by Ron Baalke

Flagstaff, AZ--The re-discovery of Hermes started early on October 15th by Brian Skiff of the Lowell Observatory Near-Earth-Object Search (LONEOS). Not seen since 1937, asteroid 1937 UB (Hermes) continues to astonish and excite astronomers worldwide. Further observations revealed late yesterday that Hermes is actually two objects--called a binary--circling around one another while about to pass by Earth again.

"This re-sighting of Hermes is the Holy Grail of near-Earth asteroid discovery," said Edward Bowell, LONEOS Director. "Its orbit has been better calculated and observers have confirmed its re-appearance and also shown its binary nature... well, an asteroid's return just does not become more profound than this."

The binary object was some 19 million miles out at the time of re-discovery last Wednesday, nearly 66 years after it was first seen. Hermes, which poses no threat to Earth, will make its closest approach on November 4th. By then it will be 4 million miles away and bright enough for amateurs to see using backyard telescopes.

The final crew of the Space Shuttle Columbia was memorialized in the cosmos as seven asteroids orbiting the sun between Mars and Jupiter were named in their honor today.

The Space Shuttle Columbia crew, Commander Rick Husband; pilot William McCool; Mission Specialists Michael Anderson, Kalpana Chawla, David Brown, Laurel Clark; and Israeli payload specialist Ilan Ramon, will have celestial memorials, easily found from Earth.

The names, proposed by NASA's Jet Propulsion Laboratory (JPL), Pasadena, Calif., were recently approved by the International Astronomical Union. The official clearinghouse of asteroid data, the Smithsonian Astrophysical Observatory's Minor Planet Center, released the dedication today.

The seven asteroids were discovered at the Palomar Observatory near San Diego on the nights of July 19-21, 2001, by former JPL astronomer Eleanor F. Helin. She retired in July 2002. The seven asteroids range in diameter from five to seven kilometers (3.1 to 4.3 miles). The Palomar Observatory is owned and operated by the California Institute of Technology, Pasadena.

"Asteroids have been around for billions of years and will remain for billions more," said Dr. Raymond Bambery, Principal Investigator of JPL's Near-Earth Asteroid Tracking Project. "I like to think that in the years, decades and millennia ahead people will look to the heavens, locate these seven celestial sentinels and remember the sacrifice made by the Columbia astronauts," he said.

The 28th and final flight of Columbia (STS-107) was a 16-day mission dedicated to research in physical, life and space sciences. The seven astronauts aboard Columbia worked 24 hours a day, in two alternating shifts, successfully conducting approximately 80 separate experiments. On February 1, 2003, the Columbia and its crew were lost over the western United States during the spacecraft's re-entry into Earth's atmosphere.

Asteroids are rocky fragments left over from the formation of the solar system about 4.6 billion years ago. Most of the known asteroids orbit the sun in a belt between Mars and Jupiter. Scientists think there are probably millions of asteroids, ranging in size from less than one kilometer (.62 mile) wide to hundreds of kilometers across.

More than 100,000 asteroids have been detected since the first was discovered back on January 1, 1801. Ceres, the first asteroid discovered, is also the largest at about 933 kilometers (580 miles) in diameter.

If an asteroid crashes into the Earth, it is likely to splash down somewhere in the oceans that cover 70 percent of the planet's surface. Huge tsunami waves, spreading out from the impact site like the ripples from a rock tossed into a pond, would inundate heavily populated coastal areas. A computer simulation of an asteroid impact tsunami developed by scientists at the University of California, Santa Cruz, shows waves as high as 400 feet sweeping onto the Atlantic Coast of the United States.

The researchers based their simulation on a real asteroid known to be on course for a close encounter with Earth eight centuries from now. Steven Ward, a researcher at the Institute of Geophysics and Planetary Physics at UCSC, and Erik Asphaug, an associate professor of Earth sciences, report their findings in the June issue of the Geophysical Journal International.

