Asteroid 2002 NY40

The Prediction

This asteroid, an irregular rock some 300 x 800m across, was forecast to pass just 1.4 Lunar distances (that is, about 527,000 km) from the Earth on the morning of 18 August 2002. This distance is nearer to the Earth than any asteroid of comparable size since 1965. You can see just how close this is on a Solar System scale by looking at its orbit - to simulate the flyby set the date as 17 Aug 2002, Interval as 1 Hour, Centre on the Earth, zoom in and run the animation.

The asteroid was discovered only in July but it was realised that it would pass very close to the Earth would potentially be visible even in binoculars as it flew past. Finder charts prepared by Sky and Telescope indicated that the asteroid would be high in the sky around midnight of 17 - 18 Aug, moving westwards between the constellations of Aquila and Lyra as it came from opposite the Sun and passed behind the Earth. Closest approach was due at 0747GMT on the morning of the 18th, daylight in Britain, but the hours leading up to it should yield a good view, weather permitting.

The high accuracy ephemeris I calculated at Jet Propulsion Laboratory Solar System Dynamics indicated that it would reach a maximum brightness of 9.5, which is well beyond naked eye visibility, but theoretically possible in good binoculars and an easy target for my 22cm telescope.

The Outcome

The 17th was a beautiful day, probably the hottest of the year so far, but towards evening became rather cloudy in the evening (of course). Never the less, having rushed home from visiting a cousin in Devizes I erected the telescope on the patio and awaited any clear spells that might pass by.

A number of clearish patches gave moderate views in the required direction, but despite extensive searching both with binoculars and telescope I was unable to locate the asteroid. However, after midnight the sky cleared for some longer spells, and by aligning on suitable guide stars I eventually located 2002 NY40 in the telescope at about 1am (0000GMT).

It appeared just as a star, for even 800m at that distance would be too small to resolve as anything other than a point of light. The immediate give-away though was its motion across the sky as you could see it moving across the background of stars in real time. At low power (x65) my telescope has a field of view of about half a degree (about the same size as the Moon), a distance the asteroid covered in just 5 minutes. It was amazing to think that this point of light was really a half mile lump of rock hurtling past the Earth at 20km a second. Perhaps in the future it will hit the Earth, so it pays to know your enemy!

I tracked it on and off (with interruptions due to cloud) for just over an hour, finally losing it at 2.10am when the clouds rolled in for good.

I had hoped to photograph it but in the event I was thwarted for a number of reasons, though I have learned valuable lessons should the opportunity to capture a similar event happen again:-

• My Minolta Dimage 7 camera can reach stars of magnitude 6 (about the same as the naked eye can reach under perfect conditions) with its longest exposure of 32s and the camera mounted on the telescope mount so it tracks the rotation of the sky. This was achieved at an optimum setting of ISO400, the highest setting of 800 being plagued by too much noise in the image, not helped by the sky glow from London and Crawley. Results were similar with an anti-pollution filter because although the sky was darker the filter cuts out some starlight too. Unfortunately this performance was not sufficient to reach the asteroid on this occasion.
• Magnitude 9 is too faint to show in the viewfinder with the camera attached to the eyepiece of the telescope, though would show clearly on the resulting picture. The telescope should give a brightness gain of at least 6 magnitudes (about 250 times).
• The trick is to use a guide star and wait for the asteroid to pass by. The camera's field of view when attached even to a low power eyepiece is very much less than using the telescope visually - this is perfect for high power shots of the planets, which are bright, but hopeless for locating faint objects unless very near the guide star. Of two stars I considered suitable, one (Gamma - Sagitta) was passed before I located the asteroid, and the other (Alpha - Vulpecula) only 10 minutes after I located it, giving insufficient time to judge its motion and prepare for it.

Instead we have to content ourselves to see the results others managed to achieve - please see the links below for some pictures and movies. Some of these commentaries also say how difficult the asteroid was to keep up with, so I don't feel quite so bad about failing to snap it!

External Links...

	Space Weather: Gallery of 2002 NY40 Flyby
	Sky and Telescope: Gallery of 2002 NY40 Flyby

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