On October 20, 2010, comet Hartley 2 (also known as 103P/Hartley) passed within just 11 million miles of Earth. It was an exciting event for amateur astronomers as well as an important precursor to a close approach by a NASA probe, the Deep Impact spacecraft.
Hartley 2 was first spotted in 1986 by an Australian telescope and estimated at 1.2-1.6 kilometres in diameter (0.75-1.0 miles). Subsequent detailed studies by large telescopes, like the Spitzer Space Telescope and the Arecibo Observatory, established important facts about its nucleus and its rate of rotation, which is about 18 hours (compared to a 24-hour day on the much larger planet Earth). Its orbit lasts about 6.5 years, and sends it swinging from roughly the distance of Earth to the Sun out to roughly the orbit of Jupiter.
In astronomical terms, 11 million miles (20 million kilometres) is a near miss, but it's still a comfortably safe distance from a planetary safety perspective. By contrast, the orbit of the nearest planet, Mars, lies about 50 million miles away, although the actual distance between the two planets ranges from slightly smaller to much greater depending on where they lie in their relative orbits. The approach is close enough for astronomers to enjoy, but small asteroids regularly tumble past at far closer distances, including some which could cause considerable damage if they ever impacted the Earth.
Hartley 2 is significant for another reason, however. On November 4, just after it has passed by Earth, it will be approached by NASA's Deep Impact space probe. The Deep Impact probe will close to just 620 miles (about 1000 kilometres) and take a large number of pictures and scientific readings. Deep Impact was initially launched in 2005 and used to study another comet, Tempel 1. After that approach (during which it fired a small impactor into the comet to study the resulting debris cloud), Deep Impact moved onto an extended mission called EPOXI, during which it took readings of several known extrasolar planets (planets around other stars) and prepared for a second cometary close approach.
Deep Impact was supposed to visit Boethin, a comet discovered in 1975, sighted again in 1986, and due for a visit in 2008. However, in a mysterious episode, NASA and other space organizations were unable to locate the comet on schedule several years ago. They theorized that it had broken up somewhere in its orbit (an uncommon but not unheard-of event for a comet), and set their sights on Hartley 2 instead.