March 16, 2880, is the day the asteroid known as 1950 DA, a huge rock two-thirds of a mile in diameter, is due to swing so close to Earth it could slam into the Atlantic Ocean at 38,000 miles per hour. The probability of a direct hit is pretty small, but over the long timescales of Earth's history, asteroids this size and larger have periodically hammered the planet, sometimes with calamitous effects. The so-called K/T impact, for example, ended the age of the dinosaurs 65 million years ago.

"From a geologic perspective, events like this have happened many times in the past. Asteroids the size of 1950 DA have probably struck the Earth about 600 times since the age of the dinosaurs," Ward said.

Ward and Asphaug's study is part of a general effort to conduct a rational assessment of asteroid impact hazards. Asphaug, who organized a NASA-sponsored scientific workshop on asteroids last year, noted that asteroid risks are interesting because the probabilities are so small while the potential consequences are enormous. Furthermore, the laws of orbital mechanics make it possible for scientists to predict an impact if they are able to detect the asteroid in advance.

"It's like knowing the exact time when Mount Shasta will erupt," Asphaug said. "The way to deal with any natural hazard is to improve our knowledge base, so we can turn the kind of human fear that gets played on in the movies into something that we have a handle on."

Although the probability of an impact from 1950 DA is only about 0.3 percent, it is the only asteroid yet detected that scientists cannot entirely dismiss as a threat. A team of scientists led by researchers at NASA's Jet Propulsion Laboratory reported on the probability of 1950 DA crossing paths with the Earth in the April 5, 2002, issue of the journal Science.

"It's a low threat, actually a bit lower than the threat of being hit by an as-yet-undiscovered asteroid in the same size range over the same period of time, but it provided a good representative scenario for us to analyze," Asphaug said.

For the simulation, the researchers chose an impact site consistent with the orientation of the Earth at the time of the predicted encounter: in the Atlantic Ocean about 360 miles from the U.S. coast. Ward summarized the results as follows:

The 60,000-megaton blast of the impact vaporizes the asteroid and blows a cavity in the ocean 11 miles across and all the way down to the seafloor, which is about 3 miles deep at that point. The blast even excavates some of the seafloor. Water then rushes back in to fill the cavity, and a ring of waves spreads out in all directions. The impact creates tsunami waves of all frequencies and wavelengths, with a peak wavelength about the same as the diameter of the cavity. Because lower-frequency waves travel faster than waves with higher frequencies, the initial impulse spreads out into a series of waves.

"In the movies they show one big wave, but you actually end up with dozens of waves. The first ones to arrive are pretty small, and they gradually increase in height, arriving at intervals of 3 or 4 minutes," Ward said.

The waves propagate all through the Atlantic Ocean and the Caribbean. The waves decay as they travel, so coastal areas closest to the impact get hit by the largest waves. Two hours after impact, 400-foot waves reach beaches from Cape Cod to Cape Hatteras, and by four hours after impact the entire East Coast has experienced waves at least 200 feet high, Ward said. It takes 8 hours for the waves to reach Europe, where they come ashore at heights of about 30 to 50 feet.

Computer simulations not only give scientists a better handle on the potential hazards of asteroid impacts, they can also help researchers interpret the geologic evidence of past events, Ward said. Geologists have found evidence of past asteroid impact tsunamis in the form of inland sediment deposits and disturbed sediment layers in the seafloor that correlate with craters, meteorite fragments, and other impact evidence. An important feature of Ward's simulation is that it enabled him to calculate the speed of the water flows created by the tsunami at the bottom of the ocean--more than 3 feet per second out to distances of several hundred miles from the impact.

"That's like a raging river, so as these waves cross the ocean they're going to stir up the seafloor, eroding sediments on the slopes of seamounts, and we may be able to identify more places where this has happened," Ward said.

He added that the waves may also destabilize undersea slopes, causing landslides that could trigger secondary tsunamis. Ward has also done computer simulations of tsunamis generated by submarine landslides. He showed, for example, that the collapse of an unstable volcanic slope in the Canary Islands could send a massive tsunami toward the U.S. East Coast.

A tsunami warning system has been established for the Pacific Ocean involving an international effort to evaluate earthquakes for their potential to generate tsunamis. Ward said that asteroid impact tsunamis could also be incorporated into such a system.

