Space Oddity

The Earth has a tiny companion that orbits the sun exactly in tune with our planet, astronomers at York University in Ontario, Canada, announced Wednesday.

Calling the 6-mile-diameter rock the “Mona Lisa of asteroids” for the subtle beauty of its sympathetic vibrations with Earth, astronomer Paul Weigert said it was a “thrilling discovery” akin to “finding a diamond in your own backyard.”

Similar odd couples orbit within the solar system in harmonic resonances, including several moons of Saturn and Jupiter. However, out of hundreds of asteroids that orbit near Earth, only the rock called Asteroid 3753 is held captive by our planet’s gravitational pull in a way that keeps it pulsing in a regular rhythmic dance.

Unlike the Earth’s most familiar companion, the moon, this asteroid orbits the sun--and at times finds itself 1,000 times farther away from Earth than the moon, Weigert reports in today’s issue of the journal Nature. However, it is still considered a “companion” because of the exact mathematical one-to-one relationship of its orbit to Earth’s.

The mystery is: How did it achieve such an unlikely orbit? “That’s one of the big questions,” Weigert said. “We’re working on it.”

The asteroid was discovered 11 years ago, but its unusual horseshoe-shaped orbit (as viewed from Earth) was only unraveled recently with the help of powerful computers. “It was waiting to be discovered, but it took someone to have the intuition to check it out,” said planetary scientist William Bottke, a postdoctoral researcher in planetary science at Caltech. “It’s an interesting celestial oddity.”

The asteroid, scientists say, poses no threat to Earth--at least for 100 million years, when it could stray from its present path and careen unpredictably. Before that time, hundreds of other asteroids would be more likely to smash into the planet. “There are plenty of other enemies out there,” said astronomer Brian Marsden of Harvard University, who called the discovery “interesting,” especially because “there are no other asteroids like this.”

Only because the asteroid orbits in resonance with the Earth does it avoid colliding with our planet, the astronomers said. Two things “resonate” when they vibrate in the same period, or in tune with each other. Asteroid 3753’s “year”--the period it takes to orbit the sun--is about the same as the Earth’s. The asteroid gains stability from its resonance with Earth just as air blown into a flute produces a tone that gains stability when its vibration matches the natural resonant frequency of the instrument.

However, the path that the asteroid takes around the sun is wildly different from Earth’s. At its closest approach to our planet, it’s still about 40 times farther away than the moon; at its farthest point, about 25 times that distance.

Moreover, while the Earth and the vast majority of bodies orbit the sun in a flat plane, like the ridges on a phonograph record, Asteroid 3753 dips far under the plane, and then far over it.

Still, the asteroid remains enthralled in Earth’s gravitational grasp through a complicated celestial choreography. As the asteroid begins to approach the planet, Earth’s gravity flings it out to a larger orbit. Since objects orbiting far from the sun orbit more slowly than objects nearby--just as an ice skater spins more slowly when she extends her arms--the asteroid slows down.

When the asteroid gets too far behind Earth, the planet pulls it back onto an inside track, where it can orbit faster, and catch up to Earth again.

“It just keeps pingponging around the Earth,” Weigert said. “The Earth and the asteroid chase each other around.” The dance is an adagio, taking place over many centuries.

Many other pairs of objects in the solar system orbit in resonance--for example, Pluto makes two orbits around the sun for every three of Neptune’s--a resonance that keeps them from colliding. However, the one-to-one orbital relationship between the asteroid and Earth allows for one of the most complex paths ever seen, Weigert said.

Called a “horseshoe” orbit because the asteroid appears to shift direction in mid-swing, it is a phenomenon seen only in one other place in the solar system--on two moons of Saturn. Astronomers have known since the 1980s that the moons Janus and Epimetheus pull on each other in a similar pattern. But as Marsden pointed out, that system is different because the two moons are relatively equal in mass and each pulls equally on the other.

In contrast, while the Earth changes the orbit of Asteroid 3753, the rock’s mass is too tiny too affect the Earth. “The Earth doesn’t care about that object at all,” Marsden said.

Before Weigert and his York colleague Kim Innanen figured out the asteroid’s orbit, scientists thought it would be impossible for such strange behavior to remain stable. Three-body problems, as scientists call them, are notoriously complex. While it is easy enough to figure out the effects of two bodies on each other--say, the Earth and sun--throwing a third into the equation makes it all but impossible to solve.

In essence, Weigert got around the problem by brute force, letting a computer calculate the future orbit of the asteroid based on information about its present motion through the sky. Astronomers at Turku University in Finland corroborated the results.

Just how the asteroid got there in the first place remains an enigma. Since asteroids orbiting near Earth usually hang around for no longer than 100 million years, it’s hard to imagine how 3753 could have formed with the Earth at the birth of the solar system almost 5 billion years ago.

On the other hand, for the combined gravitational influences of the planets and sun to nudge the asteroid into its present orbit at a later time would be as unlikely an operation as “threading a needle,” Innanen said.

Asteroid 3753 should be visible in the Southern Hemisphere this fall at its closest approach to Earth, but only through telescopes. Like most asteroids, this most unusual rock around the sun is exceedingly dim.

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Space Oddity

The asteroid is held captive by Earth’s gravitational pull in a way that keeps it locked in a kidney- shaped danced tied to Earth’s path.

1. The asteroid’s path as it would appear if the Earth were stationary.

2. A view of its orbit compared to the plane of the Solar System--the spiral is an additional motion.