While people worry about the risks of flying or bird
flu, an asteroid strike is far more likely.
In
Egyptian myth, Apophis was the ancient spirit of evil and destruction, a
demon that was determined to plunge the world into eternal darkness.
A fitting name, astronomers reasoned, for a menace now
hurtling towards Earth from outer space. Scientists are monitoring the
progress of a 390-metre wide asteroid discovered in 2004 that is
potentially on a collision course with our planet, and are imploring
governments to decide on a strategy for dealing with it.
NASA has estimated that an impact from Apophis, which
has an outside chance of hitting the Earth in 2036, would release more
than 100,000 times the energy released in the nuclear blast over
Hiroshima. Thousands of square kilometres would be directly affected by
the blast but the whole of the Earth would see the effects of the dust
released into the atmosphere.
And, scientists insist, there is actually very little
time left to decide. At a meeting in 2005 of experts in near-Earth
objects (NEOs) in London, scientists said it could take decades to
design, test and build the required technology to deflect the asteroid.
One expert, Monica Grady, an expert in meteorites at the Open
University, said: "It's a question of when, not if, a near Earth object
collides with Earth. Many of the smaller objects break up when they
reach the Earth's atmosphere and have no impact. However, a NEO larger
than 1km wide will collide with Earth every few hundred thousand years
(on average) and a NEO larger than 6km, which could cause mass
extinction (as they have 65 million years), will collide with Earth
every hundred million years (very approximately). We are overdue for a
big one."
Apophis had been intermittently tracked since its
discovery in June 2004 but, in December 2004, it started causing serious
concern. Projecting the orbit of the asteroid into the future,
astronomers had calculated that the odds of it hitting the Earth in 2029
were alarming. As more observations came in, the odds got higher.
Having more than 20 years warning of potential impact
might seem plenty of time. But, at December 2005 meeting, Andrea Carusi,
president of the Spaceguard Foundation, said that the time for
governments to make decisions on what to do was now, to give scientists
time to prepare mitigation missions. At the peak of concern, Apophis
asteroid was placed at four out of 10 on the Torino scale - a measure of
the threat posed by an NEO where 10 is a certain collision which could
cause a global catastrophe. This was the highest of any asteroid in
recorded history and it had a 1 in 37 chance of hitting the Earth. The
threat of a collision in 2029 was eventually ruled out at the end of
2004, but the risk of hitting the Earth in 2036 increased.
Alan Fitzsimmons, an astronomer from Queen's
University Belfast, said in 2005: "When it does pass close to us on
April 13 2029, the Earth will deflect it and change its orbit. There's a
small possibility that if it passes through a particular point in space,
the so-called keyhole, ... the Earth's gravity will change things so
that when it comes back around again in 2036, it will collide with us."
The chance of Apophis passing through the keyhole, a 600-metre patch of
space, is 1 in 5,500 based on current information, but that is vastly
higher than any other asteroid known.
There are no shortage of ideas on how to deflect
asteroids. The Advanced Concepts Team at the European Space Agency have
led the effort in designing a range of satellites and rockets to nudge
asteroids on a collision course for Earth into a different orbit.
No technology has been left unconsidered, even
potentially dangerous ideas such as nuclear powered spacecraft. "The
advantage of nuclear propulsion is a lot of power," said Prof
Fitzsimmons in 2005. "The negative thing is that ... we haven't done it
yet. Whereas with solar electric propulsion, there are several
spacecraft now that do use this technology so we're fairly confident it
would work."
The favored method is also potentially the easiest -
throwing a spacecraft at an asteroid to change its direction. ESA plans
to test this idea with its Don Quixote mission, where two satellites
will be sent to an asteroid. One of them, Hidalgo, will collide with the
asteroid at high speed while the other, Sancho, will measure the change
in the object's orbit. Decisions on the actual design of these probes
will be made in the coming months and years, with launch expected some
time in the next decade. One idea that seems to have no support from
astronomers is the use of explosives (thermonuclear or otherwise).
Prof Fitzsimmons. "If you explode too close to impact,
perhaps you'll get hit by several fragments rather than one, so you
spread out the area of damage."
In September 2005, scientists at Strathclyde and
Glasgow universities began computer simulations to work out the
feasibility of changing the directions of asteroids on a collision
course for Earth. In spring 2006, there was another opportunity for
radar observations of Apophis that helped astronomers work out possible
future orbits of the asteroid more accurately.
However, since they cannot rule out an impact with
Earth in 2036, the next chance to make better observations will not be
until 2013. NASA has argued that a final decision on what to do about
Apophis will have to be made at that stage.
"It may be a decision in 2013 whether or not to go
ahead with a full-blown mitigation mission, but we need to start
planning it before 2013," said Prof Fitzsimmons. In 2029, astronomers
will know for sure if Apophis will pose a threat in 2036. If the
worst-case scenarios turn out to be true and the Earth is not prepared,
it will be too late. "If we wait until 2029, it would seem unlikely that
you'd be able to do anything about 2036," said Mr. Yates.
