Asteroid Found Locked Into Earth's Orbit: Meet Our Traveling Companion

By: Jenny Marder

The object, which for now is dubbed 2010 TK7, is a “Trojan” asteroid, meaning that it is held in a delicate balance between a gravitational tug from the sun and an equal tug from the Earth.

It’s long been known that Jupiter, Neptune and Mars have Trojans orbiting alongside them, but this is the first time one has been found alongside our planet.

“An image of dancers is helpful,” said Paul Wiegert, a professor of physics and astronomy at the University of Western Ontario and an author of the study, published online Wednesday in the journal, Nature. “The Earth goes around the sun in a circle, and the asteroid dances in front of it, getting closer and farther away as it orbits.”

TK7 is about 90 degrees from the Earth right now, and its distance from Earth ranges from 49.7 million miles to 12.4 million miles. The composition and surface characteristics of TK7 are unclear, though it is believed to be several hundred feet in diameter — “on the largish side” for a near-Earth asteroid, Wiegert said.

Because it typically remains on the sunward side of the Earth, venturing only occasionally into the night sky, it has been difficult to detect until now. Trojan asteroids commonly dwell on the daylight sides of their planet. And its existence indicates that there are probably others, even less accessible by telescope.

“It is very hard to survey the entire area that could contain these objects,” said Timothy Spahr, director of the NASA-funded Minor Planet Center at the Smithsonian Astrophysical Laboratory. “So when we find one object, it’s likely that we didn’t find the only one.”

Trojans are found in areas called Lagrange triangular points, where the gravitational pull from the host planet and star — in this case, Earth and the sun — balance out. This Trojan was found at L4, which is 60 degrees ahead of the Earth.

It has been predicted for a while that such objects existed, said Spahr, who was not involved in the research. “I doubt that there’s anything different about this than any other near-Earth asteroid, other than that it’s locked in orbit around the Earth. But don’t get me wrong, I think this is totally cool.”

Scientists first glimpsed TK7 in October 2010 with NASA’s Wide-field Infrared Survey Explorer (WISE) satellite. The team confirmed the object as a Trojan asteroid using optical images from the Canada-France-Hawaii Telescope in April. Subsequent computer modeling indicates that it will “happily stay in its orbit for at least 10,000 years,” Wiegert said.

The authors found the initial signs of the asteroid from a public data set. “I love to think of a data set being pored over like that,” Spahr said.

Mike Brown, a planetary astronomer at California Institute of Technology, called the finding a “great technical achievement”, but said it will be hard to understand where the asteroid came from and the implications it has for better understanding Earth and the solar system.

“The object is indisputably there,” he said. “The only weakness is not a weakness of the study, it’s of reality. We don’t know what it means.”

The Trojan could be a chunk left over from the event that caused the formation of the moon, he said. The prevailing theory is that the moon was formed when a Mars-sized body hit the Earth, flinging material outward.

“That could have blasted other pieces that got stuck in these Lagrangian points,” he added. “But if I had to guess, I would say it’s just a little asteroid that got temporarily trapped in that location, and if you came back in a million years, it would be gone.”

The first Trojan asteroids were discovered around Jupiter, and named after ancient Greek and Trojan heroes: 588 Achilles, 659 Nestor, 911 Agamemnon and 1143 Odysseus are examples.

Such findings could tell us more about the risk of impacts from asteroids near Earth, Wiegert said: “This particular asteroid is one of the safest of asteroids that orbit in the vicinity of the Earth. It’s not dangerous. But it tells us about asteroids that live near the Earth, and it tells us about the risks of impacts.”

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