Hey, it's not my fault they named the wayward asteroid 1950 DA....
;-D

A Rocky Bicentennial

Asteroids come of age
Ron Cowen

... 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....

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....

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....

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....

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 Côte 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....

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....

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....

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)....

References:

Chambers, J.E., and G.W. Wetherill. 2001. Planets in the asteroid belt. Meteorites & Planetary Science 36(March):381.

Giorgini, J.D., et al. 2001. Asteroid 1950 DA: Long-term prediction of its Earth close-approaches. Asteroids 2001 meeting. June. Santa Flavia, Italy.

Richardson, D.C. 2001. Giants in the asteroid belt. Nature 411(June 21):899.

Further Readings:

Cowen, R. 2001. Happy landing: Craft descends onto Eros. Science News 159(Feb. 17):103. Available at http://www.sciencenews.org/20010217/fob6.asp.

______. 2000. Nudging asteroid fragments toward Earth. Science News 158(Oct. 21):264.

Perkins, S. 1998. A surprising encounter of the NEAR kind. Science News 153(Jan. 3):7.

Peterson, I. 1999. Gauss's orbits. Science News Online (April 17). Available at http://www.sciencenews.org/sn_arc99/4_17_99/mathland.htm.

Solar system orbit diagrams and distribution graphs can be found at http://ssd.jpl.nasa.gov/orbit_diagrams.html.

Sources:

Richard P. Binzel
Massachusetts Institute of Technology
Department of EAPS
77Massachusetts Avenue
Mail Stop 54-410
Cambridge, MA 02139-4301

William Bottke
Southwest Research Institute
Department of Space Studies
1050 Walnut Street, #426
Boulder, CO 80302

John E. Chambers
NASA Ames Research Center
Mail Stop 245-3
Moffett Field, CA 94035

Andrew F. Cheng
Johns Hopkins Applied Physics Laboratory
Johns Hopkins Road
Laurel, MD 20723

Jon D. Giorgini
NASA Jet Propulsion Laboratory
MS 301-150
4800 Oak Grove Drive
Pasadena, CA 91109

Derek C. Richardson
Department of Astronomy
University of Maryland, College Park
College Park, MD 20742

From Science News, Vol. 160, No. 4, July 28, 2001, p. 61.