Sun’s light touch explains asteroids flying in formation behind Mars

The power of sunlight appears to be simultaneously creating and destroying families of asteroids, according to a new study of Mars’s Trojans, asteroids that accompany the planet like planes flying in formation. The result, reported yesterday at the American Astronomical Society’s Division for Planetary Sciences meeting in Provo, Utah, solves a minor mystery and could explain the creation of asteroid families in other parts of the solar system.

Mars’s Trojans share the same orbit as the Red Planet but always stay either 60° ahead of or behind it, at the so-called fourth and fifth Lagrange points (L4 and L5). There, the asteroids orbit the planet at the exact same rate as it orbits the sun. As a result, the asteroids’ orbits are stabilized by gravitational interactions with both the sun and the planet. Trojan asteroids are most commonly associated with Jupiter, which has more than 6000. Among the inner planets, only Mars has known Trojans—10 of them, the biggest measuring about 2 kilometers across.

Mars’s Trojans have puzzled astronomers. Rather than being randomly dispersed, nine out of 10 lie in L5, trailing the planet. What’s more, all but one of these trailing Trojans whiz along in very similar orbits, suggesting that they were once bits of the largest member of the group, Eureka. Normally, such a family of asteroids would be attributed to another asteroid colliding with the biggest member. But the orbits of Eureka’s family are so similar that only an incredibly gentle and, hence, unlikely collision could have done the trick, says Apostolos Christou, a research astronomer at Armagh Observatory and Planetarium in Armagh, Northern Ireland.

Now, however, Christou and his colleagues say they have solved the mystery. Instead of emerging from an impact, Eureka’s progeny appear to have formed through a well-known phenomenon called the Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect, in which an asteroid “spins up” to ever faster rotation speeds because of an imbalance of radiation pressures: from sunlight striking its surface and from the infrared light its warm surface radiates back into space. “Eventually there comes a time where all the bits that make up the asteroid cannot stay together and start flying off,” Christou says. “For lack of a better term I call these bits YORPlets.”

But if this happened with Eureka, why don’t Mars’s two other Trojan asteroids also have families? In one case, Christou says, the asteroid appears to be tumbling chaotically, preventing the relatively small forces of the YORP effect from adding up. In the other case, the asteroid may be spinning fast enough to throw off YORPlets, but any would quickly disperse into unrelated orbits because its parent asteroid lies close to the edge of the Trojan stability zone. Their migration out of this zone would be the result of another radiation-pressure effect known as the Yarkovsky effect, which, instead of changing an asteroid’s spin, changes its orbit. “So radiation forces, namely Yarkovsky and YORP, can create, but also evict, asteroids from the martian Trojan clouds,” Christou says.

The new explanation "sounds very reasonable to me,” says Humberto Campins, an asteroid researcher at the University of Central Florida in Orlando, who was not involved in the study. The YORP effect, he adds, may also play a role with near-Earth asteroids, explaining why so many of them come in pairs or groups of three.

Learning more about the martian Trojans might also prove useful for future space exploration, Campins notes. “These could be resources for trips to Mars," he says. “If they have hydrated minerals then you can exploit those for fuel. [They] could useful on the way to Mars—or on the way back.”