[meteorite-list] City-Sized Asteroid to Pass Earth This Fall (Toutatis)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon May 3 13:09:08 2004
Message-ID: <200405031708.KAA26925_at_zagami.jpl.nasa.gov>

City-Sized Asteroid to Pass Earth This Fall
By Robert Roy Britt
space.com
03 May 2004

A minor rumor has hatched on the Internet that a large and deadly asteroid
will strike Earth this fall. Bulletin board discussions cite a 63 percent
chance of impact, while concerned readers have e-mailed SPACE.com wondering
if it is true.

Astronomers know of no such impending doom.

The rumors are likely rooted in a real event, however. On Sept. 29, 2004 an
asteroid the size of a small city will make the closest known pass of such a
very large space rock anytime this century.

While not dangerous for now, asteroid Toutatis is incredibly strange. And
scientists are quite familiar with it, having bounced radar off the tumbling
stone on previous flybys to generate computer renderings of its weird shape
and movement.

Toutatis looks something like a dumbbell hurtling awkwardly through space.
It has a crazy rotation that makes normal days impossible. Scientists can't
explain the shape or the spin, but they're eager to learn more in September
when, during the close pass, even backyard skywatchers will be able to spot
the asteroid.

Well known path

The orbit of Toutatis is pinned down with better precision than any other
large asteroid known to cross Earth's orbit. Toutatis' 4-year trek around
the Sun ranges from just inside the Earth's path out to the main asteroid
belt between Mars and Jupiter. The asteroid visits us every four years.

This fall, it will zoom by our planet within a million miles, or about four
times the distance to the Moon.

That's close by cosmic standards for an object that could cause global
devastation. Toutatis hasn't been so near since the year 1353 and won't be
that close again until 2562, NASA scientists have calculated. No other
asteroid so large is known to have come so close in the past, though
accurate tracking of space rocks is a fairly recent, high-tech skill that
still leaves wide margins of error for many objects.

Toutatis is about 2.9 miles long and 1.5 miles wide (4.6 by 2.4 kilometers).

Many smaller space rocks have passed by much closer, well inside the Moon's
orbit. Other asteroids in the size range of Toutatis have surely navigated
that window, too, but were unseen in eras when the skies were not scanned so
fully as today.

And throughout history, several asteroids and comets have hit the planet. In
fact, an object the size of Mars hit Earth when it was very young, creating
the Moon, scientists believe. But experts say the odds of a major collision
in any year are extremely small. Any other near-Earth asteroid as big as
Toutatis would almost surely be spotted decades or centuries before any
possible impact.

The prediction of any such event would make huge news rather than small
rumors.

Not dangerous, just bizarre

Asteroid Toutatis, officially numbered 4179, was discovered by French
astronomers in 1989. Researchers can't predict far enough into the future to
rule out Toutatis ever slamming into Earth, so it is listed officially as a
Potentially Hazardous Asteroid. NASA says it won't hit for at least the next
six centuries.

Meanwhile, previous close approaches have allowed intriguing radar
examinations of one of the oddest things in space.

"The vast majority of asteroids and all the planets spin about a single
axis, like a football thrown in a perfect spiral," explains Scott Hudson of
Washington State University. "But Toutatis tumbles like a flubbed pass."

The result is a lack of anything resembling a normal day or night on the
giant, pockmarked space rock.

Instead of a fixed north pole, Toutatis' axis of rotation wanders around in
two separate cycles of 5.4 and 7.3 Earth-days. Stars seen from any location
on the asteroid "would crisscross the sky, never following the same path
twice,'' Hudson says.

More study planned

Steven Ostro at NASA's Jet Propulsion Laboratory has, with Hudson, studied
Toutatis via radar on previous flybys. Ostro told SPACE.com that the
population of near-Earth asteroids -- hundreds bigger than 0.6 miles (1
kilometer) have been found in the past six years or so -- are now known to
come in "a zoo of shapes." And there are other asteroids that don't rotate
on a single, main axis.

"But Toutatis remains the only non-principal-axis rotator in the solar
system whose shape and spin state are well defined," Ostro said. More radar
observations this year will try to further refine the spin rate and orbit.

There is more to learn. For starters, scientists also can't yet say if
Toutatis has a hard surface or a thick layer of loose dirt similar to the
Moon.

"I'd very much like to know whether Toutatis' strange shape and ponderously
slow, wobbly rotation are the result of collisional breaking apart or a
gentle merger of the asteroid's two lobes, and when the responsible
phenomena happened," Ostro said.

Answers to all these big questions might require an as-yet-unplanned visit.

"Because of the radar investigations, our physical characterization of
Toutatis is the best we have for any Potentially Hazardous Asteroid," Ostro
said. "But a spacecraft rendezvous could tell us a great deal more, and I
would love to see this happen."

Looking both ways

On Sept. 29, backyard skywatchers on Earth can find Toutatis, providing they
know where to look.

Toutatis won't be visible to the unaided eye. Ordinary binoculars should be
sufficient for spotting it if the sky is clear and dark, says Alan Harris,
of the Space Science Institute in Boulder, CO.

"However, to actually know what your seeing, a small telescope would be
useful," Harris says. That will allow you to detect the slow motion of
Toutatis against background stars. The asteroid will appear as a point of
light, much like a star. It is too far for surface details to be visible.

It's also interesting to ponder what Earth would look like form Toutatis.
Ostro points out a simple relationship between the distance of Toutatis at
this close approach and the size of the Moon. Toutatis will be four times
farther than the Moon; the Moon is about ? the size of Earth.

"If you were on Toutatis and looked at Earth during the close approach, the
Earth would look as large as the Full Moon does to us."
Received on Mon 03 May 2004 01:08:50 PM PDT