The asteroid Donaldjohanson is seen in this depiction.
(Image credit: NASA/Goddard/SwRI/Johns Hopkins APL)
Last year, NASA's Lucy spacecraft encountered a bi-lobed asteroid that is a chunk of an even larger rocky body that was smashed apart in an almighty collision 155 million years ago. This little pitstop happened on Lucy's way to a rendezvous with the Trojan asteroids that shadow Jupiter around the sun.The asteroid 52246 Donaldjohanson, better known as "DJ" to Lucy's mission scientists and named after the paleoanthropologist who discovered the Lucy hominin fossil in Ethiopia in 1974, orbits the sun in the inner part of the main asteroid belt between Mars and Jupiter.The true Lucy fossil dates back in time 3.2 million years and is an important link in the evolutionary chain that led to homo sapiens. Likewise, primitive asteroidal bodies are somewhat like fossilized remnants of the building blocks of the solar system's planets, including Earth. Understanding the make-up of these asteroids and where they formed versus where they are now can provide crucial insights into how Earth was assembled and where its organic materials and water may have come from.Lucy flew past DJ in April of 2025. It is a pretty primitive asteroid, meaning that it has, or once upon a time had, certain volatile materials such as water-ice, as well as plenty of carbon — all things that can be removed thermally over time. Most objects that contain volatiles originate in the outer solar system, where it is cold enough that the volatiles do not sublimate away.Within DJ's composition, Lucy detected iron-bearing phyllosilicates, which are a mineral formed in the presence of liquid water."Phyllosilicates are an indication that water was present and there was some degree of aqueous alteration," Simone Marchi, a planetary scientist from the South-west Research Institute and lead of the study into DJ, told Space.com.However, for DJ to have had water, it must have formed further out from the sun, possibly in the outer asteroid belt."But DJ belongs to the inner asteroid belt so that's already intriguing," said Marchi.The spectral evidence also indicates DJ was only partially altered by water, which Marchi says tells us something about its history."The aqueous alteration terminated early, and though we don't know why, we can speculate. In order to have aqueous alteration there needs to be some internal heating [usually via radioactive elements] and if something forms later than everything else then there will be less heat [since many of the radioactive elements will have already decayed]. Or perhaps there was just less water to start with where it formed."
