The unusually close flyby of one of the largest NEOs is arousing considerable public and media interest. Aside from some crazies who are (as usual) predicting the end of the world, this event has provided an opportunity for many media stories about the asteroid impact hazard and the progress of the Spaceguard Survey. Following is a selection of four such stories.
THE CLOSEST WHIZ-BY OF TOUTATIS
Sky & Telescope, 25 September 2004 By Alan M. MacRobert
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.
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 Astérix le Gaulois cartoons and comic books. 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. 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. The asteroid 4179 Toutatis turned out to be long and lumpy when Steven Ostro and colleagues got radar images of it during a flyby in December 1992. 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, 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.
MONSTER ASTEROID FLYBY EXCITES DOOMSTERS, SKYGAZERS
Agence France-Presse, 26 September 2004
PARIS (AFP) - On Wednesday, Earth will get its closest known shave this century from a major asteroid, a monster big enough to extinguish billions of lives were it ever to collide with our home.
But, in contrast to the warnings of a handful of doomsayers, scientists say the peril from this rock is beyond negligible. In fact, they say this particular risk is zero and will remain so for several centuries, thanks to an increasingly successful effort to spot space bruisers and calculate their future orbits around the Sun.
The asteroid in question is 4179 Toutatis, a behemoth some 4.6 kilometers (2.9 miles) long by 2.4 kilometers (1.5 miles) across. Spinning like an anarchic dumbbell, Toutatis will be at its closest to Earth at 1337 GMT on Wednesday, when it will be 1,549,719 kilometers (962,951 miles) away, according to the Near-Earth Object (NEO) Programme run by NASA's Jet Propulsion Laboratory (JPL).
Six months ago, panicky rumours spread on the Internet that there was little point to booking next year's summer holidays -- that NASA had got it wrong or lied, and we were all heading for The Big One. Websites run by Christian zealots and individuals in contact with aliens predicted the Second Coming of Jesus or a secret US nuclear missile strike to neutralise the asteroid.
Were Toutatis to collide with Earth, the energy released would be equivalent to tens of thousands of hydrogen bombs, kicking up dust clouds which would shield out the sunlight, plunging the planet and its inhabitants into a lethal "impact winter."
Earth's atmosphere protects us from NEOs up to a diameter of 40m (130 feet), an impact energy of about three megatonnes. Beyond that size, the news is bad. NEOs between 40m and one km (0.6 of a mile) across can inflict local damage equivalent to thousands of nuclear bombs, as evidenced by the massive explosion in Tanguska, Siberia in 1908.
The NEO that whacked into what is now Mexico 65 million years ago, ending the long reign of the dinosaurs, is estimated to have been between five and 15 kms (three and 9.5 miles) across, packing a 30- to 100-million-megatonne punch.
The good news, though, is that big advances are being made to spot the biggest threats. Spaceguard Survey, a US programme, is already two-thirds towards its goal of identifying by the end of 2008 90 percent of the estimated 1,000-1,100 asteroid NEOs that are more than one km (0.6 miles) across -- the climate killers.
That does not include, however, comets that take centuries to loop around the Sun and whose paths thus remain uncharted. Yet only a tiny number of these are likely to be any potential threat.
"Any NEO that is going to hit the Earth will swing near our planet many times before it hits, and it should be discovered by comprehensive sky searches like Spaceguard," says NASA expert David Morrison. "In almost all cases, we will either have a long lead time," he points out, "or none at all."
Asteroids are commonly thought of as chunks of rock left over from the formation of the Solar System 4.5 billion years ago. The vast majority safely trundle around the Sun in the asteroid belt between Mars and Jupiter. But the gravitational tug of giant planets such as Jupiter, as well as collisions with other asteroids, can nudge rocks out of the belt. Those whose orbits may regularly bring them close to Earth are classified as NEOs.
Discovered in 1989, Toutatis is probably one of the most studied asteroids of all because its most recent circuits have brought it so close to Earth. It takes four years to loop around the Sun, although it has a very odd, almost chaotic spin quite unseen in any other asteroid.
It has not been so close to Earth since 1353 and won't be this close again until 2562, says the specialist website space.com.
Toutatis owes its name to a trio of French astronomers, who baptised it after a Celtic god well-known in France for the comic book hero Asterix. Protected by Toutatis, Asterix and his friends fear nothing except the idea of the sky falling on their heads. On Wednesday, too, they should have nothing to worry about.
TOUTATIS: ALMOST TOO CLOSE FOR COMFORT
Astrobiology Magazine, 27 September 2004 by Leslie Mullen
Early Wednesday morning, a 5,500 million pound asteroid measuring 5 kilometers in length will pass very close to Earth.
An asteroid two to three times that diameter is credited with causing the extinction of 85 percent of the world's species, including the dinosaurs, when it hit our planet 65 million years ago.
Luckily for us, asteroid Toutatis is only a tourist, and doesn't plan to stop here. It will come within 1.5 million kilometers (960,000 miles) of Earth, or four times the Earth-moon distance. Toutatis is the largest asteroid to come that close in more than a century.
Many smaller asteroids often pass well inside the moon's orbit. The Earth is also hit continually with tiny meteors that often become "shooting stars" as they harmlessly burn up in the atmosphere. But if a rock the size of Toutatis hit, the atmosphere would do little more than slow it down a bit before it slammed to Earth. The impact would create a vast crater, and toss so much dust and vaporized minerals into the air the skies would darken. Seismic waves created by the explosion would generate tsunamis and earthquakes, and red-hot rocks falling back to Earth would ignite forest fires.
Toutatis, also known as asteroid 4179, is 4.6 kilometers (2.9 miles) long and 2.4 kilometers (1.5 miles) wide. Although Toutatis looks like a large peanut, radar images revealed it is actually composed of two rocks that are in close contact. One of the rocks is approximately twice as large as the other.
