Killer asteroids!

Last night I attended the January meeting of Knutsford SciBar. A SciBar meeting is open to anyone and held in a bar or cafe (Knutsford's meetings are at the Sports Centre) and attended by a leading scientist who talks on his topic, followed by questions from the floor. Last night's meeting was entitled 'Killer Asteroids', and presented by Professor Andy Newsome of Liverpool John Moores University.

Andy told us that most asteroids live in the asteroid belt, orbiting the Sun inside the orbit of Jupiter. Some live in clumps beyond the asteroid belt also orbiting the Sun, but either side of Jupiter's orbit having been drawn out of the asteroid belt by Jupiter's gravity. The asteroids in the asteroid belt are left over from the time the solar system was formed, and if allowed to coalesce would form a planet slightly smaller than Earth. But every time they start to coalesce along comes Jupiter and its gravity rips them apart again. These asteroids just go around the Sun in circles and are no threat to us.

There are, however, 'rogue' asteroids that orbit the Sun in eliptical orbits, cutting across the orbits of the planets, including that of the Earth. These are the ones to worry about - they have the potential to hit us, with catastrophic results.

Asteroids vary in size from dust specks to many kilometers across and thousands hit the Earth each day. Thankfully these are almost all very small and burn up in the upper atmosphere. Every day about 50 tons of material is deposited on Earth from this source. A look at the moon through a telescope will reveal a heavily pock-marked surface, evidence of asteroid collisions. The Moon is immediately adjacent to Earth and Earth is bigger with more gravitational pull, so it's safe to say Earth has had even more asteroid hits than the Moon. But the Moon has no atmosphere to burn up the smaller objects, no weathering to erase evidence of old craters, and no oceans and vegetation to hide such evidence, as Earth has.

There is plenty of evidence on Earth of asteroid impact. Here's an asteroid impact crater in Arizona:

Asteroid crater, Arizona, about 2km diameter

Meteor Crater was formed about 50,000 years ago by a 40m wide asteroid weighing several hundred thousand tons and impacting at about 26,000 miles per hour. The asteroid vaporised on impact but is believed to have comprised mostly iron, as the crater and surrounding area are rich in iron of a type not found on Earth.

Flattened trees at Tunguska, Siberia, are evidence of a 1908 air-burst asteroid

Another well known asteroid site is Tunguska in Northern Siberia. This is believed to have been caused by a piece of a comet, which comprise ice and dust, that exploded in the atmosphere due to frictional and compressive heating, and flattened trees over an area of 2,150 square kilometres.

Thankfully, the chance of impact by large asteroids is small, but of course could happen at any time at any point on Earth. Life-extinction asteroids are thankfully extremely rare. The last one to hit Earth was responsible for the extinction of the dinosaurs and allowed homo sapiens to evolve. It's not the direct effect of impact that is harmful with these events (unless you are somewhere close to it!) its the dust cloud generated. This blocks out the Sun and plunges the Earth into deep winter, freezing the oceans, and which lasts for decades. Another one of those would kill off most life on earth; bad news for us, probably good news if you're a cockroach, as they seem to be able to survive anything.

Is there anything we can do about it?

Yes and no. If we keep looking we can identify rogue asteroids. Having identified them we can track them to determine their orbits for the next 150 years or so. We can then be sure whether or not they pose us a danger. But this isn't seen by governments as a priority, so it's not funded. Scientists do use telescopes to hunt for asteroids in any spare telescope time between funded observations, and those we discover are tracked to determine if they are a danger, but this is only happening in the Northern hemisphere, and the hunt for asteroids even in the northern hemisphere is spasmodic so some will be missed.

If a rogue is identified far ahead enough ahead for us to do something about it, what could be done? Well, it would certainly concentrate the minds of world governments because if nothing is done everyone will die. Blowing it up, Hollywood style, isn't an option - the result would be like being shot by a shotgun rather than a rifle bullet as all the bits hit earth. Nearly blowing it up might work -  exploding a nuclear device adjacent to the asteroid. There are no shock waves in the vacuum of space so the asteroid should remain intact, but the side of it near to the explosion would be radically heated up. The heat radiation should alter the orbit sufficient for it to miss Earth. 

Another possibility is a 'gravitational tractor'. This is a space vehicle parked close to the asteroid and powered so the asteroid's gravity doesn't cause it to be drawn to the object. It maintains its station relative to the asteroid, which is gravitationally drawn towards the 'tractor'.

'Gravitational Tractor' attracts the asteroid towards it by the pull of gravity

Or we could paint it white! If one side was splattered in white paint (asteroids are very dark) the reflected photons from the sun would exert a tiny force on it and slowly move its orbit. The orbit doesn't have to be altered much for it to miss Earth by millions of miles, provided we have some years of notice before calculated impact.

But - we are not looking all the time. We definitely are not looking everywhere (we concentrate on the ecliptic, for instance, as that's where the biggest threat is), and we miss about 1/3 of the sky by not looking in the southern hemisphere. And anything coming from the direction of the sun will always be in daylight so we wouldn't see that.

And then there are comets! These strange beasts originate on the very edge of the solar system in the Oort Cloud about 0.8 light years out from the Sun, thousands of times further out than Pluto's orbit. The temperature there is pretty much absolute zero, and the gravitational pull of the Sun is so weak that comets can be disturbed from orbit by objects outside the solar system and commence an extremely elongated narrow orbit in towards the Sun. Comets comprise ice and dust and most break up as they near the Sun; having been used to living in absolute zero they don't like being subjected to several thousand degrees as they near the Sun. Also, the massive gravity of Jupiter acts like a solar system vacuum cleaner and causes many comets to break up be absorbed by the gas giant.

If a Comet is headed for us it's coming in very, very fast, and it may well be coming from the direction of, and very close to, the Sun so we may not see it until it's very close. These Oort cloud comets should not be confused with the 'tame' ones like Halley's which have been captured in close-in circular or nearly circular solar orbits with periods of tens of years (which is why we keep on seeing them). Oort cloud comets, if they survive their first pass through the solar system, won't return for millions of years.

There was much more that Andy told us last night than I have recounted here, and it was all fascinating, not least his explanation as to why the media gave the impression recently that astronomers are rubbish at predicting whether an asteroid will actually hit Earth. The newspapers reported "10% chance of Earth impact, say astronomers". Then "15%". Then, wow, one in five chance! (20%). Then.... zero. It's going to miss us, implying the astronomers' earlier estimates were completely wrong because it was never going to hit us at all.

What was actually happening was that the astronomers were refining their predictions with time and getting ever more accurate results. The initial measurements gave an area of possible positions for the asteroid in space on reaching the proximity of the Earth which included not only the earth but also 90% of empty space adjacent to Earth (so a 10% chance of it hitting Earth). This 'target area' was subsequently refined down by ever more accurate readings of the object's trajectory to a smaller and smaller target area but an area that included the earth. Finally the accuracy became good enough to show the Earth was actually outside the now much smaller target area, so the risk instantly fell to zero.

Accurate reporting of this refinement of defining where the asteroid would track, eventually enabling astronomers to confirm it would pass rather than hit the Earth, doesn't always sell the most number of papers, but the price in this case may be a public perception that astronomers don't understand asteroid prediction so what's the point in funding it?

Thank you very much Professor Andy Newsome, and Knutsford SciBar. I'm looking forward to next month's meeting.

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