’” says Gangestead. “And by Earth-like they don’t just mean the size, they mean: ‘are there planets with nitrogen and oxygen atmospheres that are covered with water?’ Of all the planets that have been discovered, we’re pretty sure that lots of them look like Jupiter or Saturn, but what we really care about is whether or not any of them look like Earth. TESS is one step along the way in definitively identifying if one of those planets is really out there.”
Persinger is enthusiastic about TESS’s future, and proud of the hard work that went into its development. “TESS is the simplest, lowest-risk satellite I’ve ever worked on,” says Persinger. “But it took five years to get it through the NASA selection process and to be selected for flight and now we’re putting up a satellite that the U.S. public will be very excited about. We want to find those Earths and super-Earths and know where they are around the celestial sky so that others in the future can go study them and have new careers. That, to me, is probably one of the best personal adventures I’ve had in 35 years.”
Though years in the making, it seems that TESS’s adventure is only just beginning.