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Why This New Exoplanet-Hunting Telescope Blows Kepler Out of the Water
TESS will have a brand-new orbit, visualized here.
Credit: NASA

With just a 30-second window, NASA launched its next exoplanet-searching satellite today (April 18) toward a never-before-used orbit that was discovered in the 1990s. 

The goal? To get the ultimate view of the night sky from a perfect orbit around Earth.

Once inside that perfect orbit — it should take about a month to get there — the observatory will have its eyes on 85 percent of the sky. Everything had to be lined up perfectly to get the 700-lb. (318 kilograms) observatory, called the Transiting Exoplanet Survey Satellite or TESS, on its way toward this "sweet spot" around Earth. 

Its forerunner, the venerable Kepler space telescope, found over 2,300 exoplanets by staring at an area about 380 times smaller than that. 

Now, with a dwindling fuel supply, Kepler is easing into retirement and slowly drifting farther and farther from Earth. Soon, TESS will embark on a squeaky-clean, never-before-used path — a loop around Earth that the mission's principal investigator, George Ricker, called the "Goldilocks orbit" in a 2013 statement from NASA.

"We were looking for an orbit that would let us stare at a patch of sky for a long time," said Stephen Rinehart, a TESS project scientist. When designing the mission, the NASA team chose what's called the P/2 lunar-resonant orbit. That means that for every revolution the moon makes around Earth, this spacecraft makes two. And for much of the 13.7 days of TESS' orbit around Earth, the spacecraft will be looking out into the cosmos for dimming star lights — an indication that a host star has been eclipsed for a brief moment by a resident planet.

The SpaceX Falcon 9 rocket lifts off carrying NASA's TESS spacecraft on April 18, 2018.
The SpaceX Falcon 9 rocket lifts off carrying NASA's TESS spacecraft on April 18, 2018.
Credit: NASA TV

When not scanning the sky, TESS will spit its data back to Earth. 

The orbit's specific ellipse shape allows the satellite to get far enough away from Earth while surveying the sky so that our planet mostly stays out of the craft's view. (Earth will probably sometimes photobomb it.) But the satellite can also move close enough to the planet to dump all the data back home without needing powerful, expensive equipment. 

But what makes the orbit ideal is that, once the satellite is in it, very little fuel and maneuvering will be needed to keep it there, Rinehart said. This P/2 orbit is stable because of the pull of the Earth and the moon. Every time TESS is at the farthest point from Earth, it will be at a 90-degree angle from the moon — first on one side of its orbit and then on the other. Therefore, any effect on the spacecraft's trajectory caused by the pull of the moon when on one side will get canceled out by the pull on the other side.

"If you drifted a little bit, the gravity of the moon shepherds you back into the orbit," Rinehart told Live Science. "The combination of the Earth's gravity and the moon's gravity constantly works together to keep you on that path."

But getting on that path in the first place is a delicate affair, and TESS won't arrive at its orbit for about a month. 

"If we get it wrong, the orbit will eventually either come too close to the Earth or come too close to the moon," Shane Hynes, the mission system engineer for TESS, told Live Science. If that were to happen, the satellite would lose resonance — the oscillations caused by the pull and push of our planet and the moon that keep it in its stable trajectory. To push the spacecraft back onto the correct path using propellers would require a lot of fuel, Hynes added.

To make sure TESS makes it to the P/2 orbit, the NASA team had to wait for the right 30-second windows in which to launch the spacecraft — times when the moon was in a particular spot so its gravity could serve as TESS' slingshot.

"It's kind of like being on a kid's scooter and grabbing a passing car and then letting go," said Hynes. "The passing car speeds you up, and when you let go, you can go in a slightly different direction or speed." In that same way, the gravitational pull of the moon will help launch TESS into the Goldilocks orbit around Earth, he said.

This orbit also comes with additional benefits, such as being above Earth's radiation belts, which could damage the satellite's onboard electronics, Hynes said. And rarely will TESS fall under the shadow of the sun, which could cause changes in temperature and damage to the cameras, he said. Because this is the first time a P/2 orbit has been occupied, there isn't any space junk or debris to worry about on its path, Rinehart added.

"There are all these considerations, but they found a nice orbit that allows them to keep the costs at a minimum while maintaining the characteristics necessary to perform an all-sky survey that isn't impeded by [the Earth]," said Adam Burrows, an astrophysicist at Princeton University who is not involved in the mission. "Getting it in that orbit is a feat, but it's possible." 

Burrows isn't worried. "It's a complicated dance," he said, but it's "low-risk." 

The NASA scientists said their spirits are high. "I think everybody's really excited about finding planets close to the Earth," Hynes said. "TESS is probably going to find more planets each month than Kepler found in its whole mission." 

The key is to navigate the satellite to the correct cosmic highway.

Originally published on Live Science.