[meteorite-list] Incoming!

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:16:35 2004
Message-ID: <200308221906.MAA07910_at_zagami.jpl.nasa.gov>

http://www.popsci.com/popsci/aviation/article/0,12543,473545-1,00.html

Incoming!

The killer-asteroid movies are almost forgotten, but
the threat is real and unquantified. Will it fall to
hobbyists to save the world?

by Gregory Mone
Popular Science
September 2003

Its name is 1950DA, it's the size of a small mountain,
and it's headed for Earth. According to one grim
scenario, 1950DA will hit its target-most likely water,
since there is more water than land on our planet-and
plunge to the seabed in a fraction of a second. When the
asteroid meets the ocean floor, it will explode,
excavating a crater 11 miles wide. A column of water
and debris will shoot a few miles into the sky-to the
height of a low-flying jetliner. Then skyscraper-high
walls of water will head for shore, eventually breaking
in the shallows and flooding the coast. The rest you
know, if you saw the weepy 1998 asteroid movie Deep
Impact.

Worse things may already have happened: One theory
credits an 11-kilometer-wide asteroid with roasting
dinosaurs alive 65 million years ago. The enormous
impact sent debris flying back into space-some of it
halfway to the Moon. When the asteroid bits reentered
the atmosphere, the heat that was generated
flash-baked plant and animal life. (Had that not
happened, mind you, we probably wouldn't be here
today.) 1950DA is minuscule by comparison, though
even a still smaller asteroid could take out an entire
city with a direct hit. And make no mistake, there are
plenty of space rocks out there; one missed Earth by
only 75,000 miles in June 2002-and wasn't spotted
until after it had whizzed by.

Now for the good news. First, 1950DA is 877 years
away and a 300-to-1 long shot for actually striking the
planet and doing the damage in the scenario above,
which is part of a simulation recently created by
planetary scientists Steven Ward and Erik Asphaug of
the University of California, Santa Cruz. And although
there are more 1950DAs out there-maybe bigger,
maybe due to arrive much sooner-the search for
potential killer asteroids is at least under way, though
sorely underfunded. Furthermore, a small band of
scientists, many of them fueled more by passion than
by actual government grants, is working on novel
methods to deal with asteroids before they get too
close to be diverted or destroyed. (The time spans
involved give a new definition to advance thinking: As
the foldout on the previous pages shows, some
diversion operations would require centuries to work.)

NASA is more than halfway through a search for
asteroids and comets that come within striking distance
of Earth-called "near Earth objects," or NEOs-and are
wider than a kilometer. Experts calculate that the
chance of an object that size hitting Earth in the next
century is only one in several thousand, but the result
would be global havoc.

After astronomers spot an asteroid in their telescopes,
they use radar tracking to get a more precise picture of
where it's headed, how fast it's moving, and whether
its orbit around the Sun will intersect with Earth's
orbit. Before 1950DA's predicted encounter with Earth
in 2880, the asteroid will swing around the Sun almost
400 times, while Earth will complete 876 orbits.

Of the 600-plus large NEOs tracked thus far, only
1950DA poses any threat at all. But at this stage of the
search, there are an estimated 400 potential global
killers left to find, not to mention over a million
hard-to-spot smaller asteroids capable of regional
destruction. (A rock that exploded over Tunguska,
Siberia, in 1908 leveled a thousand square miles of
remote forest; it was a mere 60 meters wide.) Making
the tallying work more tricky are a few long-period
comets, which only swing by every few hundred years
and are much more difficult to track.

The search is only the beginning, and as Jay Melosh, a
planetary scientist at the University of Arizona points
out, "The question is, If we find one with our name on it,
can we do anything?"

NASA's search effort receives a paltry $3 million per
year, just a fraction of the $25 million that NASA
earmarked last year to fix the doors on the Kennedy
Space Center's vehicle-assembly building. "I'd like to
see more money spent," says David Morrison of
NASA's Ames Research Center. But as yet, there's no
official program either to build or to test
asteroid-deflection technologies. If Earth gets whacked
by a significant asteroid within the next few centuries,
survivors might find themselves marveling that their
ancestors, with tools in hand, did little to prevent a
cataclysm.

The asteroid interception and diversion experts are
mostly hobbyists-planetary scientists, astronomers
and engineers who think up these strategies on their
own time. But the ideas are plentiful: As our gatefold
shows, the path from detection to mitigation could
include low-thrust engines, solar sails, standoff nuclear
explosions and more.

