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“Divorced” asteroids go their separate ways

Artist's rendering of an asteroid pair

Artist's rendering of an asteroid pair during its short-lived proto-binary phase, which might last less than 1 million years. Shortly after its splitting, the smaller asteroid will "make a break for it", resulting in two asteroids that share the same orbit around the Sun.

  • If an asteroid rotates fast enough it can split in two
  • The two parts hang around each other as a pair
  • But if one part is small enough, it can drift away on its own

While the common perception of asteroids is that they are giant rocks lumbering in slow orbits around the Sun, a new study shows they actually are constantly changing “little worlds” that can give birth to smaller asteroids that split off to start their own lives.

Astronomers have long known that small asteroids get “spun up” to fast rotation rates by sunlight falling on them, much like propellers in the wind.

The new research shows that when asteroids spin fast enough, they can undergo “rotational fission,” splitting into two pieces which then begin orbiting each other. Such “binary asteroids” are fairly common throughout the Solar System.

The new study, led by Petr Pravec of the Astronomical Institute in the Czech Republic and involving 16 other institutions around the world, shows that many of these binary asteroids do not remain bound to each other but go their separate ways, forming two asteroids in orbit around the Sun when there previously was just one.

The researchers studied 35 so-called “asteroid pairs,” separate asteroids in orbit around the Sun that have come close to each other at some point in the past million years—usually within a few kilometres—at very low relative speeds. They measured the relative brightness of each asteroid pair, which correlates to its size, and determined the spin rates of the asteroid pairs using a technique known as photometry.

“It was clear to us then that just computing orbits of the paired asteroids was not sufficient to understand their origin,” said Pravec. “We had to study the properties of the bodies.”

Time to split

The team showed that all of the asteroid pairs in the study had a specific relationship between the larger and smaller members, with the smallest one always less than 60 percent of the size of its larger companion.

Illustration of an asteroid splitting in two

Over tens of millions of years, a rubble-pile asteroid less than 10 kilometres in diameter can spin up to a speed that will make it split in two. If the smaller one is less than 60 percent the size of the larger asteroid, they will gently separate from each other and eventually become a "divorced" asteroid pair.

The measurement fits precisely with a theory developed in 2007 by study co-author and University of Colorado at Boulder aerospace engineering sciences Professor Daniel Scheeres.

Scheeres’ theory predicts that if an asteroid splits in two by rotational fission, the pair can only escape from each other if the smaller one is less than 60 percent of the size of the larger asteroid.

When one of the asteroids in the pair is small enough, it can “make a break for it” and escape the orbital dance, essentially moving away to start its own “asteroid family,” he said. During rotational fission, the asteroids separate gently from each other at relatively low velocities.

“This is perhaps the clearest observational evidence that asteroids aren’t just large rocks in orbit about the Sun that keep the same shape over time,” said Scheeres. “Instead, they are little worlds that may be constantly changing as they grow older, sometimes giving birth to smaller asteroids that then start their own life in orbit around the Sun.”

Asteroid Gaspra

There are probably 1 million asteroids larger than 1 kilometre in diameter.

From vermin to endangered species

The asteroids that populate the Solar System are primarily concentrated in the main asteroid belt between Mars and Jupiter some 320 million kilometres from the Sun, but extend all the way down into the inner Solar System, where they are known as near-Earth asteroids.

There are probably about a million asteroids larger than 1 kilometre in diameter. Last month, researchers using NASA’s WISE spacecraft announced the discovery of 25,000 never-before-seen asteroids in data collected over just six months.

Astronomers think most asteroids are not solid “mountains”, but rather piles of rock that come in shapes ranging from snowmen and dog bones to potatoes and bananas, with each asteroid essentially glued together by gravitational forces.

“Sunlight striking an asteroid less than 10 kilometres across can change its rotation over millions of years, a slow motion version of how a windmill reacts to the wind,” said Scheeres, who has studied asteroids for the past decade.

“This causes the smaller asteroid to rotate more rapidly until it can undergo rotational fission. It’s not hard for these asteroid pairs to be pushed over the edge.”

CU-Boulder doctoral student Seth Jacobson of CU-Boulder’s astrophysical and planetary sciences department, a co-author on the Nature paper, said the most surprising part of the study was showing that sunlight played the key role in “birthing” asteroids.

“There was a time when most astronomers referred to asteroids as vermin,” said CU-Boulder doctoral student Seth Jacobson. “But the more we learn about them, the more exciting they are. They are not just big chunks of rock, but have the dynamic ability to evolve.”

Adapted from information issued by University of Colorado at Boulder / Pravec, et al. / ESO / M. Kornmesser / L. Calçada.

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