On October 11, 2010, asteroid 2010 TD54 zipped by Earth at 28,000 miles distance, well within the distance of the Moon's orbit (about a quarter of a million miles). This particular rock would not have posed a threat to Earth even if it had been on a collision course, but it does serve as an important reminder that the space around us is not empty and another, larger rock could have our names on it.
Asteroid 2010 TD54 made its closest approach to Earth at 10:51 a.m. Greenwich time (or 6:51 Eastern time, in North America) on October 12, and closed to within just 28,000 miles, which, on the scale of the solar system, certainly qualifies it as a near miss. According to an announcement by NASA, the asteroid was discovered by the Catalina Sky Survey just days before it swung by our planet. When new small objects are discovered in our solar system, they are initially given a name including the year they were discovered (hence 2010) and a designation (TD54). Common names typically follow only if the object in question has some particular significance.
Fortunately, 2010 TD54 is not particularly significant. NASA initially estimated the size of the rock at 5-10 metres. Such rocks fly by Earth very regularly, and even enter the atmosphere on occasion. When they do, however, they burn up harmlessly at very high altitude. At best, they become unusually bright shooting stars. NASA's Near-Earth Object Office keeps a list of upcoming close approaches, although all of those currently listed are going to miss the Earth by tens, hundreds, or thousands of times as great a distance as 2010 TD54. A few weeks ago, the Catalina survey's newest rival, the military-funded Pan-STARRS project, spotted a much larger, 50-metre asteroid it named 2010 ST3. However, that one is still on approach and scheduled to miss by a comfortable 4 million miles.
However, the dividing line between a harmless light show and a serious threat is a frighteningly thin one - which is why increasing attention has been given to spotting near-Earth objects like this one. An asteroid a few dozen metres across could break apart and release energy equivalent to a nuclear bomb - which is what happened over Tunguska, Russia, in 1908. An asteroid ten times that could devastate an entire region - and an asteroid ten times that size could pose a threat to the entire world. The asteroid that slammed into Chicxulub, Mexico, 65 million years ago was as much as 10 kilometres in diameter. Plans for what to do if we discover such an asteroid on its way are, currently, no more than informed speculation.
Still, the purpose of the Near Earth Objects Office is to build as comprehensive a list as possible, so as to give us as much warning as possible. Near-earth asteroids are assigned a score on the Torino Scale, an index which rates asteroids from 0 (no risk) to 10 (imminent risk of global destruction). Right now, the organization's risk table includes just one object at risk level 1, 2007 VK184, which would cause regional destruction in the very unlikely event it strikes Earth in 2048. The highest point on the scale (level 4) was reached by 300-metre Apophis in 2004, following initial calculations of a 3% chance of collision in 2029 or 2036. Apophis was subsequently cleared by further calculations, but the European asteroid project NeoDYS lists asteroid 1-kilometre-wide 1950 DA as a Level 2 on the Torino Scale due to a potential collision in 2880.