[meteorite-list] Near-Earth Asteroids Could Be Stepping Stones To Mars

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 13 Feb 2007 09:16:58 -0800 (PST)
Message-ID: <200702131716.JAA02521_at_zagami.jpl.nasa.gov>

http://www.usatoday.com/tech/science/space/2007-02-12-asteroid_x.htm

Near-Earth asteroids could be 'steppingstones to Mars'
By Dan Vergano
USA TODAY
February 13, 2007

Asteroids are big hunks of space dust and rock that will eventually
smack into Earth and end life as we know it. Or they represent the new
frontier of space exploration.

Or both. It depends on how you look at it.

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."
Received on Tue 13 Feb 2007 12:16:58 PM PST