[meteorite-list] The Threat is Out There (Asteroid 99942 Apophis)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Sun, 3 Dec 2006 22:01:13 -0800 (PST)
Message-ID: <200612040601.WAA20383_at_zagami.jpl.nasa.gov>

http://www.popularmechanics.com/science/air_space/4201569.html

The Threat is Out There

More than 100,000 asteroids hurtle past our planet. But only one - that we
know of - may hit us in the next 30 years.

BY David Noland
Popular Mechanics
December 2006 issue

Friday the 13th of April 2029 could be a very unlucky day for planet
Earth. At 4:36 am Greenwich Mean Time, a 25-million-ton, 820-ft.-wide
asteroid called 99942 Apophis will slice across the orbit of the moon
and barrel toward Earth at more than 28,000 mph. The huge pockmarked
rock, two-thirds the size of Devils Tower in Wyoming, will pack the
energy of 65,000 Hiroshima bombs- enough to wipe out a small country or
kick up an 800-ft. tsunami.

On this day, however, Apophis is not expected to live up to its
namesake, the ancient Egyptian god of darkness and destruction.
Scientists are 99.7 percent certain it will pass at a distance of 18,800
to 20,800 miles. In astronomical terms, 20,000 miles is a mere stone's
throw, shorter than a round-trip flight from New York to Melbourne,
Australia, and well inside the orbits of Earth's many geosynchronous
communications satellites. For a couple of hours after dusk, people in
Europe, Africa and western Asia will see what looks like a medium-bright
star creeping westward through the constellation of Cancer, making
Apophis the first asteroid in human history to be clearly visible to the
naked eye. And then it will be gone, having vanished into the dark
vastness of space. We will have dodged a cosmic bullet.

Maybe. Scientists calculate that if Apophis passes at a distance of
exactly 18,893 miles, it will go through a "gravitational keyhole." This
small region in space - only about a half mile wide, or twice the diameter
of the asteroid itself - is where Earth's gravity would perturb Apophis in
just the wrong way, causing it to enter an orbit seven-sixths as long as
Earth's. In other words, the planet will be squarely in the crosshairs
for a potentially catastrophic asteroid impact precisely seven years
later, on April 13, 2036.

Radar and optical tracking during Apophis's fly-by last summer put the
odds of the asteroid passing through the keyhole at about 45,000-to-1.
"People have a hard time reasoning with low-probability/high-consequence
risks," says Michael DeKay of the Center for Risk Perception and
Communication at Carnegie Mellon University. "Some people say, 'Why
bother, it's not really going to happen.' But others say that when the
potential consequences are so serious, even a tiny risk is unacceptable."

Former astronaut Rusty Schweickart, now 71, knows a thing or two about
objects flying through space, having been one himself during a spacewalk
on the Apollo 9 mission in 1969. Through the B612 Foundation, which he
co-founded in 2001, Schweickart has been prodding NASA to do something
about Apophis - and soon. "We need to act," he says. "If we blow this,
it'll be criminal."

If the dice do land the wrong way in 2029, Apophis would have to be
deflected by some 5000 miles to miss the Earth in 2036. Hollywood
notwithstanding, that's a feat far beyond any current human technology.
The fanciful mission in the 1998 movie Armageddon - to drill a hole more
than 800 ft. into an asteroid and detonate a nuclear bomb inside it - is
about as technically feasible as time travel. In reality, after April
13, 2029, there would be little we could do but plot the precise impact
point and start evacuating people.

According to projections, an Apophis impact would occur somewhere along
a curving 30-mile-wide swath stretching across Russia, the Pacific
Ocean, Central America and on into the Atlantic. Managua, Nicaragua; San
Jose, Costa Rica; and Caracas, Venezuela, all would be in line for
near-direct hits and complete destruction. The most likely target,
though, is several thousand miles off the West Coast, where Apophis
would create a 5-mile-wide, 9000-ft.-deep "crater" in the water. The
collapse of that transient water crater would trigger tsunamis that
would hammer California with an hour-long fusillade of 50-ft. waves.

BUT DON'T EVACUATE just yet. Although we can't force Apophis to miss the
Earth after 2029, we have the technology to nudge it slightly off course
well before then, causing it to miss the keyhole in the first place.
According to NASA, a simple 1-ton "kinetic energy impactor" spacecraft
thumping into Apophis at 5000 mph would do the trick. We already have a
template for such a mission: NASA's Deep Impact space probe - named after
another 1998 cosmic-collision movie - slammed into the comet Tempel 1 in
2005 to gather data about the composition of its surface. Alternatively,
an ion-drive-powered "gravity tractor" spacecraft could hover above
Apophis and use its own tiny gravity to gently pull the asteroid off
course.

In 2005, Schweickart urged NASA administrator Michael Griffin to start
planning a mission to land a radio transponder on Apophis. Tracking data
from the device would almost certainly confirm that the asteroid won't
hit the keyhole in 2029, allowing everyone on Earth to breathe a
collective sigh of relief. But if it didn't, there still would be time
to design and launch a deflection mission, a project that Schweickart
estimates could take as long as 12 years. It would need to be completed
by about 2026 to allow enough time for a spacecraft's tiny nudge to take
effect.

