Comet 103P/Hartley 2 was discovered by Australian astronomer Malcolm Hartley, who calculated that the object orbits the sun about every 6.5 years. Until now, however, gravitational interactions with Jupiter kept shifting the comet's path, sending it closer to the sun and thus farther from Earth during each subsequent return.

This year comet Hartley 2 is on course to make its closest pass by Earth at a mere 11 million miles (17.7 million kilometers) on October 20—and a dark, moonless sky in mid-October will help create ideal viewing conditions, astronomers say.

(Find out about two asteroids that recently swung between Earth and the moon's orbit.)

"Before mid-October, Northern