Didymos spins so fast that rocks break off at the equator and go into orbit

The asteroid Didymus spits rocks into space.

Last fall, when NASA’s DART mission impacted Didymos’ moon Dimorphos in a dramatic (and successful) attempt to change the object’s orbit, DART took a quick look at the Didymos system before intentionally smashing the probe.

Besides demonstrating the ability to prevent the future asteroid On Earth, DART has also collected new information about the dynamics of the asteroid pair. The data collected indicates that Didymos is actively dumping material into space, and there are likely millions of other materials. small asteroids It does the same thing across the solar system, all the time.

The popular image of an asteroid as a rocky, solid, unchanging envelope has evaporated in recent years, as we have come to learn more about these objects. While some asteroids fit this classification, many do not. Asteroids are the relics left over from the formation of the solar system, many of which are little more than piles of loose rubble, held together tenuously by gravity.

The asteroid Bennu, which was visited by NASA’s OSIRIS-REx mission in 2020, is a prime example of this. When Osiris-Rex descended to take a sample, it sank nearly two meters into the loose surface like a child in a ball pit. The spacecraft also unexpectedly imaged material being ejected from the asteroid into space, indicating that these objects are more energetic and dynamic than previously thought.

Didymus has been under scrutiny for a while now 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 about its shape, mass, and spin.

One thing they learned is that it spins, very quickly, completing one complete revolution every 2 hours and 16 minutes. At those speeds, Didymos is an asteroid “on the brink of settling,” according to a recent preprint on ArXiv.

At the equator, where the effects of rotation are strongest, rocks and dust are able to lift the surface, levitate, or go into orbit.

“Massive particles are likely to hover for some time, land on the surface and launch again, repeating such cycles over and over again, or land only at latitudes from which no more can be released,” the authors wrote.

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

Interestingly, large objects tend to stay afloat longer than small ones. This is because on the asteroid’s day side, solar radiation pressure would quickly push the tiny grains to the surface.

These conclusions are somewhat preliminary, as they are based on the best estimates of the asteroid’s size, composition, and shape, which the HERA mission should be able to prove when it arrives in 2027. But the principle in action applies across Solar System: If the Earth were spinning fast enough (once every 84 minutes), it would be possible to jump from the equator into orbit in the same way that these rocks lift fast-spinning asteroids like Didymos.

The extremely fast rotation of Didymus – and other asteroids like it – is a solar-powered phenomenon.

These asteroids are subject to a YORP effect, in which the sun heats different parts of an asteroid to different temperatures depending on its albedo. This heat is subsequently radiated away, creating thrust. It’s a tiny effect, but it builds up over time and can eventually push an asteroid around faster and faster like winds turning a windmill.

Astronomers have even seen asteroids tear themselves to pieces through a YORP impact, like asteroid P/2013 R3 in 2013.

Didymos is unlikely to see such a quick and unplanned breakup anytime soon. 97% of particles lifted off the surface fall back within five hours. But it’s something mission planners may have to take into consideration spacecraft in the future approaching fast-spinning asteroids, if they want to avoid harming the probe.

The study has been published on arXiv Prepress server.