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.
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Jodrell Bank has been identified as a prime contender to win World Heritage Site status under new rules designed to recognise the importance of scientific installations.
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.
The 27-metre diameter asteroid 2008 AF3 will tonight pass by the Earth as close as the Moon (see its orbit here).
In fact, it is passing so close that you can see it for yourself (from a dark site, with a large telescope). At magnitude 14, you will need a 12″ diameter telescope to spot it as it passes through Ursa Major (the Plough) this evening (you can plot its position from your locations using Tom’s Asteroid Flybys Webpage).
But the worry is that it was only discovered 3 days ago, highlighting the difficulty of finding asteroids that are coming straight for us!
If an asteroid is larger than 100m, and is expected to pass the Earth within 20 times the Earth-Moon separation, then it is considered a potentially hazardous asteroid. Fortunately, none are expected to hit the Earth at the moment, but new ones are being discovered on a regular basis.
According to SpaceWeather.com there are currently 917 potentially hazardous asteroids out there. A full list is maintained by the Centre for Astrophysics at Harvard.
The Orion Nebula is a wonderful sight, in binoculars or a telescope – but it is often difficult to compare what you see with your own eyes through a telescope with photographs of the same region. Hopefully, the changing image beneath will help you compare the two views of Orion with ease!
That is because the human eye is not very sensitive, equivalent to just a few seconds of a photographic exposure. So when you look with your own eye through a telescope, you only see the brightest part of the gas cloud, at the very heart of the nebula. But long-exposure photographs pick up even the faintest details.
The two photographs above, taken by Martin Morgan-Taylor (assisted by his two cats and myself), show both these extremes. The short exposure photograph was only a 30 second exposure, and this is similar to the view that amateur astronomers witness when looking down the eyepiece of their telescope. The longer exposure is 900 seconds long, showing much more detail in the nebula than can be seen with the eye.
Astronomers will be running a workshop about using telescopes at the Observatory on the evening of 19th January from 5pm – 8pm. Perhaps you’ve received a new telescope for Christmas and would like some advice on getting the most from it, or you’re considering buying a telescope but are unsure what to look for? This workshop will let you discuss the different telescopes available on the market, try out different models and find out what they can do. Tickets cost £15 (£11 concessions) and can be booked in advance via bookings@nmm.ac.uk.
A recently discovered asteroid, which passed close to the Earth in November, may strike Mars on 30 January 2008. Asteroid 2007 WD5 will pass very close to Mars at the end of the month. Astronomers had originally estimated a 1 in 75 chance that it would hit the planet. Recent observations have increased this to a 1 in 28 chance of an impact. As more observations are made, we may see that the most likely path of the asteroid will pass by Mars completely.
2007 WD5 was discovered on 20 November 2007 by the Catalina Sky Survey in Tucson, Arizona. It had passed within 7.5 million km of the Earth on 1 November 2007 and is moving away from the Earth towards Mars. Its orbit could bring it back to the Earth, years or decades from now, but there is no apparent risk of an impact with our planet.
If the asteroid were to strike Mars, it would produce an explosion equivalent to about 3 megatons of TNT. It would not be the first impact that we have seen in our solar system. In 1994, the fragments of Comet Shoemaker-Levy 9 struck Jupiter over several days, producing a series of explosions that were visible through small telescopes.
The main highlights for this month are Orion, Mars and Saturn.
There is lots to see in Orion, which is visible around the South throughout the evening (click on the map to the left for a bigger version).
Betelgeuse (a name that is often thought to translate from ‘armpit of the central one’!) is a red supergiant star, to the top left of Orion. It is so big that if the Sun were to be replaced by Betelgeuse, the outer layers of the star would lie in the asteroid belt between Mars and Jupiter. The outer layers of the star also have the density of air.
To the bottom right is Rigel, a white supergiant star about 40,000 times brighter than the Sun.
Between the two are the three belt stars of Orion. The far left star in the belt is Alnitak, and close to that star is the Horsehead nebula, unfortunately too faint to be seen with the unaided eye (nebula is a Latin word meaning cloud).
One nebula that is visible to the unaided eye is in the dagger stars that hang down from the three belt stars. Take a look through binoculars and – wow – what a sight! Even better through a small telescope. I will talk more about the Orion nebula in a follow-up posting soon.
To the top right of Orion is the constellation of Taurus, the Bull, with the bright red giant star Aldebaran forming the eye of the bull. To the top right of Aldebaran are the Pleiades – a beautiful cluster of stars that is a delight in binoculars. Don’t bother looking at the Pleiades through a telescope however, unless you have a very wide angled lens, since this star cluster is far too big, being bigger than the Moon!
But what about the planets?
Well, the five main planets have all been known about since antiquity, thanks to them being so blindingly bright. Mars is especially so at the moment, due to it being on the opposite side of the Earth to the Sun. It is around the south all night – just look for the brightest red object you can find in the sky!
After admiring Mars, and if it is later than 8pm, look to your left by 90 degrees to see another bright planet, Saturn. How do you know if you’ve found it? Well, take a look in binoculars, and you may just be able to see the rings around Saturn – although you will need a small telescope if your eyes are older than 30.
To the top right of Saturn is the bright star Regulus, and above Saturn is the bright star Algieba – a beautiful orange-yellow double star system, well worth a look with a telescope while looking for Saturn’s rings.