[meteorite-list] The Closest Whiz-by of Toutatis

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon Sep 20 15:58:56 2004
Message-ID: <200409201957.MAA22336_at_zagami.jpl.nasa.gov>

http://skyandtelescope.com/observing/objects/asteroids/article_1336_1.asp

The Closest Whiz-by of Toutatis
By Alan M. MacRobert
Sky & Telescope

Maybe a nervous friend, knowing of your interest in astronomy, has
already asked you about the latest e-mail chain letter: a killer
asteroid being tracked by astronomers has a 63 percent chance of
smashing into Earth this fall, and the authorities, of course, are
hushing it up.

You can tell your friend not to believe chain letters. But there's a
grain of reality here that will offer a fine observing challenge for
telescope users in late September, especially in the Southern Hemisphere.

Toutatis heads south across Capricornus, in view from most of the world,
just before its closest approach to Earth. These finder charts (above
and below) show its exact path for five nights as seen from various
sites worldwide; choose the city closest to you, or interpolate. Each
track segment begins at local dusk and ends when Toutatis sets, with
times and dates given in Universal Time.

http://SkyandTelescope.com/howto/basics/article_259_1.asp#UT

Click on each chart for an expanded view. Stars are plotted to 11th
magnitude; Toutatis brightens from magnitude 10.7 to 9.7 between
the 21st and 25th. S&T chart from Tycho-2 Catalogue and Minor
Planet Center data.

The object is genuine: it's the near-Earth asteroid 4179 Toutatis,
discovered in 1989 in France and named for the ancient Gallic/Celtic god
by whom characters swear loud oaths in the French Ast?rix le Gaulois
cartoons and comic books. Amateur astronomers followed the fast-moving
little body during two previous flybys of Earth (Sky & Telescope:
December 1992, page 673, and December 1996, page 76). In 2004 Toutatis
is about to make its closest predicted pass yet, scooting a very safe
1.5 million kilometers - four Earth-Moon distances - south of Earth on
September 29th. Indeed, after taking planetary perturbations into
account, astronomers have determined that this year's flyby distance is
Toutatis's closest approach at least as far back as 1353 and the closest
until at least 2562.

Toutatis will be at its brightest, about magnitude 8.9, on September
28th. Unfortunately for most readers, it will then be crossing the
far-southern constellation Telescopium; Southern Hemisphere observers
will find it nicely placed high in the evening sky. A few days earlier,
however, Toutatis will be as bright as 10th magnitude while visible
practically worldwide - in southern Capricornus, due south in
midevening. Anyone with at least a 4-inch telescope will have a good
shot at it.

The problem will be locating the speedy little point. When closest,
Toutatis will race across the sky at 30? per day, making it impractical
for us to print sufficiently detailed finder charts for that time
covering more than a few hours. Instead, the charts above cover
September 21st through 25th Universal Time
<http://SkyandTelescope.com/howto/basics/article_259_1.asp#UT>
(including the evenings of September 20th through 24th in the Americas),
when the asteroid is moving more slowly and is still visible from most
of the world. During these five days it should brighten from about
magnitude 10.7 to 9.7, as it nears from 8 million to 4 million kilometers.

On September 29th, Toutatis makes its closest approach to Earth, at
which time it happens to pass within 1? of Alpha Centauri for observers
in Australia and New Zealand. At this time of year Alpha spends most of
the night below the south celestial pole, so a good southern horizon
will be needed. Toutatis will be moving at its greatest apparent speed,
nearly 1.5' per minute of time, making it fairly easy to identify the
10th-magnitude visitor in a telescope each time it goes by a background
star. The neighborly visit does come at the time of full Moon and all
its glare. It's fortunate for would-be observers that Toutatis and the
Moon are well apart in the sky.

There's another complication. The asteroid will pass so near to us that
your location on Earth will affect its apparent position on the
background sky. This topocentric parallax may shift the asteroid's track
as much as 0.1? away from the standard, geocentric track calculated on
the charted dates for a hypothetical observer at the center of the
Earth. That's partway across a low-power eyepiece's field of view. The
tracks on the map show the exact paths as seen during evening hours from
several cities around the world. Fortunately, Toutatis's rapid motion
against the stars - from 3 to 10 arcseconds per minute on the map's
dates - should betray it in the eyepiece soon enough.

If you want to catch Toutatis on other dates, or create an exact chart
for your location (by plotting on a detailed star atlas such as
Uranometria 2000.0 or the Millennium Star Atlas), you can get a list of
positions from the IAU Minor Planet Center's ephemeris service
<http://cfa-www.harvard.edu/iau/MPEph/MPEph.html>. (Instructions: Use
Internet Explorer only. Choose "Return Ephemerides." For name, enter
Toutatis. Enter the starting date for your list of positions in the form
2004 09 22 0633 if you want, for instance, 6:33 Universal Time Sept. 22,
2004. Be sure to enter east longitude, not west; if you know your west
longitude, subtract it from 360? to get east. (For example, 90? W in the
middle of North America is 270? E.) Enter a north latitude as +, a south
latitude as -. Under Format, choose None.)

A Dizzy Tumbler

Toutatis follows a four-year, eccentric orbit that carries it from the
asteroid belt to just inside the orbit of Earth. It travels around the
Sun in nearly the same plane as Earth, so close flybys are frequent.
Nevertheless, radar ranging by Steven Ostro (Jet Propulsion Laboratory),
Scott Hudson, <http://www.eecs.wsu.edu/~hudson/Research/Asteroids/4179/>
and others has pinned down Toutatis's orbit so precisely that we know
for certain that it will not hit Earth during at least the next 65
years, and that the chance of a collision is extremely minuscule for at
least several centuries after that.

Ostro's radar observations have also mapped its three-dimensional shape
in detail. Toutatis is an extremely irregular, almost bowling-pin-shaped
body 4.6 by 2.4 by 1.9 km in size. It may be two or three large rocks
resting against each other with rubble filling the chinks. Ostro's group
also discovered that Toutatis is slowly tumbling in a complex manner. It
rotates with a period of 5.41 days around its long axis, which in turn
precesses around another axis every 7.35 days on average. As a result,
the Sun and stars careen around the Toutatian skies in patterns that
never repeat.

This kind of tumbling is expected after asteroids collide. All minor
planets probably behaved this way in the past, but centrifugal force and
internal friction have "worked" most of them into simple rotation,
around a fixed axis aligned with the asteroid's shortest physical axis.
Most asteroids rotate with periods of about 5 to 20 hours. But Toutatis
turns so slowly that the "damping time" for its weird gyrations is
greater than the age of the solar system. Ostro and his colleague Scott
Hudson have said, "The rotation of Toutatis is a remarkable,
well-preserved relic of the collision-related evolution of an asteroid."

Ostro's group plans to carry out more radar observations during
September's flyby. They expect to measure the distance between Toutatis
and the Arecibo radio telescope in Puerto Rico to within a few tens of
meters. That should be ample precision for measuring a 6-km displacement
expected in the asteroid's distance due to the Yarkovsky effect, which
arises from the absorption and reradiation of solar heat (S&T: April
2004, page 22). Because the size and shape of Toutatis are so well
known, "we will constrain the mass by means of the Yarkovsky
perturbation since the first radar observations" in 1992, says Ostro.
Thus, "We will get bounds on its density."

The radar ranging will also yield an even better orbit, probably adding
more centuries of not having to worry about a collision.

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Sky & Telescope senior editor Alan MacRobert plans to observe Toutatis
from his backyard observatory in the suburbs of Boston.
Received on Mon 20 Sep 2004 03:57:17 PM PDT