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A Rocky Bicentennial

Asteroids come of age

10:44am, July 24, 2001
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Built atop a 12th-century tower in the royal palace of Sicily, the Palermo Observatory has a commanding view of domed cathedrals, an ornate opera house, and an ancient gateway with rooftop gallery and colonnade. But what really draws the eye are the craggy mountains on the horizon.

At noon, they frame the aqua-blue ocean as in a picture postcard; at dusk, they surround the Italian city like the walls of a fortress.

It was shortly after nightfall on Jan. 1, 1801, that Giuseppe Piazzi pointed his state-of-the-art telescope over those mountains at stars in the constellation Taurus. There he observed an unfamiliar point of light.

Piazzi thought the object might be a new star in the firmament, but over several nights, he found that it moved. He reasoned that the body, which he called Ceres in honor of the patron goddess of Sicily, might be a comet. But unlike a comet, Ceres had neither a tail nor a nebulous shroud.

"It has occurred to me several times that it might be something better than a comet," Piazzi told a colleague a few weeks later.

By 1807, astronomers at other observatories had found three more, similarly tantalizing bodies in the same region as Ceres–the void between the orbits of Mars and Jupiter (see box, below). Struck by their starlike appearance, British astronomer William Herschel had in 1802 begun calling these objects asteroids after aster, the Greek word for star. Today, scientists have cataloged more than 26,000 of these wandering bodies, doubling the number in just the past 2 years.

Last month, planetary scientists convened in Santa Flavia, Italy, a seaside resort near the Palermo Observatory, to commemorate Piazzi's discovery by sharing the latest news and insights about asteroids. Topics at the meeting, dubbed Asteroids 2001, included details about the structure of asteroids, their role in planet formation, the hazard that renegade asteroids pose for Earth, and speculation that ancient denizens of the asteroid belt might have transported enough water to account for our planet's oceans.

Close-up images

Earlier this year, a spacecraft landed on an asteroid for the first time (SN: 2/17/01, p. 103; Happy landing: Craft descends onto Eros). The close-up images taken by it and other craft over the past decade have confirmed what astronomers have proposed since about 1950: Despite their starry moniker, asteroids are rocky objects that may have a structure similar to the mountains that surround Palermo.

"We're getting to know asteroids as tangible objects, on the same scale and geologic sense that we know mountains on Earth," says Richard P. Binzel of the Massachusetts Institute of Technology.

Asteroids' resemblance to mountains may go well beyond their surface. As solid as a mountain appears, its interior is highly fragmented. Just as mountains may be composed of fragments, so might asteroids.

The shape of a mountain is preserved by the physical strength of the rock, friction, and Earth's gravity, notes William Bottke of the Southwest Research Institute in Boulder, Colo.

Were a mountain to be suddenly hurled into space, its shape would change dramatically, he says.

With Earth's tug dropping to near zero, the mutual gravity of the mountain's fragments would dominate. The components would slide, shift, or even roll over one another to form a new shape.

The notion that asteroids may not be solid rock dates back to the late 1970s, when Clark R. Chapman, now at the Southwest Research Institute, and Donald R. Davis of the Planetary Science Institute in Tucson showed that the energy required to break up an asteroid is much smaller than that required to completely disperse all its fragments. As a result, if an asteroid were to be jolted into pieces by a collision with another asteroid, it would be more apt to reassemble itself into a loose collection of fragments–a so-called rubble pile–than to dissipate.

Over the past few years, astronomers have found mounting evidence that many asteroids indeed are rubble piles. The pieces of that rubble can range in size from sandlike grains to kilometer-wide boulders. Asteroids "may actually be very weak structures," says Bottke.

If many asteroids are indeed an agglomeration of fragments, there would be far-reaching consequences. "If you were to hit one side of a rubble pile with another asteroid, the shock wave will have a very difficult time propagating through the entire object," says Bottke.

Diverting an asteroid from a collision course with Earth would pose a much tougher problem than envisioned by recent Hollywood movies. And if most asteroids were made of fragments when the solar system was young, models of planet formation may need to be revised.

The rubble-pile model

"The idea that asteroids are pieces of rock held together by gravity rather than physical strength is counterintuitive," notes Bottke. "You pick up a stone on Earth and it feels solid, and you figure that asteroids must be the same way."

At the Santa Flavia meeting, Derek C. Richardson and Zoë M. Leinhardt of the University of

Maryland in College Park summarized the case in favor of the rubble-pile model.

Some evidence comes from battle scars–giant craters–gouged into asteroids. Images of the asteroid 253 Mathilde, for instance, reveal at least five "honking big craters," some of which overlap, notes Richardson. Each of these craters has a diameter of about 20 kilometers–half the length of Mathilde. All that pummeling would almost certainly have shattered a solid rock, but a rubble-pile asteroid could have survived.

