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Wednesday, 24 July, 2002, 15:47 GMT 16:47 UK
Asteroid threat: Ask Dr David Whitehouse
An asteroid discovered just weeks ago has become the most threatening object yet detected in space.

A preliminary orbit suggests that 2002 NT7 is on an impact course with Earth and could strike the planet on 1 February, 2019 - although the uncertainties are large.

Astronomers have given the object a rating on the so-called Palermo technical scale of threat of 0.06, making NT7 the first object to be given a positive value.

Although astronomers say the object definitely merits attention, they expect more observations to show it is not on an Earth-intersecting trajectory.

How much of a threat does the asteroid pose to Earth? When will astronomers know more about the asteroid?

BBC News Online science editor Dr David Whitehouse answered a selection of your questions on the asteroid threat. His answers appear below.

Do we currently possess the technical ability to destroy or divert this rock if, after more precise calculation, it turns out to be pointed straight at us?
Julian Morrison, Banbury, England

Scientists have many ideas about how they could try to deflect an asteroid such as exploding a nuclear device to push it to one side, or putting a long-lasting rocket engine of some sort on one side to give it a small but constant push. They might even consider blowing it up completely if they could be sure it would shatter into really small pieces that would burn up in the Earth's atmosphere. But it would be fair to say that no one knows if these methods would work in principle. However, with over a decade to do it, and with the resources that mankind could muster - if it's survival at stake then the incentive would be great.

Would the world be completely destroyed if the asteroid hit?
Mathilda Lückner, Mariedal, Sweden

Not this one. 2002 NT7 is only 2 km across which at impact velocity of 28 km per second would devastate a medium-sized country. Something about 20 km in size could devastate a continent whereas anything that measures 200 km across would pack enough energy to completely melt the Earth's surface rocks.

Assuming that further observations confirm that the object will indeed strike the Earth on the 1st of February 2019, what can realistically be done to destroy or deflect the object given the number of years we have before the impact?
Sasha Sharpley, Albufeira, Portugal

It is very unlikely that 2002 NT7 will strike the Earth. More knowledge of its orbit is bound to confirm that it will miss us by quite a long way. But if it didn't we would have to act very quickly. After all the US managed to put a man on the moon inside a decade, so we can do such things when inspired to.

Do you know which continent of the earth would be struck?
Trevisen, Johannesburg, South Africa

No, the prediction for the objects trajectory is not that accurate.

I have heard of other asteroids which were not detected years earlier having 'near misses' with Earth. Apparently, one nearly hit us a month or two ago. Would you not consider a hit from an 'undetected' asteroid a greater risk than one that may not strike for years? Why are our sensors and telescopes not able to detect all incoming objects of a certain size?
Nancy Beiman, Savannah USA

Asteroid experts say that at long last the problem of surveying the sky for threatening objects is being tackled but there are regions of the sky, from where an object could threaten us, that remain under-surveyed. It is true that given this scrutiny astronomers are finding more objects that pass close by, and realising that some objects went by unnoticed in the past. Experts say this is a good thing as it means we are more aware of what is moving around out there.

When will we receive confirmation of whether this asteroid is on course to collide with us?
Kalle Eskelinen, Espoo, Finland

This asteroid will be easily visible for the next 18 months or so, so a precise orbit for it should only be a few weeks away.

What do we know about the composition of this asteroid? Do we know if it is a solid body, or a looser collection of rocks held together by gravity? If it is on course to strike the Earth, then I assume knowledge of its composition will be crucial in devising any counter measures. Can we investigate it with either ground or space-based radar?
John Franklin, London, UK

You are right, knowledge of the density of the asteroid is crucial. The space probe that landed on asteroid Eros in 2000 showed that Eros's density was about the same as that of the Earth's crust whereas asteroid Mathide which the NEAR space probe passed in 1997 seems to be a loose rubble pile. It would be easier to blow up a rubble pile.

Assuming a close scrape rather than an impact occurs, how close would it have to be for us to feel some "peripheral" effects, and what would some of those effects be?
Rob Billeaud, Duluth, Georgia, USA

A space rock just 2 km in size would only affect us if it struck us or glanced through the atmosphere. Anything else and it would have no affect.

Is there proof that an asteroid has made impact with Earth in the past?
Joseph McLaughlin, Belfast Ireland

The evidence that the Earth's geological history has been punctuated in the past by giant impacts that wiped out a great deal of life and redirected evolution is overwhelming. Unmistakable chemical traces have been found at the geological layer of the time when the dinosaurs were wiped out 67 million years ago.

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