Luna's Little Sister: Cruithne|
by Ace in the Hole
Man oh man, are Astrologers gonna have a field day with this one! Guess what, after all those years of having a bad day for no reason, you may now be able to blame it on a rogue moon, orbiting the earth in a bizarre orbit. Believe it or not, although people had seen a frieky little asteroid named Cruithne, nobody ever imagined it might actually orbit the Earth, making it a second moon!
Earth has a second moon, named Cruithne, and could have many others! 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.
They discovered the little space rock is actually caught in Earth's gravity well and will orbit us for at least 5,000 years, says the British team. The gravitational forces of our planet and of the Sun meet, swirl and sort of cancel each other out in places in space called Lagrange points. Like gravity quicksand, Lagrange points allow Earth to capture passing asteroids. Cruithne, which is three miles (5-km) across and completes its eccentric horseshoe orbit every 770 years, is one of these Trojan asteroids.
Dr Fathi Namouni, Dr Apostolos Christou and Prof Carl Murray say it is "almost impossible" for asteroids in such orbits to hit us. "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 Dr. Namouni.
"Eventually these same channels provide the moons with escape routes. So the
main difference between the moon (we’ve 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."
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.
Cruithne's path is much more complicated than simple satellite motion. In addition to going around the Earth, it also goes back and forth in smaller wiggling loops. The whole thing makes for a lot a fancy diagrams you can look at by clicking on the big picture below. That image, by the way, shows the simple horseshoe shape of Cruithne's orbit, but it is really much more complicated. I would like to see how this puppy ends up on Madame Claire Voiante's star tables....
Once simply called asteroid 3753 (1986 TO), Cruithne was named after the first Celtic racio-tribal group to come to the British Isles, appearing between about 800 and 500 B.C., and coming from the European continent. They were also known as the Picts. The honor of naming the new moonlet went to discoverers of the asteroid, a process which is regulated by the International Astronomical Union's Minor Planet Center naming conventions.
The scientists say that the strange orbit they have tracked for this moon suggests that there are other moons in similar trojan orbits. Are there other moons out there in Lagrange range? Will Cruithne stay well heeled on Earth's gravitational leash? We will keep you posted!