Simulations by Erik Asphaug of the University of California, Santa Cruz and Willy Benz of the University of Bern in Switzerland indicate that to withstand the collisions, Mathilde must already have been a rubble pile. Mathilde's low density, measured in 1997 by the NEAR (Near Earth Asteroid Rendezvous) Shoemaker craft, supports the notion that the asteroid is a porous collection of gravitationally bound chunks.

Some of the most compelling evidence in favor of the rubble-pile model comes from studies of the rotation of asteroids. The largest asteroids tend to spin slowly. At the meeting, Petr Pravec of the Astronomical Institute in Ondrejov, the Czech Republic, and Alan W. Harris of NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif., pointed out that no one has ever observed an asteroid larger than 150 meters whose rate of rotation exceeds one revolution every 2 hours.

That's just what the rubble-pile model predicts, notes Richardson. If a 150-m-wide rubble-pile asteroid were to spin any faster, the centrifugal force would overcome gravity and the fragments would fly apart.

The only asteroids that spin faster are those that are 100 m or less in diameter. "The fast-rotating, very small asteroids are apparently solid chunks held together by the strength of the rock," says John Chambers of NASA's Ames Research Center in Mountain View, Calif.

Astronomers still lack hard evidence that any asteroids are made up of fragments, cautions Chapman. "It is very difficult to determine the internal constitution of a body [in space], so no asteroid is yet proven to be a rubble pile," he notes. Moreover, some of these rocks may truly be solid objects, planetary scientists argue. Studies of the large asteroid 433 Eros, on which the NEAR Shoemaker spacecraft landed in February, suggest that it's monolithic, although it has major fractures.

Revised theories

If the rubble-pile model is an accurate depiction of most asteroids, theories of planet formation may need revision. According to the standard model, the terrestrial planets–Mercury, Venus, Earth, and Mars–arose when millimeter-size dust particles that surrounded the young sun coalesced into house-size boulders, then asteroid-size objects, and finally planets. The asteroids observed today would be leftovers from that process.

If asteroids in the early solar system consisted of loosely bound collections of smaller rocks, the rate at which they would collide and stick may be different than current estimates. However, theorists aren't sure whether rubble-pile asteroids would be more or less efficient in building a planet.

Scientists at the meeting also discussed how the fragmented structure of asteroids might resolve a case of missing mass.

To form the solar system's planets, the swirling disk of dust and gas that surrounded the newborn sun had to have just the right distribution of material–a higher density nearer the sun to make the closely spaced terrestrial planets and a lower density farther away, to produce the much more spread-out gaseous planets, including Jupiter and Saturn.

According to that prescription, the asteroid belt, which today contains only about one-twentieth the moon's mass, once must have possessed several times as much mass as Earth.

Where did all that material go?

Planetary scientists had suggested that collisions between asteroids had ground down the belt and that the material escaped as fine dust. But if asteroids are rubble piles, collisions would leave the asteroids pretty much in place and intact, simulations suggest. In the March Meteoritics and Planetary Science, Chambers and George W. Wetherill of the Carnegie Institution of Washington (D.C.) suggested an alternative solution.

They propose that several planetary bodies, some as large as Mars, formed in the asteroid belt, just as they did elsewhere in the infant solar system. In the meantime, just outside the belt, the fledgling planet that would become Jupiter grew more massive.

Researchers have established that Jupiter's gravity helps material escape from the asteroid belt–but only if the orbit of the asteroidal material has a special relationship, called a resonance, with Jupiter's orbit. The ratio of the orbital periods of the asteroidal material and of Jupiter must be a whole number. For example, material in the belt that lies 3.5 times

Earth's distance from the sun is in resonance with Jupiter because it goes around the sun twice for every time that Jupiter makes one complete revolution.

An object in such a resonance receives periodic kicks from Jupiter, causing its orbit to elongate and increasing its chances of leaving the belt. These resonances act as escape hatches, but they're extremely narrow. It seems unlikely, therefore, that Jupiter alone could deplete the belt of much material.

That's where the planetary embryos envisioned by Chambers and Wetherill would come into play. These embryos would have stirred up the asteroid belt. This action, in tandem with Jupiter's tug, could have pushed many asteroids into the escape hatches. In this way, the belt could have been cleared of a massive amount of material, the researchers calculate.

Eventually, most of the planetary embryos themselves would have drifted into the escape hatches, and the asteroid-clearing process would have slowed. It is only by chance that no planet remains in the asteroid belt today, notes Richardson in the June 21 Nature.

Taking the scenario a step further, other astronomers speculate that a few of the planetary embryos had a fateful interaction when they left the belt. One or more of them may have smacked into Earth, depositing enough water to account for our planet's oceans, the team proposes. Alessandro Morbidelli of the Observatory of the Cte d'Azur in Nice, France, and Jonathan I. Lunine of the University of Arizona in Tucson presented their team's model at the asteroid meeting.

