Asteroid shares Earth's orbit shows USU-built telescope

By John Hollenhorst
KSL News
July 29, 2011

LOGAN — The discovery of an asteroid that's hitching a ride on Earth's orbit is drawing international attention and is a point of pride in Cache County.

Images of Earth's first known Trojan asteroid were captured by the Wide-field Infrared Survey Explorer space telescope that was designed and built for NASA at Utah State University.

A Trojan asteroid is one that shares an orbit with a larger planet or moon at a stable point, due to the combined gravitational forces of, in this case, the Earth and the Sun. Scientists long suspected Earth has one or more of them, but this is the first that's been confirmed.

"There's a whole family of Trojan asteroids in the Jupiter orbit and I think they found them in a couple other planets," said John Elwell, WISE program manager at USU's Space Dynamics Laboratory. "Matter of fact, there's actually Trojan asteroids around moons of Saturn."

Although our planet is vulnerable to giant rocks plowing through our neighborhood, scientists say this one poses no threat.

The small green circle on the lower right of this photo shows Earth Trojan asteroid 2010TK7's position.

"Asteroids in a Trojan orbit are some of the ones you don't have to worry about because they're in a very stable orbit," Elwell said, "60 million miles away from the earth." The asteroid is believed to be several hundred yards in diameter.

Regular Earth-based telescopes couldn't see the Trojan asteroid because it's hiding in broad daylight. It's too close to the angle of the Sun, as seen from Earth. "The telescopes tend to get blinded by the sun," Elwell said.

WISE, though, uses infrared light instead of visible light. It was designed to map the entire universe from WISE's own orbit around the Earth. In doing so, the infrared cameras on WISE captured images of a very dim star-like object. Now, Canadian scientists have determined it's a Trojan. Their finding is featured this week on the cover of the prestigious journal Nature.

The Trojan actually follows Earth's orbit like a drunk driver wandering over the center line. As it travels along the Earth's orbital path ahead of our planet, it traces a gigantic corkscrew or spiral pattern.

"The only word I can use to describe that orbit is 'bizarre,'" Elwell said. "It's hard to imagine in your mind how an object could orbit around the sun in a spiral like that."

So why do they call it a Trojan asteroid? It is not because Earth let the enemy through the gates, like the Trojan horse. It's because when scientists started finding them near other planets a century ago, they began naming them for characters in the Trojan War.

The first was named Achilles. All of them are generically known as Trojans.