Miércoles 25 de Julio de 2007, Ip nº 200

Tiny asteroid 'groupie' found trailing after Mars
Por David Shiga

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.

Mars is one of just three planets known to have such "Trojan" objects in its orbit. About 2200 are known to accompany Jupiter in its orbit, and a handful have been discovered in Neptune's orbit as well.

Jupiter and Neptune may have collected their Trojans about 3.8 billion years ago, at a time when the orbits of these planets were shifting and their gravity was flinging vast numbers of comet-like objects around the solar system (see The solar system, but not as we know it).

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, says Trojan researcher Alessandro Morbidelli of the Observatoire de la Côte d'Azur in Nice, France.

Temporary companions
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 gravity.

"They did not move, but the planet did," Morbidelli told New Scientist.

After the discovery of 2007 NS2, astronomers found the asteroid in old images from the LONEOS and LINEAR near-Earth object surveys dating back to 1998. This has allowed researchers to calculate a more precise orbit for the object.

Calculations by Aldo Vitagliano of the Universita di Napoli Federico II in Naples, Italy, show the asteroid is stable at Mars's L5 point. The newly found space rock has now been added to the list of Mars Trojans on the website of the Minor Planet Center in Cambridge, Massachusetts, US.

Earth 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.

If some wayward objects from the asteroid belt did become trapped as Trojans around Earth, the objects would not be easy to spot, Vitagliano says.

Their position relative to Earth means they would not reflect much light towards us, making them relatively dim. And because of their position, they would appear fairly close to the Sun, which means they would only be visible above the horizon for a short period of time at night.

  23/07/2007. New Scientist Magazine.