Morbidelli and Lunine note that an embryo originating in the outer, chillier part of the asteroid belt could have incorporated a substantial amount of water within its rocky interior. Moreover, the deuterium-to-hydrogen ratio in carbon-rich meteorites–material believed to be fragments of asteroids–is similar to that found in Earth's oceans. In contrast, measurements indicate that the water inside comets, another possible source of Earth's oceans, usually has a much higher ratio, notes Lunine.

"Maybe we're here [on Earth] not because of comets but because of asteroids," says Bottke.

Crediting asteroids as the source of life-giving water contrasts with the lowly status these bodies had a century ago. At that time, photographers were just beginning to record the heavens and dubbed these orbiting rocks "vermin of the sky" because they left ugly streaks on beautiful pictures of galaxy nebula.

In one sense, asteroids might still be regarded as vermin. A wayward asteroid 1 km in diameter would cause a global climactic catastrophe if it crashed into Earth. An asteroid 10 times larger would eliminate most life on our planet.

Altering the path

Paradoxically, if the rocky body hurtling toward our planet is fragmented rather than solid, it would be harder to deter. Even if the asteroid consists of just two big chunks stuck together, it isn't easy to blow up. Detonating a bomb on one half of a two-lobed asteroid, for instance, may have no effect on the other half, notes Steve Ostro of JPL.

But if the asteroid is small enough and its projected collision is far enough in the future–a thousand years or so–there may be alternatives to explosives. In what some call "the green solution," a robotic craft might paint the surface of the asteroid with a substance that would change the rock's reflectivity or thermal conductivity. A large enough change would, over time, modify the asteroid's interaction with sunlight and alter its path.

One candidate for such treatment might be the asteroid 1950 DA. At the meeting, Jon D. Giorgini of JPL and his colleagues reported calculations that the 1-km-wide body might have as much as a one in 10,000 chance of striking Earth on St. Patrick's Day in 2880.

Lack of knowledge about the asteroid's mass, rotation, and thermal conductivity make any prediction highly uncertain, Giorgini emphasizes. But even if a collision 879 years from now were a dead certainty, there would be little public support to spend the billions needed to avert a disaster so far in the future, asserts Ostro. More likely, the asteroid would be the subject of casual study today, leaving future generations to deal with a more immediate hazard.

To make an informed decision about what asteroid-averting technique to use, Ostro says researchers or their robots will have to generate a gravity map of any rock that poses a hazard and directly probe its interior.

As a prelude to such studies, Japan next year plans to launch the spacecraft MUSES-C, which will visit a 0.5-km-wide asteroid called 1998 SF36. During its 5-month sojourn orbiting the asteroid, MUSES-C will fire bullets into it and collect fragments shooting out from the impacts. The spacecraft will carry the samples back to Earth in 2006.

The rubble-pile model may also have a bearing on the tally of asteroid families, each of which arose as chips off a different parent rock in the asteroid belt. The number of well-documented families observed today is about 20. Scientists used to think that more existed when the solar system was only a few hundred million years old but that many were destroyed in subsequent collisions.

If asteroids are rubble piles not easily shattered during collisions, however, then the number of families seen today may not be much different than what existed early on, notes Bottke. "We are probably seeing all the asteroid families that have ever been in the asteroid belt," he says.

What we dont know

After the asteroid meeting, Ostro said he was struck by "the overwhelming magnitude of what we don't know" about the residents of the asteroid belt. Other scientists marveled at the progress that researchers have made over the past 200 years, including radar images constructed by Ostro and his colleagues (see story, Bow-wowing them with radar, this issue).

"Asteroids are no longer points of light; they are real worlds to be explored and studied," said Binzel in his closing remarks at the meeting.

Later that evening, in a lush Mediterranean garden across from Palermo Observatory, Binzel led a toast. As more than 300 planetary scientists raised their glasses to Piazzi and the starlike object he identified, the same seacoast mountains that framed Piazzi's world loomed in the growing darkness.

The celestial police

The discovery of Ceres in 1801 drew the attention of a group of astronomers who called themselves the celestial police and had been looking for a missing planet. The group put their faith in a simple mathematical formula that seemed to account for the distances between the sun and each of the seven planets then known. The formula indicated that an as yet undiscovered planet should exist at 2.8 times Earth's distance from the sun. Ceres seemed to fill the bill.

But early in 1802, while studying Ceres, amateur astronomer Heinrich Olbers discovered another object, which he dubbed Pallas, nearby. Now, two objects resided where there should be only one. In quick succession, astronomers found two more bodies–Juno in 1804, and Vesta in 1807. Moreover, all these objects seemed too puny to qualify as planets. The mathematical formula, known as the Titus-Bode law, fell into disrepute and the celestial police gave up their hunt.

Today, we know that Ceres has plenty of company. But at 930 kilometers in diameter–one quarter the size of Earth's moon–Ceres remains the largest asteroid known.

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