Toutatis has a strange rotation --instead of the spinning on a single axis, like the planets and most other asteroids do, Toutatis tumbles so erratically that its orientation with respect to the solar system never repeats. "The vast majority of asteroids, and all the planets, spin about a single axis, like a football thrown in a perfect spiral," says Scott Hudson of Washington State University, "but Toutatis tumbles like a flubbed pass." Toutatis's four-year orbit around the sun is also eccentric, extending from just inside the Earth's orbit to the main asteroid belt between Mars and Jupiter.
Astronomer Christian Pollas discovered Toutatis on January 4, 1989. Pollas spotted the asteroid on photographic plates taken by Alain Maury and Derral Mulholland, who had taken the photos while observing Jupiter's satellites.
Toutatis flew close by Earth in 1992 and 1996, but it hasn't come this near to us since 1353. The next time it will pass this close again will be in the year 2562. The asteroid's orbit around the sun is so eccentric that it can't be predicted with much certainty for more than a few hundred years in the future. Since researchers can't say Toutatis will never hit Earth, it is currently listed as a Potentially Hazardous Asteroid.
There is a rumor circulating on the Internet that the asteroid will strike Earth during this 2004 flyby. However, astronomers have been tracking the path of Toutatis ever since it was discovered, and they are certain it will pass safely by Earth.
Throughout history, several asteroids have hit Earth. The solar system was cluttered with asteroids while the Earth was young, and the face of the moon and other dead planetary bodies shows how frequent such impacts were. Impacts by large rocks are much less frequent today, but they can still occur.
There are thought to be more than 300,000 nearby small asteroids (asteroids about 100 meters across). Such asteroids should statistically hit Earth once every few thousand years. The most recent such asteroid strike occurred in 1908, when an asteroid measuring about 60 meters in diameter hit Russia. The "Tunguska" bolide exploded in the atmosphere and flattened about 700 square miles of Siberian forest.
Large (1 kilometer or greater) asteroids are far more rare and infrequent. There are only about 1,100 nearby large asteroids, and they are predicted to strike the Earth every half million years or so. But when these asteroids strike, they can cause catastrophic changes in the global climate. Asteroids that cause mass extinctions are thought to be 10 kilometers or greater in diameter.
The Spaceguard Survey was established to track large asteroids and comets that might pose a direct threat to Earth. So far, the Spaceguard Survey has found about half of these NEOs, and they expect to find the majority of them by 2008.
Although Toutatis will be in the far southern sky when it is closest to Earth, the asteroid is expected to brighten a few days prior to a 10th magnitude point of light visible from the Northern Hemisphere. Sky-watchers should look for it near the bright star Delta Capricorni.
Toutatis won't be visible to the naked eye, but binoculars should suffice for spotting it in the night sky. A telescope would provide the best viewing, because it would allow the viewer to detect the slow motion of Toutatis against the background stars.
ASTEROID CLOSE ENCOUNTER COMING WEDNESDAY
National Geographics News, 24 September 2004 John Roach
Tumbling through space like a fumbled football, a peanut-shaped asteroid named 4179 Toutatis is expected to pass within a million miles (1.6 million kilometers) of Earth on Wednesday. "The September 29 ... approach is the closest in this century of any known asteroid at least as big as Toutatis," said Steven Ostro, who studies asteroids at NASA's Jet Propulsion Laboratory in Pasadena, California.
Radar images of the three-mile-long (4.6-kilometer-long) asteroid suggest it could be composed of two or three space rocks held together by gravity. But to know for sure would require drilling through the object, Ostro said.
Toutatis makes an elliptical four-year trek around the sun that takes it from just inside Earth's orbital path to the asteroid belt between Mars and Jupiter.
Scientists say the asteroid also has one of the strangest rotation states yet observed in the solar system. Instead of spinning on a single axis-as do most asteroids and the planets, including our own-Toutatis wobbles around two. The asteroid rotates around one axis once every 5.4 Earth days and, in turn, rotates around the other axis once every 7.3 Earth days. As such, "the orientation of the asteroid never repeats exactly," Ostro said.
French astronomer Christian Pollas discovered Toutatis in 1989 as he was examining photographic plates of Jupiter's faint satellites. Pollas named the asteroid after a Celtic god whose name is invoked in the hugely popular French comic book series Asterix. (Toutatis is the protector of Asterix and his companions, who fear nothing except that the sky may someday fall on their heads.)
Ostro and his colleagues have studied the orbit of Toutatis more closely than any other known near-Earth object its size. The scientists say with confidence that the asteroid poses no risk of impacting Earth at least through 2562, when Toutatis will pass within 250,000 miles (400,000 kilometers) of Earth. "We can't see the future beyond that close approach, and we must know exactly how close it will be to [project] the orbit out further [in time]," Ostro said.
So is there any chance that Toutatis may hit Earth in the future? "The answer is obviously, yes," Ostro said. "There's a good chance [of Earth impact] in the next several tens of millions of years. But you could make that statement about any one of the near-Earth asteroids."
Early next month Ostro and his colleagues will make additional radar observations of Toutatis using the Arecibo telescope in Puerto Rico. Their main goal is to determine the asteroid's mass using an experiment based on the Yarkovsky effect.
The Yarkovsky effect is a force produced by the manner in which an asteroid absorbs energy from the sun and reradiates it into space as heat. "If we can model the force adequately, then we can [determine Toutatis's] mass by analyzing very precise radar measurements," Ostro said.
Measuring the asteroid's Yarkovsky effect will also help astronomers to better refine their predictions of the asteroid's orbit. With that information, scientists can calculate if, and/or when, Toutatis poses a real risk of impacting Earth.