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Sun, 10 Feb 2013


How to spot asteroid 2012 DA14


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The view looking east-north-east on Friday 15 February at 20.00, with times for 2012 DA14


Amateur astronomers are eagerly anticipating the approach on Friday 15 February of an asteroid, 2012 DA14, which will be the closest 'near miss' to Earth than any other in recent years. At about 19:45 on that day the 50-metre asteroid will be just 35,000 km from the UK, which is just within the ring of geostationary satellites.

It will be possible to view the object from the UK, weather permitting, using amateur instruments, even binoculars. The map above shows the part of the sky where it will appear, looking north-east at 8 pm. But it won’t be a walk in the park, because the object will not be particularly bright, at 8th magnitude. Although in theory it will be visible using only small binoculars, in practice from most UK sites this will be close to the limit of what you can easily see. And it gets fainter (9th magnitude) as it gets higher in the sky – the magnitude scale runs in reverse from what you might expect.

As it’s moving more quickly through the sky than any other predicted natural object we’ve observed, viewers will need to look in precisely that right place at any particular time to be sure of spotting it.

The map above shows its path, but you’ll need much more information than just that. Typically, when observing a comet or similar body, observers will use a sky mapping program to give them details of the object’s position. But on this occasion, the asteroid will be so close to the Earth that most programs won’t be able to cope. This is because most objects are so far away that it doesn’t matter whether the observer is in the UK or South Africa, so the programs just take the centre of the Earth for the observer's location. But in this case a makes a huge difference to both the track and the timing.

The popular website www.heavens-above.com gives positions which take this into account, and for your own personal prediction you can register and log on to that site free of charge, choose your location, and see a star map with the asteroid’s path for your location superimposed. For convenience, we have provided maps for central England below, based on the heavens-above maps, with permission.

There are three maps, covering the track as seen between 20.10 and 21.50 on 15 February, as seen from the middle of the UK. In each case, a 1 degree field (equivalent to the low power view of many telescopes) is shown. At its fastest, the asteroid will be crossing this field of view in just over a minute, which shows how careful you will need to be in finding the right point in the sky. We have also indicated with red arrows stars similar in brightness to the asteroid at that time. The top of each map joins onto the bottom of the one below it.

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The track of the asteroid past the Coma Star Cluster
Use Cor Caroli to find this part of the sky (above)
The brightest stars in the middle of the map are in The Plough


Tips for observing
Each map is based on a part of the sky which you should be able to find with a fair knowledge of the constellations or using the map at the top, which shows the sky for 20.00. The stars are rising in this part of the sky, so the constellations will have moved higher in the east by 9 pm.

Find The Plough and Leo, and draw a mental line between Alkaid in the Plough and Denebola in Leo. One third the way down is Cor Caroli, and two thirds the way down is the Coma Star Cluster, easily visible in binoculars if not the naked eye. The first map below is based on the Coma Star Cluster, the second includes Cor Caroli, and the third includes part of The Plough.

If it’s clear beforehand, familiarise yourself with that part of the sky, and try to identify the stars along the track and in particular the stars with a similar brightness. That will help you to work out whether whatever instrument you’re using will show the asteroid.

You may have to choose particular waypoints along the track that you can find, and which the asteroid will cross at a specific time.

Though these maps are for a specific location, they will be adequate for most sites within the UK. The map below shows how part of the track varies between Dover (green), Penzance (red) and Thurso (black). As well as a 1º field of view, a typical 5º field of view of binoculars is shown. Click for a larger version.

To get maps for your own location, go to www.heavens-above.com.

What instrument is best?

This depends on where you are observing from and the conditions at the time. From a dark, country site binoculars will be fine, but ideally they should be 10 x 50 or larger. The asteroid will be very tiny in smaller instruments, and even 10 x 30 image-stabilised binoculars will not give a good view of an 8th-magnitude object from a typical UK site with some light pollution. 15 x 70 binoculars will be ideal, as they should both show the asteroid clearly and have a wide enough field of view that you will not have to keep shifting position. However, they will need to be tripod-mounted or held firmly.

Telescopes will also show the asteroid, but use the lowest magnification or you might miss it. Finding the right spot in the sky will not be a trivial task, particularly if you don't have a good finder on your telescope. Youi'll need to pick out the correct field of view just before the asteroid passes through it. At least with binoculars you can see the neighbouring stars, but most telescopes have quite a small field of view and picking the right area is tricky.

Those with plenty of experience of star-hopping, going from one star to another to find a fainter object, will have plenty of practice, but if you only view bright objects or use a Go To telescope, you will need to get plenty of practice in advance, and also get to know your handset. Most of these don't include the fainter stars in their databases, so you'll need to discover which stars your scope can actually find that are close to the track, and then move the telescope from there having located the stars. Alternatively, use the heavens-above.com listing of asteroid positions for your location and drive the telescope to a selected location in advance. Not easy!

Make sure your finder is accurately aligned with your telescope in advance. This is an occasion when a good optical finder, such as a 9x50, will be much more use than a red-dot finder, as it will help you to find the actual field of view.

A telescope linked to a computer sky program will be ideal, but you will still need to use a separate map as the software will probably not display the asteroid's track accurately even if its orbital elements are entered in advance.

 

Added by: Robin Scagell