快猫短视频

FOUR MONTHS AGO, astronomers issued a chilling warning. A rogue lump of rock
the size of an office block appeared to be on a collision course with Earth.
Expected time of arrival: Saturday 21 September 2030. The chances of it hitting
were a slender 1 in 500. But if it did, this object could explode in the
atmosphere with an energy of several atomic bombs. Goodbye New York perhaps. Or
Paris. Or Beijing.

That time, at least, it was a false alarm. After a little more homework to
pin down the object鈥檚 course, astronomers found it would pass by with several
million kilometres to spare. It was relegated to a long list of cosmic
might-have-beens, and everyone breathed easy again.

Sooner or later, though, one of these warnings will stick. There are plenty
of killer comets and asteroids out there, and it鈥檚 only a matter of time before
one of them really does head our way. Faced with such a catastrophe, what will
we do?

Despite a stream of studies of the threat from asteroids and comets, and even
hearings on the hazard in the US Congress, no one yet knows. But a new report
commissioned by the British government aims to change that. It presses for
significant international efforts to identify and understand cosmic hazards, and
work out how to knock them out of harm鈥檚 way. The report goes much further than
its predecessors in calling for urgent international action. 鈥淚t鈥檚 a potentially
momentous development,鈥� says Clark Chapman, a planetary scientist at the
Southwest Research Institute in Boulder, Colorado.

There is already a host of space missions in the pipeline that should help us
understand our enemy. Last month, NASA concluded its mission to an intriguing
asteroid called Eros. And the agency is now cooking up an ambitious plan to drop
half a tonne of copper on a comet. When the results from these and a slew of
other projects come in, we will be far better placed to thwart the invaders from
space.

The new British report comes from the Task Force on Potentially Hazardous
Near Earth Objects (NEOs), a committee of experts commissioned by science
minister Lord Sainsbury to investigate the risks to Earth of impacts by rocky
asteroids and icy comets. The team was asked to assess how likely such
collisions are and what their effect would be, and recommend a plan of action.
Its report, completed in August 2000, includes a census of near-Earth asteroids
and predicts the effects of an impact depending on the asteroid鈥檚 size
(see Table).

For instance, asteroids measuring 10 kilometres or more threaten mass
extinctions similar to the one that wiped out the dinosaurs 65 million years
ago. Studies of craters on the Moon and estimates of the numbers of large
asteroids in space suggest these bruisers come our way only once every 100
million years or so. But objects don鈥檛 need to be that big to do serious harm.
The NEO report stresses the dangers of bodies just 50 or 60 metres across, which
arrive once every couple of centuries. When an asteroid this size exploded in
the sky above the Tunguska river valley in Siberia on 30 June 1908, the blast
wave flattened around 2000 square kilometres of forest. A similar strike above
London would wipe out everything within the M25 ring road.

If astronomers saw one of these heading towards us tomorrow, what would we
do? Probably not much, says Brian Marsden, director of the Minor Planet Center
in Cambridge, Massachusetts, which gathers data on asteroids and comets. Current
telescopes wouldn鈥檛 be able to spot an object that size until it had come within
a few million kilometres of Earth鈥攊f they saw it at all. That sounds
fairly remote, until you consider that it would be zooming towards us at 20 or
30 kilometres per second. 鈥淭he hit would likely occur in a matter of days,鈥� says
Marsden.

Marsden thinks it would take at least a day to identify and report the
object, and confirm a collision course with any certainty. At that point, radar
observations鈥攊f they could be arranged in time鈥攃ould pinpoint the
impact site, and other observations could gauge the asteroid鈥檚 size. If the
expected impact site was inhabited, it might be possible to evacuate the people,
or send up a missile to try a last-ditch and probably very risky attempt to
deflect the asteroid.

But any likely measure would be half-baked, according to Marsden, because no
action plan is in place. There aren鈥檛 even any official channels for reporting a
dangerous asteroid. 鈥淲e at the Minor Planet Center are likely to get the
information first, but nobody鈥檚 ever told me who to call if we find something,鈥�
says Marsden. 鈥淚f any nation is going to do anything about it, it鈥檚 probably the
US. So do I contact someone in the State Department, or in the Defense
Department? It has never been made clear to me.鈥�

