A 250m-diameter (600ft) asteroid passed by the Earth this morning (29 January, closest approach at 0833 GMT). The asteroid, 2007 TU24, came as close as 538,000km (334,000 miles), just beyond the Moon’s orbit.
NASA estimates that there are around 7000 near-Earth asteroids as large as 2007 TU24, and a similar object can be expected to pass this close to Earth about every five years or so. However, Earth impacts for an object this size only occur every 37,000 years on average.
According to NASA’s Near Earth Objects (NEOs) fact sheet, ‘There are no known NEOs on a collision course with the Earth. There is a possibility that an as yet undiscovered large NEO may hit the Earth, but the probability of this happening over the next 100 years is extremely small.’
An asteroid is considered potentially hazardous if it is larger than 100m, and is expected to pass the Earth within 20 times the Earth-Moon separation. The Earth-Moon distance is about 0.0026 AU (1 AU = 149.6 million km).
It’s hoped that detailed observations of 2007 TU24 will reveal how the asteroid is composed – i.e. whether it is a single solid object or a loose collection of rubble, information which could help plan our defence against future hazardous asteroids.
As mentioned in a recent post, the considerably smaller asteroid 2008 AF3 (27-metre diameter) passed by the Earth as close as the Moon just over two weeks ago. A 600m-wide asteroid, 2004 XP14, flew past the Earth at roughly the same distance just over six months ago.
The nights of 24-25 January are a great time to see the planet Mercury.
The best time to look is within 10 minutes of 17:10 local time. Any earlier, and the bright glow of the Sun gets in the way; any later, and Mercury will be too low in the sky. At this time, Mercury will be 9 degrees above the horizon – about the size of your outstretched hand at arm’s length above the horizon. You can find it in the south-west. It is visible to the unaided eye, but it can be a difficult planet to spot.
But, by the 30th of January, Mercury has disappeared, hidden in the glare of the Sun once again.
The reason why Mercury is so difficult to see (can you spot it in the photograph to the left?) is that it is the closest planet to the Sun, and so when we look in the night sky, Mercury is always pretty close to the Sun. But every 3 months there are a brief few days when Mercury gets to the extreme of its orbit, as viewed from the Earth.
Note to the expert amateur astronomers: the highest altitude of Mercury after sunset is different to the day when Mercury is at its maximum eastern elongation. So even though Mercury got to its most easterly point in its orbit on 22 January, it gets slightly higher in the sky at sunset for the following few days.
The next time Mercury is around the evening sky is in mid May, when it will be 4 degrees higher in the sky at sunset, making it slightly easier to spot.
Mercury is also in the news thanks to the NASA Messenger probe. On 14 January, Messenger took some great pictures of Mercury during a close fly-by.
The two comets are Comet Holmes and Comet Tuttle – both visible to the unaided eye at the moment, if you manage to get away from all those inefficient city lights. If not, binoculars will help you spot the pair.
Comet Holmes, still visible to the unaided eye from a dark site, is still getting bigger – it is now three times the size of the full moon! The image to the left was taken by Toni Scarmato in Italy, with the comet approaching the bright star Algol. It will get closest to Algol on 22 January, so that will be a great time to take a look.
Because it is so big, you do not need any specialist equipment – just a digital camera with lens will do. For lots of examples, see the SpaceWeather.com gallery. It is visible high in the sky, in the constellation of Perseus – halfway between Cassiopeia and the face of Taurus the bull.
But the big question that everyone is asking is – will Comet Holmes outburst again? When it was first discovered in 1892, it faded for about 70 days until it suddenly brightened again. Will history repeat itself? If so, the comet may brighten again any time now. (The full history of the discovery behaviour of Comet Holmes can be found at Gary Kronk’s Cometography website.)
While Comet Holmes is very high in the sky from Greenwich, Comet Tuttle is much lower, just 25° above the horizon in the constellation of Cetus the Whale (or sea monster), well below Perseus. Images and maps of Comet Tuttle can be seen on the SpaceWeather.com website.