We are not alone

Trojan asteroid 2010 TK7

Trojan asteroid 2010 TK7 (circled in green) orbits the Sun ahead of the Earth. This single frame was taken by NASA's Wide-field Infrared Survey Explorer, or WISE. The majority of the other dots are stars or galaxies far beyond our Solar System.

  • Trojan asteroid found orbiting ahead of Earth
  • 80 million kilometres from our planet
  • No danger of it colliding with us

ASTRONOMERS STUDYING OBSERVATIONS taken by NASA’s Wide-field Infrared Survey Explorer (WISE) mission have discovered the first known ‘Trojan’ asteroid orbiting the Sun along with Earth.

Trojans are asteroids that share an orbit with a planet near stable points in front of or behind the planet. Because they constantly lead or follow in the same orbit as the planet, they never can collide with it.

In our Solar System, Trojans also share orbits with Neptune, Mars and Jupiter. Two of Saturn’s moons even have Trojans.

Hard to see

Scientists had predicted Earth should have Trojans, but they have been difficult to find because they are relatively small and appear near the Sun from Earth’s point of view.

“These asteroids dwell mostly in the daylight, making them very hard to see,” said Martin Connors of Athabasca University in Canada, lead author of a new paper on the discovery in the July 28 issue of the journal Nature.

“But we finally found one, because the object has an unusual orbit that takes it farther away from the Sun than what is typical for Trojans,” added Connors. “WISE was a game-changer, giving us a point of view difficult to have at Earth’s surface.”

Artist's impression of the WISE space telescope

Artist's impression of the WISE space telescope

Thousands of asteroids found

The WISE telescope scanned the entire sky in infrared light from January 2010 to February 2011.

Connors and his team began their search for an Earth Trojan using data from NEOWISE, an extension to the WISE mission that focused in part on near-Earth objects, or NEOs, such as asteroids and comets.

NEOs are bodies that pass within 45 million kilometres of Earth’s path around the Sun.

The NEOWISE project saw more than 155,000 asteroids in the main belt between Mars and Jupiter, and more than 500 NEOs, including 132 that were previously unknown.

Unusual orbit

The team’s hunt resulted in two Trojan candidates. One, called 2010 TK7, was confirmed as an Earth Trojan after follow-up observations with the Canada-France-Hawaii Telescope in Hawaii.

The asteroid is roughly 300 metres in diameter. It has an unusual orbit that traces a complex motion near a stable point in the plane of Earth’s orbit, although the asteroid also moves above and below the plane.

The object is about 80 million kilometres from Earth.

The following video shows how the asteroid continually loops above and below Earth’s orbital plane, while always remaining ahead of our planet:

No danger to Earth

The asteroid’s orbit is well-defined, and for at least the next 100 years it will not come closer to Earth than 24 million kilometres.

“It’s as though Earth is playing follow the leader,” said Amy Mainzer, the principal investigator of NEOWISE at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “Earth always is chasing this asteroid around.”

A handful of other asteroids also have orbits similar to Earth. Such objects could make excellent candidates for future robotic or human exploration.

Asteroid 2010 TK7 is not a good target because it travels too far above and below the plane of Earth’s orbit, which would require large amounts of fuel to reach it.

More information: WISE mission

Adapted from information issued by NASA / JPL-Caltech / UCLA / Paul Wiegert, University of Western Ontario.

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