Earth's companion: asteroid locked in game of leapfrog with planet

The Earth has an asteroid companion traveling in orbit with it around the sun, Nasa astronomers have discovered.

The small asteroid has a slightly tilted orbit, bouncing up and down through Earth’s orbital plane once a year. Over decades, as it scuttles along its path around the solar system, it twists back and forth.

The asteroid has been declared a “quasi-satellite”, too distant to qualify as an actual satellite but cinched by gravity into a relatively close orbit around the sun.

Not too close, though: at its closest the asteroid is 38 times farther away than the moon – an average 238,855 miles. Earth’s gravity is strong enough to keep the asteroid from breaking free entirely, but it at its farthest from the planet it reaches about 100 times that distance.

“In effect, this small asteroid is caught in a little dance with Earth,” Paul Chodas, of Nasa’s Jet Propulsion Laboratory, said in a statement.

“The asteroid’s loops around Earth drift a little ahead or behind from year to year, but when they drift too far forward or backward Earth’s gravity is just strong enough to reverse the drift.”

Searching for another metaphor, Nasa described the asteroid as “caught in a game of leapfrog with Earth that will last for hundreds of years”, because it spends about half its orbit closer to the sun than Earth and the other half farther away, behind the planet.

The new discovery is among the smaller asteroids of the galaxy. Astronomers on Haleakala, Hawaii, estimate that it is larger than 120ft (40m) but smaller than 300ft (100m). Scientists with the University of Hawaii found the asteroid in April, using the Pan Starrs 1 survey telescope.

The new asteroid, known as 2016 HO3, is expected to remain in its tandem orbit with the Earth for centuries. Nor is it the Earth’s only fellow traveler: the asteroid 3753 Cruithne, named after ancient Celtic people also known as Picts, is another quasi-satellite. It takes 770 years to work its way along a wobbly, horseshoe-shaped path, pulled back and forth by the Earth and the sun.

More similar to the new asteroid was an object called 2003 YN107, an asteroid that “followed a similar orbital pattern for a while over 10 years ago, but it has since departed our vicinity”, Chodas said. Not long after its discovery, Nasa declared the spinning chunk of rock and metal an example of a “corkscrew asteroid”.

The new asteroid, however, “has been a stable quasi-satellite of Earth for almost a century,” Chodas added, “and it will continue to follow this pattern as Earth’s companion for centuries to come.”

Cruithne is far larger than 2003 YN107 (which was about 20m across), and is estimated to be about three miles in diameter. Its path puts it well out of a possible collision course with the Earth, as does that of the latest asteroid identified.

Nasa scientists have hunted for years for asteroids that could strike the Earth, and estimate they have found more than 90% of those 1km wide or larger (none appear a likely risk). Smaller objects can do considerable damage, however, as exemplified by the Chelyabinsk meteor explosion of 2013 and the Tunguska event of 1908, both over Russia.

In the former, a 20m-wide meteor, traveling 12 miles per second, exploded 28 miles over Russian skies with the energy of about 500 kilotons of TNT. The explosion shattered glass for miles, sent debris crumbling off of buildings, burned skin and retinas and sent 1,210 people to hospital. Only about 0.05% of the meteor survived in a crater.

Directly beneath the meteor’s path, the shockwave was powerful enough to knock people off their feet. Windows were shattered in more than 3,600 apartment blocks, and a factory roof collapsed.

The Tunguska event involved an asteroid of unknown dimensions, possibly as small as 100ft in diameter. Its explosion over Siberia flattened trees over about 800 square miles, and is estimated to have had the force of at least several hundred atom bombs of the kind dropped on Hiroshima.

Nasa is also searching for these smaller objects, though it has had only limited success in tracking small and mid-size asteroids. The space agency has asked the public for help, proposed a new space telescope, and prepared a mission to land a robot on an asteroid, to study the possibility of deflecting the objects away from Earth.