Although the asteroid temporarily called 2010 TK7 was discovered late in 2010, we now learn in the latest issue of Nature that this object is our planet’s first known Trojan asteroid. The term refers to objects that orbit around one of the two Lagrangian points L4 and L5 — these are found 60° ahead of and behind the larger body. Trojans come in various sizes. The Saturn system actually has Trojan moons (Telesto and Calypso, which accompany Tethys, and Helene and Polydeuces, which move in orbital configuration with Dione). Jupiter, Neptune, Mars and now the Earth have all been found to have Trojan asteroids associated with them.

As the paper on this work points out, the viewing geometry poses problems for discovering Trojan asteroids moving with our planet, although we have found unusual objects like the ‘horseshoe orbiters’ 2010 SO16 and 3753 Cruithne. What made the current discovery possible was the Wide-field Infrared Survey Explorer (WISE) satellite, which searched large areas of the sky 90º from the Sun with a high level of astrometric precision (2010 SO16 was identified through the same data). Follow-up study with the Canada-France-Hawaii Telescope in 2011 allowed the orbit of 2010 TK7 to be determined, verifying its status as an Earth Trojan.

It’s worth noting that the NEOWISE component of the WISE mission focused on near-Earth objects like asteroids and comets, observing more than 155,000 main belt asteroids and tracking more than 500 NEOs, including 132 that were being seen for the first time. Trojans are still difficult to spot, but 2010 TK7 fell within the parameters of the WISE coverage area:

“These asteroids dwell mostly in the daylight, making them very hard to see,” said Martin Connors of Athabasca University in Canada, lead author of the paper on the discovery. “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. WISE was a game-changer, giving us a point of view difficult to have at Earth’s surface.”

As you can see in the image below, 2010 TK7 follows a complex path that moves it sometimes closer and sometimes farther from the Earth, with the asteroid preceding the Earth at all times as both move around the Sun.

Image: The complete path of asteroid 2010 TK7 (green) during the course of one of its 195-year cycles. The asteroid remains in front of the Earth as they both orbit the Sun. The asteroid is the white sphere, the Sun the yellow sphere, and the dark blue dots trace out the Earth’s orbit. Credit: Paul Wiegert, University of Western Ontario, Canada.

Could a Trojan like this become a tempting target for a space mission? The paper notes the problem: 2010 TK7 is in a highly inclined orbit, traveling far above and below the plane of the Earth’s orbit. The delta-v required is 9.4 km/s. This contrasts with values of below 4 km/s for other near-Earth asteroids, making other objects more likely candidates. In any case, we still have much to learn about this Trojan. Its absolute magnitude allows an estimate of 300 meters for its diameter, making it relatively large among the near-Earth asteroid population. It’s presently about 80 million kilometers from the Earth, and we also learn that no spectral or color information is yet available to tell us whether it is unusual in any way that might justify a mission.

The animation below illustrates the orbit of 2010 TK7.

Image: The movie follows Earth as it travels along its orbit (blue dots) around the sun, so Earth remains at the front of our view. The various objects are not drawn to scale. Asteroid 2010 TK7 has an extreme orbit that takes the asteroid far above and below the plane of Earth’s orbit. The motion above and below the plane is referred to as an epicycle. In addition, the asteroid moves within the plane of Earth’s orbit in what is called libration, circling horizontally around its stable point every 395 years. The clock at upper left shows how the orbit changes over time. The asteroid’s orbit is well understood — over the next ten thousand years, 2010 TK7 will not approach Earth any closer than 20 million kilometers (12.4 million miles), which is more than 50 times the distance from Earth to the moon. Credit: Paul Wiegert, University of Western Ontario, Canada.

The paper is Connors et al., “Earth’s Trojan Asteroid,” Nature 475 (28 July 2011), pp. 481-483 (abstract). See also the discoverers’ article Earth’s First Trojan Asteroid: 2010 TK7.