Earth has a second moon, of sorts, and could have many others, according to three astronomers who did calculations to describe orbital motions at gravitational balance points in space that temporarily pull asteroids into bizarre orbits near our planet.
The 3-mile-wide (5-km) satellite, which takes 770 years to complete a horseshoe-shaped orbit around Earth, is called Cruithne and will remain in a suspended state around Earth for at least 5,000 years.
Cruithne, discovered in 1986, and then found in 1997 to have a highly eccentric orbit, cannot be seen by the naked eye, but scientists working at Queen Mary and Westfield College in London were intrigued enough with its peregrinations to come up with mathematical models to describe its path.
That led them to theorize that the model could explain the movement of other objects captured at the gravitational balance points that exist between all planets and the sun.
"We found new dynamical channels through which free asteroids become temporarily moons of Earth and stay there from a few thousand years to several tens of thousands of years," said Fathi Namouni, one of the researchers, now at Princeton University.
"Eventually these same channels provide the moons with escape routes. So the main difference between the moon (weve always known) and the new moons is that the latter are temporary -- they come and go, but they stay for a very long time before they leave."
Astronomers have long known that the solar system is full, relatively speaking, of asteroids.
Most orbit the sun in a belt between Mars and Jupiter, but a handful cross Earth's orbital path -- an imaginary curve through space along which our planet travels around the sun.
Namouni and his colleagues discovered several new types of orbital motion, which showed that some asteroids that cross Earths path may be trapped in orbits caused by the gravitational dance between Earth and the sun.
The work was published in a recent issue of Physical Review Letters.
The finding is based on work by 18th century French mathematician Joseph-Louis Lagrange, whose name is affixed to five points of equilibrium (L1 to L5 in the top diagram) that occur between the gravitational forces of planets, including Earth and the sun.
Lagrange had shown that the forces at the balance points could capture objects and keep them orbiting there (NASA and the European Space Agency have taken advantage of one balance point by launching a sun-observing satellite called SOHO that currently orbits at L1). The orbits of objects at these points are exotic, often tadpole-shaped, but rarely horseshoe-shaped. The horseshoe orbit involves movement around the L3, L4 and L5 points (see diagram at top).
Cruithne takes 770 years to complete its horseshoe orbit. Every 385 years, it comes to its closest point to Earth, some 9.3 million miles (15 million kilometers) away. Its next close approach to Earth comes in 2285.
Namouni and his colleagues latched on to Cruithnes orbit and worked out models built on Lagranges work to explain its eccentric orbit and then theorized that such "co-orbital dynamics" could explain the strange movement of other objects at the Lagrangian points.
Cruithnes orbit is exceedingly strange. "What it does with respect to the Earth is it moves very slowly," said Namounis colleague Apostolos Christou. "At specific points in its orbit, it reverses its rate of motion with respect to Earth so it will appear to go back and forth."
Whats in a moon?
Co-orbital motions probably describe the orbits of many objects at the Lagrange points, Namouni and his colleagues say, but are these objects moons?
A moon typically is defined as an object whose orbit encompasses a planet, say, the Earth, rather than the sun, said Carl Murray, who worked with Namouni and Christou on the research.
But its hard to say what a "true" moon is, he said.
In his view, there are three classes of moons large moons in near-circular orbits around a planet, having formed soon after the planet; smaller fragments that are the products of collisions; and outer, irregular moons in odd orbits, or captured asteroids like Cruithne. In the past year, astronomers have reported finding such objects around Uranus.
So where does our well-known moon fall in this classification, given that scientists think it is the result of a Mars-sized object slamming against our planet soon after it formed?
"Our own moon is in many ways unique and its formation seems like a one-off event," he said. "Our moon is very different in all respects from an object like Cruithne."
There are almost certainly more temporary moons of Earth and of other planets waiting to be discovered, Murray said.
As scientists get better at discovering asteroids, they will find more that have orbits that will keep them close to Earth for a long period of time. But some of those objects are very small.
"At some stage you have to consider the definition of moon," he said. "Is a dust particle orbiting the Earth a moon of the Earth?"
As for Cruithne, Namouni said its not really a "moon" because it moves around the Earth at this time but may not forever. Earth is causing Cruithnes present trajectory, but it could eventually escape.
So its not a moon of Earth, but it might become one.
"We found that Cruithne is likely to use the new dynamical channels to become a real moon of the Earth and remain as such for 3,000 years," Namouni said.
Since there is no definitive count yet of all the asteroids in our solar system, including Earth-crossers, Namouni and his team cannot estimate how many other temporary moons may be orbiting Earth and other planets.
Still, the finding throws into question the current official counts of moons around the planets, since there may be dozens of unknown asteroids circling each planet in temporary or permanent orbits due to gravitational balance points.
For now, Namouni says there should be a new category of moons -- "temporary moons that are captured for a few thousand to several tens of thousands of years."