"Tsunamis travel fast, but the ocean is very big, so even if a small or moderate-sized asteroid comes out of nowhere you could still have several hours of advance warning before the tsunami reaches land," he said. "We have a pretty good handle on the size of the waves that would be generated if we can estimate the size of the asteroid."

Planetary scientists, meanwhile, are getting a better handle on the risks of asteroid impacts. A NASA-led campaign to detect large asteroids in near-Earth orbits is about half way toward its goal of detecting 90 percent of those larger than 1 kilometer in diameter (the size of 1950 DA) by 2008.

"Until we detect all the big ones and can predict their orbits, we could be struck without warning," said Asphaug. "With the ongoing search campaigns, we'll probably be able to sound the 'all clear' by 2030 for 90 percent of the impacts that could trigger a global catastrophe."

Rogue comets visiting the inner solar system for the first time, however, may never be detected very long in advance. Smaller asteroids that can still cause major tsunami damage may also go undetected.

"Those are risks we may just have to live with," Asphaug said.

W. H. Ryan, New Mexico Institute of Mining and Technology and New Mexico Highlands University, reports that photometric observations of (3782) Celle were obtained during 2001 Sept. 18.3-19.5, 2002 Dec. 9.3-11.4, 2003 Jan. 4.1-6.2, and Jan. 26.1-Feb. 1.3 UT at the 1.8-m Vatican Advanced Technology Telescope as part of a larger survey by a collaboration that also includes E. V. Ryan, C. T. Martinez, and L. Stewart. Analysis of these data reveals a normal rotational lightcurve (period 3.84 hr, amplitude 0.10-0.15 mag) superimposed onto deeper attenuation events lasting for about 2.5-3.5 hr that varied in amplitude from 0.15-0.3 mag. The onset of these events was observed on 2003 Jan. 5.25, 27.34, 28.10, 30.39, and 31.15. An event in progress was also observed on 2002 Dec. 9.4. The attenuations were of two distinct types that can clearly be identified as primary and secondary occultations/ eclipses, similar to those that have been previously observed in known minor-planet binary systems (Pravec et al. 2000, Icarus 146, 190). In particular, the 2003 Jan. 5.25 event was indicative of a complete occultation of the secondary component of such a binary system. The data are interpreted as clear evidence that (3782) is actually an asynchronous binary system with a primary-to-secondary diameter ratio of 0.42 ± 0.02 and an orbital period of 36.57 ± 0.03 hr. Since (3782) has been associated with the Vesta family both dynamically (Zappala et al. 1984, A.J. 107, 772) and spectroscopically (Bus and Binzel 2002, Icarus 158, 146), this is the first identified binary amongst the so-called 'Vesta chips'.

An international team of researchers led by Patrick Michel (Observatoire de la Côte dAzur CNRS, Nice) have carried out simulations of asteroid collisions. For the first time, such simulations have made it possible to provide information about the internal structure of asteroids and, in particular, have shown that the parent bodies from which asteroid families have originated must have been fragmented (and non-monolithic) bodies or stacked rocks. The formation of an asteroid family results from the break-up of such a body, which creates hundreds of thousands of fragments, certain of which could become dangerous asteroids and meteorites. These findings also show that the impact energy during a collision is highly dependent upon the internal structure of the target; this information is very useful for the development of a strategy of defense against the threat of an impact with the Earth. The researchers' results are published in the February 6, 2003, issue of Nature and are featured on the journal's cover.

In the asteroid belt, which is located between Mars and Jupiter, asteroid families are concentrated groups of small bodies that share the same spectral properties. More than 20 families have been identified, each family believed to be fragments resulting from the break-up of a large parent body in a regime where gravity, more than the material strength of the rock, is the key factor (*). The actual size and velocity distributions of the family members provide the main constraint for testing our understanding of the break-up process in this gravitational context. A new asteroid family, which bears the name of its largest member, Karin, was recently identified and studied. It is the youngest family discovered to date, and appears to have resulted from a collision around 5 million years ago. This family provides a unique opportunity to study a collisional outcome that is relatively unaffected by phenomena such as collisional erosion and the dynamic diffusion of fragments, which, over time, alter the properties resulting directly from the collision.

