At 6:42am BST on Thursday, 12 October our best orbit prediction for a 20-metre-wide space rock designated 2012 TC4 indicates that it will hurtle by Earth just 43,800 kilometres (27,215 miles) above the ocean between Australia and Antarctica. This may seem like a comfortable miss, but at closest approach the asteroid is little under 3½ Earth diameters away, just above the orbits of communications satellites.
Discovered by the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) in Hawaii on 4 October 2012, TC4 was only observed for a week before travelling out of range of asteroid-tracking instruments. Thankfully, the incredibly dim magnitude 26.8 body was recovered again by one of the European Southern Observatory’s 8-metre Very Large Telescopes (VLT) in Chile on 27 July this year. These crucial observations have enabled dynamicists to produce a better orbital prediction for 2012 TC4‘s trajectory through the Earth-Moon system.
Studies of this asteroid’s light curve found that it has a rotation period of just under 12¼ minutes with a brightness variation of almost a magnitude, which indicates that 2012 TC4 has a non-spherical (most likely elongated) shape. The spectral class is presently unknown.Viewing 2012 TC4 from the British Isles
If you thought that your chances of viewing this near-Earth asteroid from the UK would be slim owing to its small size, think again. The proximity of 2012 TC4 on the evening of Wednesday, 11 October means that as darkness falls in the British Isles (soon after 8pm BST) the magnitude +14.6 asteroid is favourably placed in the constellation of Aquarius, about ¾ degrees south of magnitude 4.8 star sigma (σ) Aquarii (α = 22h30.6m, δ = -10°41′, J2000).
But 2012 TC4 does not linger in the vicinity of sigma (σ) Aquarii because its motion against the stars of Aquarius is already picking up to 1½ degrees per hour — and accelerating as it draws closer to our planet. By 10pm BST (21h UT), the asteroid is close to the meridian and highest in the UK sky, about 22 degrees altitude in the south as seen from the heart of the British Isles. By this time 2012 TC4 is predicted to be magnitude +14.3, visible in 10-inch telescopes, and travelling at a rate close to 2 degrees per hour against the background stars of Aquarius.
At 12am BST on 12 October (23h UT on 11 October), 2012 TC4 — by now predicted to be magnitude +13.9 and travelling at 3 degrees per hour — lies ¾ degrees east of magnitude 6.4 star 29 Aquarii (α = 22h02.4m, δ = -16°58′, J2000). This is probably about the last time most UK observers with 8-inch-class telescopes will see the asteroid, low in the southwest. The rising Moon will also be low in the east-northeast at this time.Earth’s close brush with 2012 TC4 on 12 October will modify the asteroid’s orbit, further complicating the task of dynamicists keen to work out just how close it will get to our planet in the future. But for now, we can relax that it will pass by in complete safety.