Breathing space

What if the threatening object were much bigger? Because large asteroids are
bright and easily spotted at a distance, we鈥檇 almost certainly have a few
decades to play with, according to Jonathan Tate, an air defence specialist in
the British army and founder of Spaceguard UK, a non-profit organisation in
Wiltshire that exists to provide information about near-Earth objects. Take the
one about 1.5 kilometres across that sparked a false alarm in 1997. In that
case, astronomers predicted that it would hit the Earth in 2028, before they did
their sums again and worked out that it would in fact miss by a safe margin.
鈥淪uch warning times might be up to 50 or even more years, giving us plenty of
time to come up with mitigation measures,鈥� Tate says. 鈥淎fter all, the Americans
went from a standing start to the Moon in less than 10 years.鈥�

But who would take responsibility? Space agencies? The military? Or perhaps
the United Nations. Secretary-General Kofi Annan said in 1998 that international
cooperation at the UN does extend into outer space. But according to Tate, NASA
is the only organisation currently capable of hatching a plan, possibly along
with the European and Russian space agencies.

The most likely strategy, he says, is to use nuclear weapons to shove an
asteroid out of the way. But it鈥檚 no sure-fire solution. 鈥淭here is a significant
danger that such action will break the object apart, turning a cannon ball into
a cluster bomb,鈥� he says. Several small blasts might nudge the body away more
safely. Then there are political hurdles. International treaties ban weapons of
mass destruction in space, although Tate suspects no one will quibble when the
time comes: 鈥淚f global or regional destruction is the alternative, who is going
to sue?鈥�

快猫短视频s have discussed alternatives to nuclear weapons, such as placing
solar sails on the asteroid. Energy from the Sun would then chaperone it into a
safe orbit. It might even be possible to use a smaller asteroid as a 鈥渂illiard
ball鈥� to knock an asteroid off-course. But none of these strategies has come
even close to being tested, says Tate: 鈥淎s a British general said before the
Gulf War, 鈥業f you haven鈥檛 practised it you can鈥檛 do it鈥�.鈥�

The first step is to know your enemy. Using a missile to zap an asteroid can
have hugely different effects, depending on its composition, according to
computer simulations reported in 1998 by Erik Asphaug of the University of
California at Santa Cruz. For their simulations, Asphaug and his colleagues used
a peanut-shaped asteroid called Castalia, which is about 1.6 kilometres wide and
crosses the Earth鈥檚 orbit. They assumed Castalia could have one of three
structures: a solid lump of rock, two rocks joined by a pile of rubble, or a
鈥渃osmic beanbag鈥濃�攁 simple pile of rubble held together by gravity. The
team then simulated a spacecraft mission that dumped a lump of basalt the size
of a house onto the virtual Castalia, releasing about as much energy as was
generated by the Hiroshima atomic bomb.

The impact deflected the solid Castalia most, and although it was fractured,
most of the pieces stayed together. With the twin-lump Castalia, one side
shattered but the rubble pile in the centre cushioned the other side from
damage. The real problems came with the beanbag asteroid. Its rubbly interior
absorbed the shock and there was little damage or deflection (Nature,
vol 393, p 437). The beanbag would be extremely difficult to shift off-course,
Asphaug concludes.

But whether the average asteroid is helpfully solid or a troublesome cosmic
beanbag remains a mystery. 鈥淲e don鈥檛 know what they are really made of or what
they鈥檙e like inside,鈥� says Asphaug, who believes this makes any deflection
strategy near impossible. 鈥淚t鈥檚 like designing a car when you don鈥檛 know whether
it is going to be made of metal, rock or sand. It鈥檚 a wonderful academic
exercise, but you wouldn鈥檛 want to stake too much upon getting behind the
飞丑别别濒.鈥�

The hints so far suggest there is, in fact, no such thing as a typical
asteroid. In June 1997, a NASA mission originally called the Near Earth Asteroid
Rendezvous, and later renamed NEAR Shoemaker, flew past a fat asteroid more than
60 kilometres wide called Mathilde. Then in February 2000, mission controllers
successfully trapped NEAR Shoemaker in the puny gravity of Eros, a potato-shaped
chunk of rock 35 kilometres across. It circled this little world for a year
before landing on its surface last month.

The probe鈥檚 observations threw up plenty of surprises. First, they suggested
Mathilde has a low but uneven density, hinting that it鈥檚 a rubble pile.
鈥淢athilde appears to be much more difficult to disrupt than anybody could have
imagined,鈥� says Asphaug. But Eros told a different story. It has a uniform
density鈥攁bout the same as that of the Earth鈥檚 crust鈥攕uggesting it鈥檚
a solid body. Eros also has a network of grooves and ridges that suggest it is
criss-crossed with fractures.

