Most of Earth’s population would have been blissfully unaware of any cosmic events in the early hours of 28 June 2011, but at 5.14 am New Zealand time, an asteroid the size of a small house passed close to the Earth. The large lump, called 2011 MD, measured between 6.3 and 14 m in diameter and passed 12,000 km above the Earth’s surface. Such asteroids, or near-Earth objects, are not unusual. NASA reports that objects of this size come this close to Earth about every 6 years on average. However, 2011 MD is notable because it was close enough to pass between us and our global positioning system (GPS) satellite array, and its orbit was altered by the gravitational pull of the Earth.
Travelling at 6.7 km/second, the asteroid approached from the Northern Hemisphere night side, passed closest to the Earth over the southern Atlantic Ocean near the pole and left again passing over the Northern Hemisphere on the day side. Dr Pasquale Tricarico, a research scientist at the Planetary Science Institute in the United States, writes that “the orbit of the asteroid is perturbed by the Earth, so much so as to turn it around and send the asteroid almost back to where it comes from”.
Scientists are yet to calculate what this change in orbit will do to future passes of the Earth but it is likely that, even if the asteroid did enter our atmosphere on a future fly-by, it is small enough to burn up in our atmosphere, with perhaps only a few small meteorites (fragments of 2011 MD) reaching the Earth’s surface.
Near-Earth asteroid research
2011 MD was spotted a week beforehand by astronomers from the Lincoln Near-Earth Asteroid Research (LINEAR) near-Earth object discovery team observing from Socorro, New Mexico. The astronomers believe the asteroid is a fragment smashed off a much larger asteroid in the asteroid belt between Mars and Jupiter. Asteroids are varied in their composition. Most are dense, many are rich in carbon and a few (fewer than 7%) are very dense and made of iron and nickel.
The team detect potential near-Earth objects using a computerised ‘spot the difference’ approach, where images of the night sky from the same celestial position but with different time stamps are overlaid to detect moving objects against the starry background.
The asteroid managed to fly by without hitting either our satellite array or the Earth, but objects can and do collide with Earth. Fortunately, most of these are very small. Objects less than 10 m in diameter are classified as meteorites, and it is believed that several hundred of these hit the Earth each year, but only a handful are ever recovered. Fortunately, large collisions are far less frequent. Asteroids with a 1 km diameter strike the Earth every 500,000 years on average. Even larger asteroids with a 5 km diameter or more collide with Earth approximately once every 10 million years.
It is now a widely accepted theory that an asteroid impact, the Chicxulub Impact in the Yucatán Peninsula in Mexico, was responsible for a mass extinction event, including nearly all dinosaurs, in the Cretaceous–Tertiary period 65 million years ago. The crater left by Chicxulub is 180 km in diameter, with the impacting projectile estimated to be over 10 km in diameter.
Scientists have calculated that 1 known asteroid with a diameter of over 1 km, named 1950 DA, has a possibility of colliding with Earth on 16 March 2880. In 2011 this probability was calculated as approximately only 1 in 300 but calculations will be refined as the asteroid is interacted on by other objects. As of the 7 December 2015, the probability of an impact in 2880 is 1 in 8,300 (0.012%)