Tuesday, October 19, 2021

Just-Launched Satellite Lucy's Solar Panels Have a Problem

NASA's Lucy mission to the Trojan asteroids of Jupiter launched successfully early this past Saturday morning, but after deployment of its two enormous solar panels, one has not reported successfully latching into place.  

Combined, the two solar arrays have a collecting area of 51 square meters. Such large arrays are necessary because the spacecraft will spend much of its 12-year journey about five times the distance of the Earth from the Sun. Lucy's solar panels can only generate about 3 percent of the energy at a Jovian distance than they can at Earth's orbit around the Sun.

A rule of thumb number to remember is that the sun delivers around 1300 Watts per square meter to the top of Earth's atmosphere.  The 3% value they quote is 39 W/sq.m., and the 51 square meter array would give the spacecraft 1990 Watts every hour at those distances.  Lucy is traveling farther from the sun for a longer mission than any previous satellite that was completely solar powered.  Earlier satellites commonly used Radioisotope Thermal Generators, RTGs.  

The other way to think about this issue is that if it's designed to run on 3% of what's available in Earth orbit, it's got 33.3 times that right now while they troubleshoot the issue.  On Lucy's NASA project website to watch, they note:

Lucy’s two solar arrays have deployed, and both are producing power and the battery is charging. While one of the arrays has latched, indications are that the second array may not be fully latched. All other subsystems are normal. In the current spacecraft attitude, Lucy can continue to operate with no threat to its health and safety. The team is analyzing spacecraft data to understand the situation and determine next steps to achieve full deployment of the solar array.

This is yet another example of the saying that "space is hard."  Lucy is in an escape orbit and beyond the reach of any other spacecraft to help out.  The mission controllers are on their own.  I should point out that they said, "indications are that the second array may not be fully latched."  They still need to determine it's actually a problem with the latching and not the circuit that measures whether or not the latch is properly set.  If the sensor or circuit is bad, it's annoying but not a show stopper. 

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.  

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.
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 really 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 end this with a 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.




  1. I'm curious-why would you go with solar panels rather than RTGs for that environment, aside from all the green idiots who would be protesting about evil radiation?

    1. There was nothing specific about that in any of the sources I read, but I haven't done a deep dive into the history of the mission. Maybe there's something mentioned in there.

      Offhand, I assume it's exactly what you say. Living fairly close to the KSC, I remember missions that were protested by anti-nuclear loonies.

    2. I remember the protests against the Galileo mission because it used a plutonium-238 RTG. Plutonium being a code word to the loonies for evil. I'm sure that NASA administrators dictated a non-nuclear power system to preclude similar protests.

  2. I watched something last night on Lagrange points. L4 and L5 make equilateral triangles with the sun, Jupiter, and the Trojans. That means they're as far from Jupiter as Jupiter is from the sun. This, Earth being closer.

    1. I'm assuming you're referring to the video Scott Manley put up over the weekend. For everyone who'd like to see great computer visualizations of the Lagrange points, see Scott Manley's excellent video. Yeah, it's nearly 14 minutes long but worth it.

  3. They should have picked a horse name . . .

  4. In addition to wanting the mission to be green, there may have been apprehension about availability of fuel. There is, and likely was during planning / production of LUCY, a shortage of 238-Pu (the usual fuel for RTGs). There was a European Space Agency probe (Rosetta?) that went out to the asteroid belt(?) and failed(?) because the solar panels did not provide enough power.

  5. It's academic until something turns up. I do admit to having used the term as a pejorative. Hey Galileo, whatcha looking at?