Notes from a presentation at Capricon 44, Chicago, Feb. 2, 2024. Any mistakes are mine. The speaker, Bill Higgins, is not responsible for any errors here.

Bill Higgins

The two swarms of frigid Trojan asteroids, circling the Sun at the same distance as Jupiter, have never been visited by spacecraft. NASA’s Lucy mission plans to change that. Launched in 2021, Lucy’s intricate trajectory will eventually fly by eleven asteroids, including eight Trojans of varying size and composition far beyond the main Asteroid Belt. Passing through the asteroid belt on her way to the distant realm of the Trojans, Lucy’s first flyby this past November held surprises—and if all goes well, the best is yet to come.

• NASA, Japan, ESA, and China all have missions to asteroids.
• Lucy is named after the fossil. “Journey to the fossils of the solar system.” Also see the SWRI Lucy site.
• It is a mission to the Trojan asteroids, which are in two clusters at the L4 and L5 points of Jupiter’s orbit around the sun, ±60^o either side of Jupiter.
• There is some diversity in the Trojam population. This is important when thinking about planetary formation.
• Motivation.
  • Close-up look at some Trojan asteroids.
  • Clues about the formation of the solar system.
• Planetesimals made up the gas giants. A lot were ejected from the solar system early in its history. Many were collected by Jupiter.
• The Trojan asteroid are fossils of the planetary formation era.
• Lucy will visit 7-8 Trojan asteroids, passing 2-3 main belt asteroids on the way. The ambiguity is because two of the originally planned targets turned out to have moons.
• Trojans have a range of colors, from gray (C) to very red (D).
  • C’s reflect about the same over a wide range of wavelengths.
  • P’s and especially D’s reflect more in infra-red,
  • Estimate densities 0.8-2 gm/cc. Sort of like limestone
  • Some have odd shapes.
  • Some have satellites.
• What made up the giant planets? How did they form? Did they move in different orbits in the early solar system.
• Lucy is powered by two very large solar panels. At that distance the sun is 1/25 the brightness in the Earth’s orbit.
• One panel did not latch properly in deployment. After some tests the team decided to proceed with the mission.
• The spacecraft was launched in October 2021. It received a gravity assist from Earth a year later. Another such assist is coming in December 2024.
• The first encounter will be the asteroid 52246 DonaldJohanson, named after the discoverer of the Lucy fossil. Then it will fly by the L4 Trojans Eurybates (type C), Queta (type C), Polymele (type P) and its satellite, unofficially named Shawn, and Leucus (type D) in April 2028. Then it will return to perihelion and get another gravity assist from Earth.
• This will take it to Jupiter’s L5 point. There it will fly by the pair 617 Patroclus (type D) and Menoetius on March 2, 2033.
• Several of Lucy’s instruments are base on others that have flown on previous missions, e.g. L’LORRI, L’Ralph, L’TES. The “L” prefix indicates the Lucy version. See The Lucy Spacecraft for more about the instruments.
• Main belt asteroid 152830 Dinkinesh was added to the mission and Lucy flew by it on November 1, 2023, revealing first that Dinkinesh is a binary pair, and that its satellite Selam (“peace” in Amharic) is itself a contact binary.
• Lucy is powered by solar panels rather than radioisotope thermoelectric generators (RTGs).
• Solar panels worked for the Juno mission.
• See the last entry in the Lucy FAQ.
• There is currently a shortage of Plutonium-238, the usual fuel for RTGs. It is a byproduct of atomic bomb production.
• There is now a de facto ban on using Earth gravity assists for RTG powered spacecraft.