The moon isn't the only hunk of space rock that has been travelling around the sun with the Earth for ages.
Canadian scientists have discovered that the Earth is also accompanied by a "Trojan" companion — an asteroid that travels a constant distance ahead of it at all times, sharing nearly the same orbit around the sun.
Similar objects have been found travelling with other planets in our solar system, including Mars, Jupiter and Neptune.
Trojan asteroids were first predicted to exist in 1772 by Italian-French astronomer and mathematician Joseph-Louis Lagrange. He recognized that there are certain points where the gravitational pull of two objects such as the Earth and the sun on a third object such as an asteroid balance each other off, allowing the object to remain, on average, at a fixed distance from the other two masses.
Those points are now called Lagrange points, and the two where Trojans can be found are referred to as Trojan points. Trojan asteroids aren't necessarly found precisely at the Trojan point, but may instead orbit them so that the Trojan point represents their average position.
The first Trojan was observed orbiting with Jupiter in 1906.
Objects in the solar system are traditionally given names from classical Roman and Greek mythology, and the first Trojan asteroid was named Achilles, said Martin Connors. Because of his role in the Trojan war, similar asteroids have been called Trojans ever since.
"In a way, these are sort of like secondary moons," said Martin Connors, the Alberta scientist who led the discovery published in Thursday's issue of Nature.
Like the moon, the asteroid known as 2010 TK7 is under the control of Earth's gravity and has been orbiting stably with the Earth for at least 10,000 years. However, Trojans are not satellites — unlike the moon, the Trojan asteroid does not orbit the Earth. Viewed from the Earth, the asteroid sits 60 degrees from the sun.
"You would think that the gravity of the planet would just like to pull the asteroid in," said Connors. "But when it does do that, it moves closer to the sun, and then the sun's gravity makes it go faster, and it pulls away from the planet again."
Gravity balancing act
Along the Earth's orbit, it is only when an object such as the asteroid sits at one of two Trojan points at a 60-degree angle from the Earth that the Earth's and the sun's gravity balance each other off, allowing the object to remain nearly stationary relative to the Earth.
2010 TK7 is about 300 metres across, making it rather large compared to other nearby asteroids. According to NASA, it is now about 80 million kilometres from Earth. By comparison, the moon is on average 384,000 km away.
Connors, who holds a Canada Research Chair in space science at Athabasca University, has been searching the skies for nearby Trojans for about 15 years.
It isn't easy because the area being searched is 60 degrees, or very close to the sun when viewed from Earth. That means the objects are only visible for a short time just before sunrise or just after sunset.
Connors combed through data collected by researchers at the University of California in Los Angeles from NASA's WISE (wide-field infrared survey explorer) telescope. It orbits the Earth and detects asteroids by scanning for the infrared radiation they give off after warming up in the sun. That radiation doesn't make it through the atmosphere, so it is invisible from Earth.
After finding two asteroids that looked like they could be Trojans, Connors sent the data to Paul Wiegert at the University of Western Ontario, who used computer modelling to see how similar objects should behave. That made it clear that the researchers would need more observations of the asteroid to confirm it was a Trojan.
They called upon Christian Veillet, executive director of the Canada-France-Hawaii Telescope in Kamuela, Hawaii, to use the telescope to scrutinize the asteroid's orbit for six days this past April. The observations confirmed that the asteroid is a Trojan.
Connors said the discovery of one Trojan orbiting with the Earth means there are likely others. Such nearby asteroids wouldn't require much energy to visit with a space probe, since they are so close and have a similar orbit to earth, he said. However, 2010 TK7 itself isn't a good candidate for such a visit because its orbit is tilted relative to the Earth's.
Trojan asteroids are of interest to scientists because they may hold material leftover from the formation of the solar system, Connors said. While meteorites also contain such material, they typically have wandered long distances and scientists can never be quite sure where they came from. Trojan asteroids likely have remained close to the Earth for a long time, and the material they contain is more likely to originate near the Earth.