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The next era in searches for exoplanets

  TESS

The TESS mission, scheduled for launch this week, will search for exoplanets that orbit nearby nearby stars that could be targets for follow-up observations of the James Webb Space Telescope and other future observatories. (credit: NASA)





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For several years, a small NASA spacecraft has been one of the most productive tools in the search for exoplanets. Astronomers have used Kepler data to discover several thousand exoplanets by measuring faint but periodic drops in the brightness of stars when planets pass or pass through stars, as seen by the spacecraft. Kepler's mission continued even after the failure of the reaction control wheels that ended his primary mission, instead of exploring a variety of regions of the sky for exoplanets and other sciences.

"TESS is small, but it drills above its weight," Ricker said.

Kepler, however, is coming to an end as he runs out of fuel for his thrusters. As the Kepler mission nears its inevitable conclusion, astronomers have now turned their attention to the next generation in exoplanet searches. A SpaceX Falcon 9 is scheduled to launch this week on NASA's TESS mission (the launch, which was scheduled for Monday, was delayed a few hours before takeoff until no earlier than Wednesday to address a guide, navigation and control problem with The rocket).

TESS, as its name implies, uses the same approach as Kepler to discover exoplanets, in search of those deep abysses of brightness that are the signature of worlds that orbit other stars. But while Kepler's original mission was to look at a single area of ​​the sky, TESS will survey the entire sky, focusing mainly on near-bright stars that are ideal targets for follow-up observations of large space-based and future-based space telescopes. like the James Webb Space Telescope.

"It's a search environment for JWST," said George Ricker, principal investigator for TESS, at a meeting of the American Astronomical Society in the suburbs of Washington, DC in January. Only a small viewfinder helps astronomers find a target for a larger telescope, TESS will do the same for JWST and other observatories.

The search scope analogy also applies to its relative sizes and costs. While JWST is a massive spacecraft with a 6.5-meter primary mirror, and its latest delays are expected to break its $ 8 billion maximum development cost, TESS and its set of four small wide field cameras weigh approximately 360 Kilograms. , with a mission cost of approximately $ 300 million, including the launch. The spacecraft seemed particularly small last week as it was being encapsulated in the Falcon 9 payload fairing, sized for much larger satellites.

"TESS is small, but it drills above its weight," Ricker said.

  TESS encapsulation

The TESS spacecraft was encapsulated in the Falcon 9 payload size for much larger spacecraft, last week. (credit: NASA)

The launch of TESS is not much different from a Falcon 9 mission for a commercial communications satellite, with two burns on the upper stage of the rocket to place the spacecraft in a transfer orbit highly elliptical. "It looks like a GEO transfer orbit, but it goes more to the moon," said Hans Koenigsmann, vice president of construction and flight reliability at SpaceX, at a press conference prior to the launch on April 15.

Released from the upper stage about 49 minutes after take-off, TESS will perform a series of maneuvers to extend its orbit, preparing for a lunar flyby scheduled for mid-May. "We have to do peak burns and perigee burns for four or five orbits" after the launch of the satellite, said Jeff Volosin, manager of the TESS project at NASA's Goddard Space Flight Center, at that press conference. "That moment of lunar flyby will put us in that [final] orbit."

If all goes well, TESS will enter that final orbit, known as the P / 2 resonance orbit, in mid-June. That orbit places the spacecraft at a stable 2: 1 resonance with the Moon, minimizing the necessary maintenance maneuvers and allowing high data rates to transmit data to the Earth, unlike Kepler's heliocentric orbit that still drives it away more of the Earth.

"Our current estimates are that Kepler's tank will dry out in several months, but we've been surprised by its performance before," Sobeck said.

While TESS has a two-year main mission, the stability of its orbit could allow significant extensions if the rest of the spacecraft is functioning well. "The operational life of the mission could be extended for more than two decades," Ricker said in January.

The main mission of TESS will be the searches of exoplanets, but as with Kepler, whose data has been used for a wide variety of other astrophysical investigations, mission officials foresee additional uses of the data, such as astrophysics stellar and extragalactic astronomy. A research program led by NASA will support these additional lines of research.

TESS had some problems during its development, especially with its optics. Last summer, NASA acknowledged that they had discovered that the focus of the cameras would deviate slightly as they cooled off after launch. While the agency played down the issue, some astronomers worried that it would prevent the mission from fulfilling some of its scientific goals.

Ricker and other project scientists are more confident now that the drift of the approach will not be a problem. "The subsequent tests we did from this summer and then in the fall indicated that there is a model" to explain the change, he said in January. That model concluded that there is a "very reproducible crystallization effect" in one of the materials used in the chambers that cause the drift. That drift stops after a while, and Ricker called it a "change of focus" all at once instead of a "focus drift", which sounds more continuous.

Because TESS is primarily a photometry mission, measuring the brightness of the stars, the focus drift, or the change in focus will not affect the science of the mission, Ricker argued at the January conference.

"This is a photometry mission, not an image mission," he said. "What this means is that it's not important to have a clear focus on the whole field of vision, it was never part of the design, but it's important that the approach is stable, and that's what we've been able to establish."

The launch of TESS comes when Kepler reaches the end of his life. The spacecraft is running out of propellant used by its propellers to maintain its attitude since the failure of two of its reaction control wheels several years ago. When that thruster runs out, the mission of the ship will come to an end, since it will not be able to control its signaling.

A NASA statement in March on the end-of-mission plans led some to conclude that Kepler's demise is imminent. But without a fuel meter, spacecraft operators rely on their best estimates of the amount of propellant left, with some margin of error.

"Our current estimates are that Kepler's tank will dry out within several months, but we've been surprised by its performance before!" Charlie Sobeck, Kepler's systems engineer, said in that statement. "So, although we anticipate that the flight operations will end soon, we are prepared to continue as long as the fuel permits."

The spacecraft itself works well, apart from the low fuel level, so scientists are working to extract as much science as possible. There are plans for additional campaigns that will be extended for the rest of this year.

"TESS forms a kind of bridge between what we have learned about exoplanets to date and where we are going in the future," Volosin said.

Astronomers continue to extract data collected by Kepler for new exoplanet discoveries and other investigations, a process that will continue long after Kepler finishes. "While we're running out of fuel," said Jessie Dotson, a K2 project scientist at NASA's Ames Research Center, in January, "science is just getting started."

TESS is being billed by NASA as a successor to Kepler, continuing the track of exoplanets and finding targets for later missions. "TESS forms a kind of bridge between what we have learned about exoplanets to date and where we are headed in the future," Volosin said. "That's a big part of our mission: to allow future exploration by providing a giant data set across the sky where these exoplanets are located."

These discoveries should be exciting, based on the reaction of the public to the capture of Kepler's exoplanets. Robert Lockwood, the program manager for TESS in Orbital ATK, who built the spacecraft, said his wife, who does not work in the field, was particularly excited about this mission, and cited the "extraordinary things" he will do to find new ones. worlds

"The idea of ​​finding other things on other planets really only awakens something in people," he said at the pre-launch press conference. "I think it will look like a forest fire … TESS will take it to another level."



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