Knowing about surface features like these would be essential to any attempts
to deflect an asteroid, says Chapman. But even armed with close-up snaps like
those from NEAR, he thinks it would still be difficult to come up with a
fail-safe plan: 鈥淗ow cohesive would Eros be in the face of sudden 鈥榩ushes鈥�
against it, to ensure the whole body moves rather than comes apart in pieces? I
don鈥檛 think we know the answer to that yet.鈥�

Still, astronomers are optimistic that we鈥檒l soon understand asteroids and
comets better. Next year, a Japanese mission called MUSES-C will send a
spacecraft to an asteroid called 1998 SF36. In 2005, it will survey the asteroid
and hop across its surface to gather samples at three sites, then cruise back to
Earth to deliver the samples in 2007.

A NASA spacecraft is already hot on the heels of a comet called Wild 2.
Launched in February 1999, the STARDUST craft will reach Wild 2 in January 2004,
soak up some of the comet鈥檚 dust, and return with it to Earth in 2006. The
European Space Agency also has designs on a comet. Its Rosetta mission will
blast off to Comet Wirtanen in 2003, and check out two asteroids on the way.
鈥淭here is some chance that existing and planned missions to asteroids or comets
will reveal such profoundly unexpected aspects of these bodies that we will have
to seriously alter our perspectives on how to deal with potential impacts,鈥� says
Chapman.

Perhaps some of the most intriguing results will come from NASA鈥檚 Deep Impact
project, due to take off in 2004 for Comet Tempel 1. On its arrival the
following year, the craft will drop a half-tonne copper cylinder onto the comet
to blast a crater the size of a football field in the surface. As the debris
settles, cameras will focus on the comet鈥檚 innards, thought to be similar to the
pristine material that built the Solar System.

The copper missile鈥檚 impact will give the comet a nudge that should change
its orbit by a few hundred kilometres, says Michael A鈥橦earn, who will head the
mission鈥檚 science team. It鈥檚 such a tiny shift that astronomers won鈥檛 be able to
measure the change until the comet has completed an orbit of the Sun. It falls
far short of the force needed to deflect a comet from a collision course with
Earth. Nonetheless, the mission is sure to tell us something about controlling
rogue comets. 鈥淚 think it falls short of a realistic 鈥榯rial run鈥�,鈥� says Chapman.
鈥淏ut anything that teaches us about the physical nature of these bodies and how
they respond to active `touching鈥� can help.鈥�

Further optimism comes from the British NEO report. It presses the government
to 鈥渦rgently鈥� liaise with other governments and the International Astronomical
Union to set up a forum for open discussion of near-Earth objects along the
lines of the Intergovernmental Panel on Climate Change. This panel would look
into the science of asteroids and comets as well as impact hazards, and design
action plans that would have some official force.

Marsden is particularly pleased that the report recommends hunting down all
near-Earth asteroids bigger than 300 metres across, and monitoring their orbits
over the next few decades. As part of the effort, Britain would seek partners in
Europe to build an advanced 3-metre telescope in the southern hemisphere. This
is more ambitious than anything planned by NASA, which will only be looking for
asteroids more than 1 kilometre wide.

Marsden suspects that NASA limited its search plans so that it could promise
Congress it would find 90 per cent of its targets by 2008. Widening the search
to include asteroids 0.5 kilometres wide would have meant finding a much less
impressive 60 per cent. 鈥淚鈥檝e not been entirely happy with NASA鈥檚 view on this,鈥�
Marsden says. 鈥淚 think the UK task force鈥檚 report gave a much more balanced view
of the matter in considering also the smaller events.鈥�

The British report recommends sending an army of relatively cheap
mini-spacecraft to some near-Earth objects to find out what they鈥檙e made of. It
also proposes putting funding for the Minor Planet Center onto a firm
international footing, and calls for detailed studies of ways to deflect killer
asteroids.

鈥淭he Brits have placed themselves in a leadership position for serious
consideration of the asteroid threat,鈥� says Asphaug. His only quibble is that
the report is not ambitious enough when it comes to spacecraft missions. He
thinks we should send more sophisticated radar-equipped satellites to drop
landers fitted with seismographs. The spacecraft could blast an asteroid with
grenades and watch the aftermath for a week, then hop off to spy on another one.
It鈥檚 an aim that鈥檚 achievable now, he says.

Above all, says Asphaug, we have to find out what鈥檚 out there. 鈥淵ou need a
highly instrumented rendezvous to at least a handful of NEOs鈥攕mall
100-metre nuggets of carbonaceous rock, large 10-kilometre rubble piles, dead
comets, what have you,鈥� says Asphaug. 鈥淏ecause that鈥檚 just the point: what have
测辞耻?鈥