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.”
What we do know is that DJ was once part of a much larger asteroid that suffered a giant impact 155 million years ago, causing the parent body to break apart into a number of chunks, the largest being the 45-mile-wide (73-kilometer-wide) asteroid 163 Erigone. Consequently, the remains of this parent asteroid, including DJ, are collectively referred to as the Erigone family.
DJ’s violent origin may also explain its shape, which features two lobes joined together by a narrower and relatively smooth neck.
“We have now seen many small bodies in the solar system that appear to have this bi-lobed shape, and that’s across a wide range of sizes,” said Marchi.
For example the near-Earth asteroids 25142 Itokawa, which was visited by the first Japanese Hayabusa mission in 2005, and 4149 Toutatis that was encountered by China’s Chang’e 2 in 2012, are both bi-lobed. So too is the tiny asteroid Selam, which is a satellite of the asteroid 152830 Dinkinesh visited by Lucy in 2023. Then there are cometary bodies including 67P Churyumov–Gerasimenko visited by the Rosetta mission and comet 19P/Borrelly, imaged by NASA’s Deep Space 1 spacecraft in 1999.
These objects are all different sizes, different types and in different locations, but they all share the same structure. However, Marchi cautions that they might not all form the same way. For example, the neck between the lobes of comets such as 67P might form through erosion via sublimation and outgassing as the comet gets closer to the Sun, whereas for asteroids it might indicate a history of being involved in a giant impact, the resulting fragments coming together to be bound by gravity – a so-called contact binary.
Lucy now continues onwards, scheduled to encounter its first trojan asteroid, known as 3548 Eurybates, in August 2027. Trojans are asteroids that have been captured by Jupiter’s gravity at the L3 and L4 Lagrange points, 60 degrees in front and 60 degrees behind Jupiter itself.
“We think that the Trojans, based on our understanding of the solar system, formed further out and then were captured where they are today following the early shuffling of the planets,” said Marchi. “This shuffling could also have been the origin of DJ, so there could be a connection there between DJ and the Trojans.”
Compositionally, the majority of trojans are expected to be even more primitive than DJ, containing more carbon, water and other volatile materials that would sublimate if they got too close to the sun.
That is all except one: Eurybates.
“It is the only one of our targets that from spectroscopy appears to be relatively similar to DJ. It’s not identical, but it is closer in composition to DJ than the other Trojans, so it will be intriguing to compare them,” said Marchi.
Indeed, any similarities will help tell us how asteroids were herded around during the first few hundred million years of solar system history following the formation of the planets. Jupiter and Saturn, especially, began migrating inwards and then out again. In doing so, their gravity pushed and pulled minor bodies all over the place, as did the gravity of Uranus and particularly Neptune as they edged outwards. These migrations led to the formation of the asteroid belt and the Kuiper belt, and ejected trillions of bodies into the wide orbits of the Oort Cloud.
“The key question is, if DJ has been relocated in the inner asteroid belt, then how many other asteroids came along with it and ended up being closer to the Earth, where they could have delivered some water, some organics and other things to our planet?” asked Marchi.
Lucy will visit six of Jupiter’s trojans, which in total number over 15,300 discovered so far. Far from lumps of rock, the trojans, along with DJ and Dinkinesh (which is the Ethiopian name for the Lucy fossil), are windows into the past, and the storytellers of the Earth’s most ancient history.
Lucy’s findings from Donaldjohanson were published on Thursday (June 18) in the journal Science.
