NASA's Lucy Mission to Jupiter's Trojan Asteroids is Back on Track

Last October 16, NASA's Lucy spacecraft started its multiyear mission to Jupiter's Trojan asteroids.  Within two days, it was reported that one of the solar power arrays had not unfolded and latched properly.  In an update today, August 3rd, NASA Goddard Space Flight Center announced that the efforts to repair the satellite have succeeded and the mission will proceed as planned. 

NASA Goddard released this rendering of the satellite before work was done and it's easy to see the solar array on the right isn't fully opened.  They explained that even in this position, it was producing about 90% of its rated power, so the mission didn't seem to be in lots of trouble.  

Although not in lots of trouble, it was imperative to understand what had really happened so they could be sure it wouldn't degrade even farther on its own.

In situations like this, the standard response is to call together a "Tiger Team" or (less flashy named) anomaly response team of scientists and engineers from all the contractors involved: spacecraft builder Lockheed Martin, solar array designer Northrop Grumman, science mission leader Southwest Research Institute (SwRI); and mission operations leader NASA’s Goddard Space Flight Center.  From the description at NASA's Goddard SFC:

To evaluate Lucy’s solar array configuration in real time, the team fired thrusters on the spacecraft and gathered data on how those forces made the solar array vibrate. Next, they fed the data into a detailed model of the array’s motor assembly to infer how rigid Lucy’s array was – which helped uncover the source of the issue.

At last, they closed in on the root cause: a lanyard designed to pull Lucy’s massive solar array open was likely snarled on its bobbin-like spool. 

After months of further brainstorming and testing, Lucy’s team settled on two potential paths forward.

In one, they would pull harder on the lanyard by running the array’s back-up deployment motor at the same time as its primary motor. The power from two motors should allow the jammed lanyard to wind in further and engage array’s latching mechanism. While both motors were never originally intended to operate at the same time, the team used models to ensure the concept would work.

The second option was simply to use it as it was.  Since they were getting 90% of the expected power out of that panel and designing a critical system like that without lots of design margin Just Isn't Done, there was no reason to think they absolutely needed to open the array completely.  But they went ahead and tried.  

After months of simulations and testing, NASA decided to move forward with the first option – a multi-step attempt to fully redeploy the solar array. On seven occasions in May and June, the team commanded the spacecraft to simultaneously run the primary and backup solar array deployment motors. The effort succeeded, pulling in the lanyard, and further opening and tensioning the array.

The mission now estimates that Lucy’s solar array is between 353 degrees and 357 degrees open (out of 360 total degrees for a fully deployed array). While the array is not fully latched, it is under substantially more tension, making it stable enough for the spacecraft to operate as needed for mission operations.

The next mission milestone for Lucy is a gravity assist flyby of Earth this October.  Lucy is not scheduled to reach the first asteroid on its mission until April of  '25.  Here, I'm going to fill in some background from the piece I wrote on this mission in October '21.

Lucy is an interesting mission that seems totally academic.  The mission is to the Trojan asteroids, orbiting at the LaGrange points of Jupiter, ahead of and behind Jupiter itself in its orbit around the sun.  

The $981 million mission will fly an extremely complex trajectory over the span of a dozen years. The spacecraft will swing by Earth a total of three times for gravitational assists as it visits a main-belt asteroid, 52246 Donaldjohanson, and subsequently flies by eight Trojan asteroids that share Jupiter's orbit around the Sun.
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Lucy will fly by its first asteroid target in April 2025, a main-belt asteroid named after Donald Johanson, the American anthropologist who co-discovered the famed "Lucy" fossil in 1974. The fossil, of a female hominin species that lived about 3.2 million years ago, supported the evolutionary idea that bipedalism preceded an increase in brain size.

The mission itself is named after that fossil Lucy as well.  It's a fanciful name, but these asteroids themselves are considered to be old fossils from the formation of the solar system. 

This is Lucy's trajectory over the next 12 years.  It looks complex but it's even more complex than this shows.  This perspective is created by fixing Jupiter's position, so during the 12 years of the mission that this is showing, Jupiter and both groups of Trojan asteroids will complete a little more than one full orbit around the sun, making this a more complex plot.  Plot from the Southwest Research Institute, the scientists behind the mission.  

I'll leave you with this mind-blower.

Somewhat ironically, although Lucy is visiting the "Jupiter trojans," it will never be closer to Jupiter than when it is on Earth. This is because the Trojans trail Jupiter at a greater distance than the distance that lies between Earth and the Solar System's largest planet.