A grand total of 10,000 asteroids and comets that pass near Earth have now been discovered. The 10,000th near-Earth object – asteroid 2013 MZ5 – was detected on 18th June by the Pan-STARRS-1 telescope in Hawaii. 98% of all near-Earth objects discovered were first detected by NASA-supported surveys, including this one.
"Finding 10,000 near-Earth objects is a significant milestone," said Lindley Johnson, program executive for the Near-Earth Object Observations Program at NASA Headquarters. "But there are at least 10 times that many more to be found before we can be assured we will have found any and all that could impact and do significant harm to the citizens of Earth." During Johnson's decade-long tenure, 76 percent of the NEO discoveries have been made.
Near-Earth objects (NEOs) are asteroids or comets that can approach within 28 million miles (45 million km) of the Earth.
For comparison, the average distance from Earth to the Moon is 238,900 miles (384,500 km). They range in size from just a few feet, to as large as 25 miles (41 km) in the case of 1036 Ganymed.
Asteroid 2013 MZ5 is approximately 1,000 feet (300 metres) across. Its orbit is well understood and will not approach close enough to Earth to be considered potentially hazardous.
"The first near-Earth object was discovered in 1898," said Don Yeomans, long-time manager of NASA's Near-Earth Object Program Office in California. "Over the next hundred years, only about 500 had been found. But then, with the advent of NASA's NEO Observations program in 1998, we've been racking them up ever since. And with new, more capable systems coming on line, we are learning even more about where the NEOs are currently in our solar system, and where they will be in the future."
Of these 10,000 discoveries, nearly 10 percent are larger than 3,280 ft (1,000 m) – roughly the size that could produce global consequences should one impact Earth. However, NASA has determined that none of these larger NEOs currently pose any impact threat and probably only a few dozen more remain undiscovered.
The vast majority of NEOs are smaller than a kilometre, with the number of objects of a particular size increasing as their sizes decrease. For example, there are estimated to be around 15,000 about one-and-a-half football fields in size (460 ft, or 140 m) and over a million that are one-third of a football field in size (100 ft, or 30 m). An NEO hitting the Earth would need to be 100 feet (30 metres) or larger to cause significant devastation in populated areas. Almost 30 percent of the 460-ft-sized NEOs have been found, but less than 1 percent of the smaller NEOs have been detected. The asteroid that wiped out the dinosaurs, around 65 million years ago, is thought to have been 6 miles (10 km) in diameter. A simulation of an impact on this scale – which occurs about once every 100 million years – is shown in the video below.
Tim Spahr, director of the Minor Planet Center: "When I began surveying for asteroids and comets in 1992, a near-Earth object discovery was a rare event. These days, we average three NEO discoveries a day, and each month the Minor Planet Center receives hundreds of thousands of observations on asteroids, including those in the main-belt. The work done by the NASA surveys, and the other international professional and amateur astronomers, to discover and track NEOs is really remarkable."
Within a dozen years, the program had achieved its goal of discovering 90 percent of near-Earth objects greater than 3,300 ft (1 km) in size. In December 2005, NASA was directed by Congress to extend the search and identify 90 percent of NEOs larger than 500 ft (140 m) in size. When this goal is achieved, the risk of an unwarned future Earth impact will be reduced to a level of only 1 percent when compared to pre-survey risk levels. This lowers the risk to human populations, because once a threatening NEO is known well in advance, the object could be deflected with current technologies.