The asteroid danger is real and measures should be
taken to prevent an impact with Earth. Discovered three years ago, the
asteroid Apophis will pass exceptionally close to the Earth in 2029,
only 24,000 miles away, which is where we have most of our
communications satellites. Terrestrial gravity might cause this asteroid
to leave its trajectory and collide with the Earth in 2036. There were
plans to develop a space system that could protect the Earth from a
potential asteroid impact... by 2040. Houston, we may have a problem?
Projected Path Of Apophis on approach
in 2029. White line is range of uncertainty.
Apophis orbit.
The Foresight encounter spacecraft, shown in this
artist's conception, would help scientists determine the precise orbit
of a potentially hazardous asteroid by flying beside it and sending back
location data.
Chances are that the asteroid Apophis will sail
harmlessly past Earth in the year 2036 — but just in case, space
engineers have developed a prize-winning plan for monitoring the
potential cosmic hazard.
The Foresight mission concept was the big winner in the Planetary
Society's February 2008 contest to encourage methods for monitoring
Apophis, which is currently given a 1-in-45,000 chance of hitting Earth
on April 13, 2036.
Scientists say they can't yet eliminate the threat entirely because they
don't know Apophis' orbit precisely enough. That's what prompted the
Planetary Society, a nonprofit space interest group based in California,
to offer $50,000 in prizes for proposals to "tag" the asteroid during
earlier encounters.
Experts have
been wary of asteroids since they came to the conclusion that one of
them ended the Age of the Dinosaurs 65 million years ago. Scientists
such as Stephen Hawking warn that their relatively close proximity
presents grave dangers to humankind, a point of view supported in a
number of recent books, such as William Burrows' The Survival
Imperative: Using Space to Save Earth and British astronomer royal
Martin Rees' Our Final Hour: A Scientist's Warning.
But others consider asteroids the next landscape for scientific
discovery. "We're looking at the possibilities," says Kelly Humphries, a
spokesman for NASA's Johnson Space Center. With NASA planning a
moon-exploring spacecraft, Humphries says, "Anything robust enough to go
to the moon is going to be robust enough for lots of missions."
In December, NASA astronaut Edward Lu told Space.com that plans under
study include landing on an asteroid and retrieving rock samples for
return to Earth before 2020.
And at NASA's Ames Research Center, lab chief Simon "Pete" Worden, a
longtime advocate of such exploration, has set aside $10 million for
designing small spacecraft that could visit asteroids, according to the
Jan. 19 Science magazine.
The space agency does have a few asteroid missions already planned. In
its just-released 2008 budget, NASA said it is studying a mission,
dubbed the Origins Spectral Interpretation, Resource Identification and
Security (OSIRIS) probe, to return rock samples from an asteroid.
In June, NASA will launch the Dawn mission to orbit the two largest
asteroids, Ceres and Vesta, in the main asteroid belt between Mars and
Jupiter.
And outside NASA, others also see asteroids' scientific potential.
"They are pristine in a way, vagabonds of the solar system, leftovers
from the era of the formation of the planets," says American Museum of
Natural History astrophysicist Neil deGrasse Tyson, host of PBS' NOVA
scienceNOW, and author of the new book Death by Black Hole and Other
Cosmic Quandaries. "And as for landings, they are low-hanging fruit, or
low-hanging rocks, in this case, for space exploration."
Too close for comfort?
The International Astronomical Union has given identifying numbers to
nearly 150,000 asteroids; about 5,000 are discovered every month. A mix
of sand piles, dust balls, metal-rich rocks and burned-out comets, they
mostly congregate in the asteroid belt between Mars and Jupiter.
Closer to home, NASA has, as of November, tracked 855 potentially
dangerous Near-Earth asteroids. These pass within about 30 million miles
of Earth, with a diameter of approximately 1 kilometer (.62 miles) or
larger. Astronomers regard that size as the point at which impact with
Earth would threaten civilization, says Richard Binzel of the
Massachusetts Institute of Technology.
NASA operates a program, the Spaceguard Survey, to track this "cosmic
shooting gallery," in the words of NASA scientist David Morrison, aiming
to identify 90% of the estimated 1,100 civilization-buster Near-Earth
asteroids lurking overhead before 2009. Congress has further told NASA
to catalogue 90% of all Near-Earth asteroids less than 460 feet wide by
2020, and figure out ways to deflect any headed for Earth.
Tyson says such asteroids offer an intriguing array of midway points
between the four-day trip to the moon and the six-month voyage to Mars.
"As steppingstones to Mars, (asteroids) are a really good way to learn
to leave the comfort of the Earth-Moon system," says Binzel. "There are
literally hundreds of Near-Earth asteroids that are probably easier to
reach than the moon, in terms of the propellant you need to go there and
back."
That's because asteroids have hardly any gravity. So fuel costs for
blasting out of each one's "gravity well" are minimal. Eros, a hefty
near-Earth asteroid, some 20 miles long by 8 miles across, has such
light gravity that a person could toss a baseball off its surface and
into orbit. In comparison, a rocket needs a 5,370 mph escape velocity to
leave the moon.
