NASA Science News

Most Halloween visitors are no more alarming than, say, an earnest-looking 3-foot vampire with an eye on the candy tray — easily warded off with a bag of M&M’s. But this year there’s a more sinister Trick-or-Treater on our planet’s doorstep: the near-Earth asteroid 4179 Toutatis.
At 0430 Universal Time on Oct. 31st, the 5-km long space rock passed less than 29 lunar distances from Earth. There was no danger of a collision, say scientists, but astronomers are nevertheless keeping a watchful eye on Toutatis. It is one of the largest known "Potentially Hazardous Asteroids" (PHAs) and its orbit is inclined less than half-a-degree from Earth’s. No other kilometer-sized PHA moves around the Sun in an orbit so nearly coplanar with our own.
A group of astronomers led by Steve Ostro (JPL) and Scott Hudson (Washington State University) is monitoring Toutatis this week using NASA’s Goldstone planetary radar in the Mojave desert. They will bounce radio signals off the fast-moving asteroid to learn more about the path it follows through space and the peculiar way it spins.
Unlike planets and the vast majority of asteroids, which rotate around a single pole, Toutatis has two spin axes. It twirls around one with a period of 5.4 Earth-days and the other once every 7.3 days. The result is an asteroid that travels through space tumbling like a badly thrown football.
"Our goal for 2000 is to double the radar time base (currently 1992-1996), thereby dramatically fine-tuning our knowledge of the object’s extraordinary spin state as well as its orbit," says Ostro.
"The 2000 close approach is more distant than our encounters with Toutatis in 1992 and 1996, when the asteroid passed 9 and 14 lunar distances from Earth, respectively," added Jon Giorgini of JPL’s Solar System Dynamics group. "The radar echoes from this apparition are going to be weaker, but the ranging and velocity measurements will still be of great interest."
That’s because Toutatis follows an elliptical orbit around the Sun that just won’t hold still. Orbital resonances and close encounters with Venus, Earth, Mars and Jupiter constantly alter the shape of the asteroid’s path as it loops through the solar system every 3.98 years.
"Toutatis has a 3:1 orbital resonance with Jupiter and a 1:4 resonance with Earth," explains Giorgini. "Thus, every third time Toutatis orbits the Sun, it returns to the same spot relative to Jupiter. Every 4th time Earth goes around the Sun, it and Toutatis end up in the same relative position as well. Up until about 1922, Toutatis also had numerous close-approaches to Venus and Mars." Such gravitational encounters, which nudge the asteroid from its intended path, are orbit-altering experiences for Toutatis.
Contrary to some press reports in the late 1990’s, the variability of Toutatis’s orbit does not render the asteroid’s path unpredictable. "Actually, we know Toutatis’s orbit better than that of any other near-Earth asteroid," says Giorgini. "The radar data we collected during close approaches in the 1990’s let us usefully predict its trajectory over a few hundred years, from about 1300-2500 AD. We’re safe from collisions for at least several centuries." Clearly, though, continued monitoring is warranted.
View a 3D model of Toutatis’s orbit – from JPL:
"For our Goldstone radar observations in November we’re predicting an initial range uncertainty of plus or minus 600 meters," continued Giorgini. "If we acquire Toutatis much outside that expected uncertainty level, it could indicate the effect of unmodeled forces acting on the asteroid over the last 4 years– for example, perturbations from other asteroids. (Toutatis’s orbit extends from just inside Earth’s to a point deep within the asteroid belt between Mars and Jupiter.)
Astronomers with backyard telescopes can see Toutatis for themselves, but not this week. On Oct. 31st the estimated visual magnitude of the space rock was +28. Even big professional telescopes have trouble with objects that dim. Toutatis seems so dark because the sunlit side of the asteroid is facing away from our planet as it glides by almost directly between the Earth and the Sun.
Fortunately for asteroid-watchers, Toutatis will brighten rapidly in the days ahead. By the end of November it will become a 14.5th magnitude object in the constellation Leo, well within reach of 8-inch or larger telescopes in the northern hemisphere.
If you miss Toutatis this time around, don’t worry. Four years from now it will be back and brighter than ever. On Sept. 29, 2004, Toutatis will pass just 4 lunar distances from Earth — that’s closer than any other known PHA will come during the next 30 years. Toutatis will be so bright — 9th magnitude near closest approach — that skywatchers will be able to easily see it through binoculars. As viewed from Toutatis in 2004, the Earth will appear to be the size of the Full Moon.
"2004 should be a great year for radar observations of Toutatis," continued Giorgini. Radar maps will discern features just a few tens of meters across, substantially improving on radar images from 1992 and 1996. Data from those epochs revealed Toutatis as a strange-looking, peanut-shaped object that tumbles erratically through space. In fact, it may be two asteroids that stuck together when they gently collided in the distant past. Giorgini and his colleagues are hopeful that high-resolution radar data four years hence will reveal even more about this strange asteroid.
In the meantime, trick-or-treaters might ponder a new sort of costume for Halloween 2004 — something scarier than a vampire that might cause unsuspecting grownups to drop the candy-tray altogether. Imagine going from door to door as 4179 Toutatis! ("I like the idea of dressing up as Toutatis," says JPL’s Steve Ostro, "especially if the kids imitate the object’s spin state when you answer the door.") True, dressing up as lumpy gray rock isn’t glamorous, but ask any dinosaur, it sure is scary!
Editor’s Note: The reference to M&M’s in the introduction to this story should not be construed as an official NASA endorsement of that product.