Most of us have heard of the main asteroid belt located between Jupiter and Mars. Astronomers estimate it contains between 1.1 and 1.9 million asteroids larger than 0.6 miles (1 km), along with millions of smaller ones. Then there's the Kuiper Belt, located beyond Neptune with an estimated 70,000 icy asteroids larger than 60 miles (100 km) across. One of the largest and most famous members is Pluto, also known as a dwarf planet.
Lesser known are the two asteroid swarms associated with Jupiter. Both orbit in Jupiter's path, one group 60° ahead of the planet and the other group 60° behind it at two of the planet's Lagrange points. At these locations, the gravities of Jupiter and the sun balance out to create stable regions in space where small objects like asteroids can loll about relatively undisturbed.
They're called Trojan asteroids and named after Greek and Trojan warriors from Greek mythology. Together, the two camps total more than 7,000 members (as of 2020) with many more waiting to be discovered. About two-thirds gather ahead of Jupiter at the L4 Lagrange point, with the remainder behind the planet at L5.. The leading camp celebrates Greek heroes from the Trojan war including Achilles, Hektor and Agamemnon. The following camp is named for the champions of Troy — Priamus, Troiles, Glaukos and their comrades.
NASA's Lucy mission, which launched on Oct. 16 from Cape Canaveral, will be the first to visit these asteroids. By studying them up close, scientists hope to better understand how the planets formed and why they ended up in their current locations. Lucy's 12-year journey will take it to eight different asteroids — one in the main belt and seven Trojans from both camps. No other space mission has visited so many different destinations.
Trojans come in three basic flavors: C, B and D. P- and D-types are dark red and resemble the distant Kuiper Belt asteroids, while C-types are closer in appearance and composition to the outer asteroids of the main belt. All are believed to be rich in carbon compounds. Water and other ices may also lurk beneath layers of insulating dust and rock.
The Lucy mission is named after the fossilized skeleton of an early hominin — a pre-human ancestor dated to about 3.2 million years ago — discovered in Ethiopia in 1974 and named “Lucy” by the discovery team. The Lucy mission's goal is to deepen our understanding of the formation of the solar system by studying its "fossils" (the asteroids) the same way Lucy's fossilized remains shed light on the origins of humankind.
After gravity assists from two Earth flybys, Lucy will fly by the inner main-belt asteroid 52246 Donaldjohanson (named after the discoverer of Lucy's remains). In 2027, it will arrive at the L4 cloud and fly by four Trojans, then return to Earth for another assist and fly back out to the L5 camp, where it will visit the binary asteroid 617 Patroclus and its orbiting asteroid-let Menoetius in 2033.
Every planet has five Lagrange points along its orbit, some more stable than others. At Earth, they make great places to park orbiting observatories. A satellite or telescope launched there will stay put so long as routine course corrections are made over its lifetime. On December 18th, NASA will launch the behemoth James Webb Space Telescope, with its 21-foot-diameter (6.5-m) mirror, to the planet's L2 Lagrange point 1 million miles (1.5 million km) away on the opposite side of the planet from the sun. From this distant, stable platform it will plumb the depths of the universe.
"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.