And NASA's plans include building a rocket capable of sending astronauts
to the moon, called Ares 1, which is scheduled to be ready for flight
testing in 2014. The rocket designers aim to overcome the Earth's 39,600
mph escape velocity and deliver a 25-ton astronaut capsule to the moon,
complete with the fuel needed to return. That capability should put a
variety of asteroids within reach.
For something a bit sooner, Morrison will describe a Near-Earth Asteroid
Trailblazing (NEAT) probe, low-cost landers designed to flit among
nearby asteroids, scouting their surfaces, at a March American Institute
of Aeronautics and Astronautics meeting.
"Landing on one would be more like docking with the international space
station than a moon landing," says astronomer Daniel Durda of the
Southwest Research Institute in Boulder, Colo. Astronauts would most
likely "swim" over the surface of an asteroid, he says, so lightly do
things fall on a typical one, which essentially has zero gravity.
A proposal to rein them in
Of course, there is also the impact threat to consider. In 1980,
geologist Luis Alvarez suggested in the journal Science that a comet or
asteroid impact ended the Age of Dinosaurs. Many researchers believe the
impact landed off the Yucatan peninsula in the Gulf of Mexico, and fears
that there will be another such mass-extinction event stock the cabinet
of modern worries.
"From a practical point of view, some time in the future, one of these
things is going to threaten Earth with an impact and we'll need to do
something about it," Durda says. So why not visit one to get the hang of
herding them? he asks.
The Harvard-Smithsonian Minor Planet Center predicts there will be more
than 5,300 "close" asteroid encounters, within 18.4 million miles, by
2040.
One of the most interesting, Apophis, grabbed headlines three years ago
because of the possibility that it would smack into Earth in 2036.
Improved observations lower the odds to a 1 in 45,000 chance, Binzel
says, "nothing to lose sleep over."
But the asteroid's close approach in 2029 to within 22,600 miles of
Earth, closer than the moon, may offer an exploration opportunity.
In 2005, Lu and another astronaut, Stanley Love, proposed a "gravity
tractor" design for deflecting Apophis and other asteroids from Earth.
"Our suggested alternative is to have the spacecraft simply hover above
the surface of the asteroid. The spacecraft tows it without physical
attachment by using gravity as a towline," they wrote in the journal
Nature.
Once in orbit and gravitationally bound to a dangerous asteroid, the
space tractor would gently fire its thrusters to slowly "tug" the
threatening rock onto a safer trajectory. Apophis, for example, would
require a one-ton tug to orbit the asteroid for a month before its 2029
close pass by Earth to put it onto a safer path.
Are they hollow or solid?
One of the great uncertainties about asteroids is what they are made of,
something that might make astronauts piloting robotic surveyors more
likely than actual manned landings. Some, like Eros, appear to be fairly
solid objects, based on their gravity, albeit intensely dust-covered
ones. The slowly-rotating Mathilde, which the NEAR-Shoemaker probe flew
past in 1997, appears three times less heavy than its size would
indicate, suggesting it may be hollow. And the asteroid Itokawa is just
a rubble pile, a surprise that explains the 2005 failure of Japan's
Hayabusa probe to land there.
As Tyson says, asteroids are thought to mostly be leftovers from the era
of planetary accretion 4.6 billion years ago in the solar system.
Weathered by eons of orbits around the sun and impacts with other space
rocks, they still offer clues to the ingredients of today's planets and
moons. "Each asteroid is a piece in the puzzle of how the planets
formed," Tyson says.
Broadly speaking, inhabitants of the main asteroid belt between Mars and
Jupiter are thought to come in three flavors: dark carbon-rich
"carbonaceous" ones that make up about 75% of the total, iron-rich
"metallic" asteroids, and fairly bright "silicaceous" asteroids built
from a mix of iron and sand.
But nobody knows for sure, Binzel says, which makes exploring asteroids
an exciting prospect. A few are likely burned-out comets plying their
retirement years in the placid depths of space. "Water or ice might be
inside them," handy for space travelers, he says. "Others might have
minerals that might be useful future resources."
(Space law still has a few wrinkles to iron out first though on mining
asteroids, cautions Frans von der Dunk of the International Institute of
Air and Space Law at Holland's Leiden University. The United Nations'
Outer Space Treaty makes nations liable for mining companies and allows
mining, he says, but stops short of defining property rights, making
gold mines in space a legally dicey pursuit.)
"Asteroids have been a low priority for too long," says Burrows, The
Survival Imperative author, who calls for long-term space colonies to
serve as a refuge for humanity if there's a catastrophic collision.
"People worry about terrorism, with good reason, but while it doesn't do
to get over-excited, there are bigger threats."
Asteroid defense gets a hearing next month at an American Association
for the Advancement of Science symposium in San Francisco. With new
telescopes in Chile and Hawaii coming online, astronomers expect
Near-Earth asteroids to turn up nearly 100 times more often than today's
rate of discovery.
Asteroid scares may become more common, as a result, as presenters
including Lu and Morrison will discuss, but the opportunities for
exploration are expected to increase, as well.
"Hundreds of exciting and strange asteroids are nearby," Binzel says.
"Certainly there is scientific interest."
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