A new space rock has been found that devotedly travels around with Mars as it orbits the Sun, bringing the total number of such 'groupies' to four. But astronomers say it was Mars – not its tiny companions – that originally insinuated itself into the rock group billions of years ago.The asteroid, called 2007 NS2, was discovered by astronomers at the La Sagra Observatory in southern Spain on 14 July. Based on its brightness, it is estimated to be about 1 kilometre across.It follows Mars in its orbit, occupying a spot called L5, which lags the Red Planet by 60° as it moves around the Sun. It shares L5 with two other objects, while a fourth object orbits 60° ahead of Mars at a point called L4.Objects that wander into the L4 and L5 points of a planet tend to be confined there by the combined gravity of the planet and the Sun.
How Mars got its Trojans is uncertain, but the Red Planet may have collected them at a much earlier period, just after the dawn of the solar system a little more than 4.5 billion years ago
At that time, an embryonic Mars may have been kicked around the solar system through gravitational interactions with other planetary embryos. Any asteroids that happened to be at the L4 and L5 points of its new orbit would have been trapped there by the Red Planet's gravityEarth has no known Trojans, perhaps because our home planet is too heavy to have been knocked around the same way that Mars was, Morbidelli says. "Mars jumped around because of its small mass, but not the Earth," he says. Mars is just 11% as massive as the Earth.
Although no asteroids are known to occupy Earth's L4 and L5 points, there are a handful of so-called Earth co-orbital asteroids. These objects have corkscrew orbits, slowly looping around Earth, while following its orbital motion around the Sun. This configuration is unstable, so these objects are only temporary companions to Earth.One such object, a 200-metre-wide asteroid called 2004 GU9, has been looping around Earth this way for 500 years, but is expected to eventually drift away.
How Mars got its Trojans is uncertain, but the Red Planet may have collected them at a much earlier period, just after the dawn of the solar system a little more than 4.5 billion years ago
At that time, an embryonic Mars may have been kicked around the solar system through gravitational interactions with other planetary embryos. Any asteroids that happened to be at the L4 and L5 points of its new orbit would have been trapped there by the Red Planet's gravityEarth has no known Trojans, perhaps because our home planet is too heavy to have been knocked around the same way that Mars was, Morbidelli says. "Mars jumped around because of its small mass, but not the Earth," he says. Mars is just 11% as massive as the Earth.
Although no asteroids are known to occupy Earth's L4 and L5 points, there are a handful of so-called Earth co-orbital asteroids. These objects have corkscrew orbits, slowly looping around Earth, while following its orbital motion around the Sun. This configuration is unstable, so these objects are only temporary companions to Earth.One such object, a 200-metre-wide asteroid called 2004 GU9, has been looping around Earth this way for 500 years, but is expected to eventually drift away.
