Yesterday (Monday the 26th) I was on the BBC talking about Asteroid 2004 BL86, a fairly large asteroid which has just flown by us. It was at its closest to us at about 16:10 GMT/UT, and was expected to be just over 3 times further away than the Moon when at its closest. Thankfully it missed us, because such a large asteroid would cause wide-spread devastation were it to hit the Earth.
As the name of the asteroid implies, it was discovered in 2004 by a programme known as LINEAR, the Lincoln Near Earth Asteroid Research, named after the Lincoln laboratory at MIT, who are one of the partners along with NASA and the US Air Force. 2004 BL86 is estimated to be between 400 metres and 1km in size, considerably larger than the asteroid which exploded above Chelyabinsk in southern Russia in February 2013. That asteroid has been estimated to have been about 17 metres in size, and injured about 1,000 people in a sparsely populated part of Russia when it exploded in the atmosphere. Just imagine the devastation a 400m+ asteroid would cause, with a mass over twenty times larger its energy would also be over twenty times more (assuming the same speed)!
The orbital parameters of asteroid 2004 BL86 have been sufficiently well determined that we know that it will almost certainly never hit the Earth. The only way in which it could would be if its orbit were somehow significantly altered, but this is unlikely given its large size as it is much harder to change the direction of a large asteroid than a smaller one. It will not visit the Earth’s vicinity again for over 200 years, and in fact it is the largest known asteroid to come this close to us until asteroid 1999 AN10 passes us in 2027.
Most asteroids are found in the asteroid belt, which lies between the orbit of Mars and Jupiter. They are thought to be the small building blocks of the planets, but which were prevented from coalescing to form a planet because of Jupiter’s gravitational disruption.
However, there are other asteroids which are not in the main asteroid belt, and some of these have orbits which bring them close to Earth. Every now and again we get hit by one, but thankfully it is very very rare for Earth to be bit by a large asteroid. Although the statistics are very sparse, we think that something the size of the Chelyabinsk asteroid hits the Earth about once a century, and something the size of 2004 BL86 hitting the Earth would be much rarer, maybe once every few tens of thousands of years.
Where to see asteroid 2004 BL86
Unfortunately, this asteroid is not bright enough to be visible to the naked eye, but it is visible through a small telescope or large binoculars. It will be about 9th magnitude at its brightest, and the chart below, taken from Sky & Telescope magazine, shows its path through the sky against the background stars. Note: the times on this chart are Eastern Standard Time, so add 5 hours for GMT/UT. At the time of its closest approach to Earth it was in the constellation Hydra, and could be seen about mid-way between Jupiter and Sirius. Just before midnight GMT/UT on the 26th it passed into Cancer, and passed close to the Beehive Cluster at about 6am GMT/UT earlier this morning.