NASA, however, is taking a wait-and-see attitude. An analysis by Steven
Chesley of the Near Earth Object program at the Jet Propulsion
Laboratory (JPL) in Pasadena, Calif., concludes that we can safely sit
tight until 2013. That's when Apophis swings by Earth in prime position
for tracking by the 1000-ft.-dia. radio telescope in Arecibo, Puerto
Rico. This data could also rule out a keyhole hit in 2029. But if it
doesn't, the transponder mission and, if necessary, a last-resort
deflection mission could still be launched in time, according to
Chesley. "There's no rush right now," he says. "But if it's still
serious by 2014, we need to start designing real missions."

IN 1998, CONGRESS mandated NASA to find and track near-Earth
asteroids at least 1 kilometer in diameter. The resulting Spaceguard
Survey has detected, at last count, about 75 percent of the 1100
estimated to be out there. (Although Apophis was nearly 2500 ft. short
of the size criterion, it was found serendipitously during the search
process.) Thankfully, none of the giants so far discovered is a threat
to Earth. "But any one of those couple of hundred we haven't found yet
could be headed toward us right now," says former astronaut Tom Jones,
an asteroid-search consultant for NASA and a Popular Mechanics editorial
adviser. The space agency plans to expand Spaceguard to include
asteroids down to 140 meters in diameter - less than half the size of
Apophis, but still big enough to do serious damage. It has already
detected more than 4000 of these; NASA estimates approximately 100,000
exist.

Predicting asteroid orbits can be a messy business, as the history of
tracking Apophis in its 323-day orbit demonstrates. Astronomers at
Arizona's Kitt Peak National Observatory discovered the asteroid in June
2004. It was six months before additional sightings - many made by
amateurs using backyard telescopes - triggered alarm bells at JPL, home to
the Sentry asteroid-impact monitoring system, a computer that predicts
the orbits of near-Earth asteroids based on astronomical observations.
Sentry's impact predictions then grew more ominous by the day. On Dec.
27, 2004, the odds of a 2029 impact reached 2.7 percent - a figure that
stirred great excitement in the small world of asteroid chasers. Apophis
vaulted to an unprecedented rating of 4 on the Torino Impact Hazard
Scale, a 10-step, color-coded index of asteroid and comet threat levels.

But the commotion was short-lived. When previously overlooked
observations were fed into the computer, it spit out reassuring news:
Apophis would not hit the Earth in 2029 after all, though it wouldn't
miss by much. Oh, and there was one other thing: that troublesome keyhole.

The small size of the gravitational keyhole - just 2000 ft. in diameter - is
both a blessing and a curse. On the one hand, it wouldn't take much to
nudge Apophis outside it. Calculations suggest that if we change
Apophis's velocity by a mere 0.0001 mph - about 31 in. per day - in three
years its orbit would be deflected by more than a mile, a piddling
amount, but enough to miss the keyhole. That's easily within the
capabilities of a gravity tractor or kinetic energy impactor. On the
other hand, with a target so minuscule, predicting precisely where
Apophis will pass in relation to the keyhole becomes, well, a
hit-or-miss proposition. Current orbit projections for 2029 have a
margin of error - orbital scientists call it the error ellipse - of about
2000 miles. As data rolls in, the error ellipse will shrink
considerably. But if the keyhole stubbornly stays within it, NASA may
have to reduce the ellipse to a mile or less before it knows for sure
whether Apophis will hit the bull's-eye. Otherwise, a mission risks
inadvertently nudging Apophis into the keyhole instead of away from it.

Can we predict Apophis's orbit to the submile level far enough in
advance to launch a deflection mission? That level of forecasting
accuracy would require, in addition to a transponder, a vastly more
complex orbital calculation model than the one used today. It would have
to include calculations for such minute effects as solar radiation,
relativity and the gravitational pulls of small nearby asteroids, none
of which are fully accounted for in the current model.

And then there's the wild card of asteroid orbital calculations: the
Yarkovsky Effect. This small but steady force occurs when an asteroid
radiates more heat from one side than the other. As an asteroid rotates
away from the sun, the heat that has accumulated on its surface is shed
into space, giving it a slight push in the other direction. An asteroid
called 6489 Golevka, twice the size of Apophis, has been pushed about 10
miles off course by this effect in the past 15 years. How Apophis will
be influenced over the next 23 years is anybody's guess. At the moment
we have no clue about its spin direction or axis, or even its shape - all
necessary parameters for estimating the effect.

IF APOPHIS IS INDEED headed for the gravitational keyhole, ground
observations won't be able to confirm it until at least 2021. By that
time, it may be too late to do anything about it. Considering what's at
stake - Chesley estimates that an Apophis-size asteroid impact would cost
$400 billion in infrastructure damage alone - it seems prudent to start
taking steps to deal with Apophis long before we know whether those
steps will eventually prove necessary. When do we start? Or,
alternatively, at what point do we just cross our fingers and hope it
misses? When the odds are 10-to-1 against it? A thousand-to-1? A million?

When NASA does discover a potentially threatening asteroid like Apophis,
it has no mandate to decide whether, when or how to take action. "We're
not in the mitigation business," Chesley says. A workshop to discuss
general asteroid-defense options last June was NASA's first official
baby step in that direction.

If NASA eventually does get the nod - and more important, the budget - from
Congress, the obvious first move would be a reconnaissance mission to
Apophis. Schweickart estimates that "even gold-plated at JPL," a
transponder-equipped gravity tractor could be launched for $250 million.
Ironically, that's almost precisely the cost of making the
cosmic-collision movies Armageddon and Deep Impact. If Hollywood can
pony up a quarter of a billion in the name of defending our planet, why
can't Congress?
Received on Mon 04 Dec 2006 01:01:13 AM PST