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ASU sensor will help NASA's Lucy mission solve solar system riddle

By Nicholas Gerbis
Published: Monday, October 18, 2021 - 5:25am

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Lucy concept
Southwest Research Institute
Artist’s illustration of the Lucy concept.

On Saturday, NASA launched its Lucy mission to survey Jupiter's Trojans — asteroid clumps that precede and trail the gas giant as it orbits the Sun.

An ASU-built sensor will help the mission settle an ongoing debate about the solar system's origins.

Mounting data from planets circling distant stars bolster the idea that our sun's planets did not form in their present orbits, but rather migrated toward and away from the sun in the distant past.

"With these planet migration models, early in the solar system history, it was a pretty crazy place. And planets were moving and things were getting thrown, even scattered out of the solar system," said Phil Christensen of ASU's School of Earth and Space Exploration, who leads the Lucy Thermal Emission Spectrometer (L'TES) team.

If true, then the Trojans' makeup should reflect icy origins far beyond Neptune's orbit.

That's because the Trojans became trapped long ago in special pockets where the sun's gravity balances perfectly with Jupiter's.

These areas, where gravitational and centripetal forces balance just so, are called Lagrange points.

"It's like this gravitational hole that you fall into and you can't get out of. So they've been collecting things," said Christensen.
But the Trojans are more than ancient — they are pristine.

NASA/WMAP Science Team
At Lagrange points, the gravitational pull of two large masses precisely equals the centripetal force required for a small object to move with them. The Trojans are located in Jupiter's L4 and L5.

Many asteroids, including OSIRIS-REx's target Bennu, at some point approached closer to the sun, which cooked their surfaces and altered the thawing, movement and refreezing of volatiles such as water.

But Trojans have occupied their Lagrange points since the solar system began.

"So they've never been heated. They've never been bathed. And so they really, truly are primitive material — not just old, but primitive. It hasn't been altered from when it first condensed four and a half billion years ago."

L'TES is the latest in a long line of thermal emission spectrometers Christensen's teams have launched aboard missions such as Europa Clipper, Mars Global Surveyor and OSIRIS-REx. Some of their instruments still function 20 years after launch.

That track record exerted a major influence on what NASA asked them to provide.

"They wanted an infrared instrument, but they wanted something that was not going to cause them any trouble—that was proven, you know. So they came to me and said, 'Build an exact copy of the OSIRIS instrument.' And we did," said Christensen.

Christensen said he didn't mind reproducing his pride and joy. For him, it makes more sense than working to perfect a technology and then only making a single copy of it.

"I joke with people about, you know, imagine someone made an iPhone, and he only made one of them. It's like, wow, that was a lot of work just for one," he said.

Lucy will survey the leading Trojans in 2027 and the trailing group in 2033, with Earth flybys in between.

Because the spacecraft will seek out some of the solar system's earliest remains, it bears the name of one of the most famous hominin fossil skeletons studied on Earth. That Lucy was found in Ethiopia in 1974 by Donald Johanson and Tom Gray.

Appropriately enough, the Lucy spacecraft will fly by the main belt asteroid (52246) Donaldjohanson on its way to Jupiter.

Christensen said he looks forward to whatever surprises lurk among the floating Trojan fossils.

"You're always surprised. You know, we'll get out there and go, 'Oh, that's not what we thought.' That's the beauty of doing it," Christensen said. 

Southwest Research Institute
Lucy's orbital path (green) relative to Jupiter. After launch, Lucy has two close Earth flybys before encountering its L4 Trojan targets from 2027-2028. After diving past Earth again, Lucy will visit the L5 targets in 2033.