NASA-Goddard / SwRI / Johns Hopkins APL
On its technique to future encounters with Trojan asteroids, the Lucy spacecraft made a apply run previous tiny asteroid 52246 Donaldjohanson on April 20, 2025. Now, the Lucy workforce, led by Simone Marchi (Southwest Analysis Institute), printed the findings from the temporary encounter in Science, detailing the eventful historical past of the little binary world and proving the potential of Lucy’s devices to collect the info wanted to grasp its future tiny targets.
The primary and most evident conclusion from Lucy’s photographs of Donaldjohanson is that it’s made from two chunks caught collectively at a slender neck. Mapping craters throughout each lobes has made it clear that the pair has been united for a very long time. Each lobes are saturated with craters, to the purpose that any new crater would obliterate older ones. Primarily based on well-understood statistics concerning the sizes and frequencies of asteroid impacts in the principle belt, the Lucy workforce calculated that the floor is at the very least 40 million years outdated. That signifies that the occasion that created the fragments that got here collectively to kind Donaldjohanson occurred greater than 40 million years in the past.
“Greater than 40 million years outdated” isn’t a very particular age, however it’s at the very least per the thought (based mostly on the asteroid’s orbital path) that Donaldjohanson is a member of the Erigone asteroid household. The household consists of fragments from a dad or mum physique that was destroyed in a catastrophic collision round 155 million years in the past. Lucy’s spectroscopic measurements, which recommend the presence of iron-rich clay minerals, additionally assist the Erigone connection.
The neck that joins the lobes, nevertheless, is youthful than that. Its floor is far smoother and bears far fewer craters. The Lucy workforce estimates it’s lower than 20 million years outdated. How can the 2 lobes be the identical age whereas their connection is youthful?
The Lucy workforce discovered a clue in Donaldjohanson’s rotation interval, measured over a interval of two months earlier than the flyby. Lucy confirmed ground-based estimations of a really sluggish rotation price of 252.6 hours — but additionally within the measurements is a second rotation interval of 455.2 hours. The asteroid is tumbling – albeit very slowly – in a state referred to as “non-principal axis rotation.”
Such a sluggish and chaotic rotation state isn’t authentic; it occurred on account of forces exerted on the little asteroid by daylight’s stress and the uneven absorption and re-emission of the Solar’s warmth. Contact binaries usually have a lot greater rotation charges, measured in hours (generally, minutes), not days or even weeks. At these quicker charges, centrifugal acceleration acts to counter self-gravity, pushing materials towards the ends of the elongated asteroid.
However as Donaldjohanson’s rotation slowed to its present price, the centrifugal impact turned negligible. Self-gravity took over, pulling materials towards the middle of mass. For the reason that heart of mass is contained in the bigger lobe, free materials on the smaller lobe will, if perturbed, slide towards the bigger lobe. Certainly, the very clean floor on the smaller lobe within the neck space going through the bigger lobe is an indication of such slope failure. Each time a large influence rattles Donaldjohanson, extra materials flows from the small lobe towards the larger one, down that clean slope.
The Lucy workforce used the form and measurement of Donaldjohanson, as measured from the flyby photographs, to develop a mannequin of its form. Then, assuming a relentless density, they calculated the acceleration resulting from gravity throughout its floor to measure the orientation and steepness of its slopes beneath totally different rotation charges.
If Donaldjohanson rotated as soon as each 5 hours, materials on the floor would spin out from the big lobe towards the small lobe. There’s no proof on the floor of the asteroid of that ever taking place, however then once more, any occasion older than 40 million years has been obliterated by the cumulative results of influence cratering. A rotation price of as soon as each 10 hours is the candy spot the place centrifugal and gravitational accelerations would steadiness within the neck area. Slower than that, and materials cascades towards the bigger lobe.
So the historical past of Donaldjohanson might be like this: 155 million years in the past, a giant smash in the principle asteroid belt destroyed the dad or mum of the Erigone household. Two kilometer-sized fragments, touring in the identical route, pulled one another in to kind a contact binary physique spinning as soon as each few hours. Over time, smaller asteroids smacked into the binary, digging craters and spalling off fragments of ejecta, a few of which re-landed on the floor. The asteroid developed a deep layer of regolith over its pair of cohesive cores, and now each new crater destroys an older one, churning the floor however not rising the whole variety of craters.
On the similar time, daylight and warmth acted to sluggish its rotation. Sooner or later in its historical past, the rotation price interval slowed to longer than 10 hours, at which level gravity overcame spin and previously steady slopes within the neck area failed. Landslides fell from the oversteepened slopes on the small lobe, sliding regolith towards the big one. Because the rotation interval continued to sluggish and additional impacts shook the asteroid, the slope failure continued, constructing the graceful floor and thick neck that we see immediately.
It’s neat that we will study a lot story a few world from such a fast flyby. In fact, NASA and the Lucy workforce knew they might, or they wouldn’t have launched this mission towards a number of flybys of Trojan asteroids. A 12 months from now, Lucy can be approaching (3548) Eurybates-Queta for an much more informative encounter. Eurybates is greater than 10 occasions the dimensions of Donaldjohanson and rotates as soon as in about 9 hours, so Lucy will have the ability to detect it sooner and see all faces of it a number of occasions earlier than its August 2027 shut method, and examine it for months after the flyby because it dwindles within the distance.
