Didymos spins so fast that rocks break off at its equator and swing into orbit

Asteroid Didymos spits rocks into space.

When NASA’s DART mission slammed into Didymos’ moon Dimorphos last fall in a dramatic (and successful) attempt to alter the object’s orbit, DART got a glimpse of the Didymos system before the probe intentionally ripped into pieces became.

In addition to demonstrating the ability to prevent future asteroid impacts on Earth, DART also gathered new information about the dynamics of the asteroid pair. The data collected suggests that Didymos is actively hurling material into space, and there are likely millions of other small asteroids doing this all the time throughout the solar system.

The popular image of an asteroid as an unchanging, solid boulder has dissipated in recent years as we have learned more about these objects. While some asteroids fit this classification, just as many do not. Asteroids are debris left over from the formation of the solar system, and many of them are little more than loose piles of debris held weakly together by gravity.

Asteroid Bennu, visited by NASA’s OSIRIS-REx mission in 2020, is a prime example. When OSIRIS-REx touched down to collect a sample, it sank almost two meters into the loose surface like a child in a ball pit. The spacecraft also unexpectedly photographed material being expelled into space by the asteroid, suggesting these objects are more active and dynamic than previously thought.

Didymos has been studied for some time in preparation for DART and the European Space Agency’s follow-up mission Hera. Now that DART has seen the asteroid up close, researchers have a wealth of data on its shape, mass and rotation.

One thing they learned is that it spins, and quite quickly, completing a full rotation every 2 hours and 16 minutes. At those speeds, Didymos is an asteroid “on the verge of stability,” according to a preprint recently published on ArXiv.

At the equator, where the effects of spin are strongest, rocks and dust can lift off the surface, levitate, or move into orbit.

“Massive particles may float for some time, land on the surface and lift off again, repeat such cycles over and over again, or simply land at latitudes from which further liftoff is not possible,” the authors write.

Some of the floating rocks reach orbit, and some of these are likely to be deposited on the moon Dimorphos. Smaller particles can even escape the system and be blown away by the solar wind forever.

Interestingly, large objects tend to stay afloat longer than small ones. This is because on the day side of the asteroid, the pressure of solar radiation quickly pushes the smaller grains back to the surface.

These conclusions are somewhat preliminary, as they are based on best estimates of the asteroid’s size, composition and shape, which the Hera mission should be able to confirm upon arrival in 2027. But the principle that works holds true across the solar system: If the Earth spun fast enough (once every 84 minutes), it would be possible to jump from the equator into orbit the way these rocks take off fast-spinning asteroids like Didymos.

The excessively fast rotation of Didymos – and other similar asteroids – is a solar-powered phenomenon.

These asteroids are under the influence of the YORP effect, in which the Sun heats different parts of an asteroid to different temperatures depending on the albedo. This heat is later radiated and creates thrust. It’s a tiny effect, but it builds up over time and can eventually push an asteroid around faster and faster like the wind turns a windmill.

Astronomers have even seen asteroids tear themselves apart through the YORP effect, like asteroid P/2013 R3 in 2013.

Didymos is unlikely to see such a dramatic rapid unscheduled dismantling any time soon. 97% of particles that take off from the surface land again within five hours. But it’s something mission planners may need to consider for future spacecraft that come close to fast-spinning asteroids if they want to avoid damage to the probe.

The study appears on the arXiv preprint server.