Melosh, for example, has been focusing on the use of
solar collectors, which could concentrate sunlight on an
asteroid, vaporizing enough material to gradually nudge
the rock off course. Until recently, this idea amounted to
little more than a series of conceptual sketches
bolstered by calculations. Then Melosh learned of
L'Garde, a California company that makes smaller
versions of the exact collectors he needs. With a few
adjustments, he says, his strategy could be put to work
tomorrow.

It would take years for sunlight to redirect an asteroid,
however, so advance notice is absolutely critical. Ditto
for tactics that would involve painting an incoming
asteroid or covering its surface with white glass
beads-both approaches would make the asteroid more
reflective, increasing the tiny reaction forces produced
when sunlight is radiated back into space. Over several
centuries, the cumulative effect of these slight forces
would alter the asteroid's velocity and cause a miss.
"You let the Sun do the work," says Jon Giorgini of
NASA's Jet Propulsion Laboratory (JPL), one of the
scientists who projected 1950DA's orbit out to 2880.

"The key," says Donald Yeomans, who heads the NEO
Program Office at JPL, "is you've got to find them early.
If they're on an approach trajectory and you've [only]
got a few months, there's not much you can do."

Given ample time, an effective defense strategy might
require that a probe be launched to study the structure
of the incoming body. Not all asteroids are the solid
objects familiar from museum meteorite displays. Some
are porous, others are collections of rubble loosely held
together by gravity. Exploding a nuclear bomb nearby
might nudge a dense asteroid off track, but it could
break a brittle one into pieces, effectively multiplying
the threat by creating smaller but still lethal rocks.

Each threat, in other words, requires an adjustment of
strategy. "You need to find out [the asteroid's]
density, find out its mass, its porosity, its composition,
because all these things are important if you want to
effect some kind of mitigation or deflection," says
Yeomans. One approach uses so-called kinetic kill
vehicles-numerous small spacecraft placed in an
asteroid's path. Hit by hit, they slow it down enough
that Earth will pass through the projected collision
point before the asteroid does.


Also possible is a dock-and-push approach, in which a
spacecraft parks on the asteroid's surface, fires its
thrusters, and alters the trajectory. Robert Gold of the
Applied Physics Laboratory at Johns Hopkins
University says the probe he designed for NASA's
Near Earth Asteroid Rendezvous mission-the first to
land on an asteroid-could divert a
hundred-meter-wide object, which is large enough to
wipe out the Washington Beltway. "If you found [the
asteroid] 30 years in advance, that little 6-foot by
6-foot spacecraft could provide enough impulse to
make it miss Earth," he says.

Still, as Yeomans warns, none of this will work without
advance notice. Currently, NASA expects to find only
about 90 percent of the NEOs large enough to cause
global catastrophes. The remaining 10 percent are too
dark for today's telescopes, or too difficult to
distinguish from the many asteroids that orbit
harmlessly in the solar system's main asteroid belt
between Mars and Jupiter. Andrea Milani, of the Space
Mechanics group at the University of Pisa in Italy,
wants to find the hidden large NEOs and extend the
survey down to objects as small as 300 meters across.
Both goals require a new generation of ground-based
telescopes capable of detecting fainter objects, and
possibly space-based observatories to peer into
obscure areas of the solar system. The ground-based,
8.4-meter Large-aperture Synoptic Survey Telescope
is one possibility, but its $120 million price is the
equivalent of 40 years of the current search budget.

As the search expands, at least one group is
recommending a practice deflection. The B612
Foundation, which takes its name from the asteroid in
Antoine de Saint-Exupéry's The Little Prince, is
designing a mission to alter an asteroid's orbit using a
low-thrust, possibly nuclear-electric spacecraft. The
Houston-based organization, a group of scientists and
astronauts that includes space station crewmember Ed
Lu, won't be able to fund the mission on its own, so
B612 plans to present the plan to NASA and other
space agencies, and push for a test by 2015. Clark
Chapman of B612 won't name an exact price for the
mission, but he estimates it will fall somewhere
between the cost of a typi-cal interplanetary probe
mission and NASA's several-billion-dollar Jupiter Icy
Moons Orbiter project.

Morrison applauds the civilian effort, but also calls for
more from NASA. "We need an actual program, even if
it's a modest beginning, to demonstrate that we can do
some of this," he says. Morrison doubts that any
concerted effort will be made until a specific threat is
discovered-something like 1950DA but closer to home.
In the meantime, he and fellow asteroid spotters will
keep at it.

"It's one of the few areas where astronomy can actually
have a practical impact on us, on Earth," he says.
"Saving the world, that's not such a bad idea."

Gregory Mone, a PopSci assistant editor, is the author
of The Wages of Genius.
Received on Fri 22 Aug 2003 03:06:32 PM PDT