Patrick Michel of the Cassini Laboratory (Observatoire de la Côte dAzur CNRS) and two of his colleagues from the Universities of Bern (Switzerland) and Maryland (USA), have developed numerical simulations of collisions with the aim of determining the classes of events that make it possible to reproduce the main characteristics of the Karin family. As the results depend to a large degree on the internal structure of the parent body, they were able to show that this family must have resulted from the break-up of a body that was originally full of fracture and/or empty zones, rather than a purely monolithic body. Their findings moreover indicate that all the members of this family are aggregates formed by the gravitational re-accumulation of smaller fragments, and that certain of them could have been ejected on trajectories that cross the Earth's trajectory. Since those families that are already known and the oldest families share similar properties, the authors suggest that they are likely to have had a similar history.

This information concerning the internal structure of large asteroids also has consequences for the impact energy that would destroy them. This is useful not only to estimate the lifetime of these objects in the asteroid belt, but also in order to develop strategies that aim to redirect such a potentially dangerous asteroid.

Reference:
P. Michel, W. Benz & D.C. Richardson, Disruption of fragmented parent bodies as the origin of asteroid families, Nature Vol. 421, 608-611, 2003.

Astronomers with the Lincoln Near Earth Asteroid Research (LINEAR) project have observed the first object in the solar system (Venus and Mercury excluded) having an orbit entirely within that of Earth -- albeit barely. The minor planet dubbed 2003 CP20 has a 235-day orbit that stretches out to 0.980 astronomical unit from the Sun. At our own closest approach to the Sun, we only get within 0.983 a.u. The new asteroid is estimated to be only 2 kilometers across. And though it poses no risk to Earth (with a minimum possible distance of 28 million kilometers), the object can pass within 0.05 astronomical unit (7.5 million kilometers) of Venus. More information will be known as astronomers refine the asteroid's orbit.

In the last few years, researchers have discovered more than 500 objects in the Kuiper belt, a gigantic outer ring in the outskirts of the solar system, beyond the orbit of Neptune. Of these, seven so far have turned out to be binaries--two objects that orbit each other. The surprise is that these binaries all seem to be pairs of widely separated objects of similar size. This is surprising because more familiar pairings, such as the Earth/moon system, tend to be unequal in size and/or rather close together.

To account for these oddities, scientists from the California Institute of Technology have devised a theory of Kuiper belt binary formation. Their work is published in the December 12 issue of the journal Nature.

According to Re'em Sari, a senior research fellow at Caltech, the theory will be tested in the near future as additional observations of Kuiper belt objects are obtained and additional binaries are discovered. The other authors of the paper are Peter Goldreich, DuBridge Professor of Astrophysics and Planetary Physics at Caltech; and Yoram Lithwick, now a postdoc at UC Berkeley.

"The binaries we are more familiar with, like the Earth/moon system, resulted from collisions that ejected material," says Sari. "That material coalesced to form the smaller body. Then the interaction between the spin of the larger body and the orbit of the smaller body caused them to move farther and farther apart."

"This doesn't work for the Kuiper belt binaries," Sari says. "They are too far away from each other to have ever had enough spin for this effect to take place." The members of the seven binaries are about 100 kilometers in radius, but 10,000 to 100,000 kilometers from each other. Thus their separations are 100 to 1,000 times their radii. By contrast, Earth is about 400,000 kilometers from the moon, and about 6,000 kilometers in radius. Even at a distance of 60 times the radius of Earth, the tidal mechanism works only because the moon is so much less massive than Earth.

Sari and his colleagues think the explanation is that the Kuiper belt bodies tend to get closer together as time goes on -- exactly the reverse of the situation with the planets and their satellites, where the separations tend to increase. "The Earth/moon system evolves 'inside-out', but the Kuiper belt binaries evolved 'outside-in,'" explains Sari.

Individual objects in the Kuiper belt are thought to have formed in the early solar system by accretion of smaller objects. The region where the gravitational influence of a body dominates over the tidal forces of the sun is known as its Hill sphere. For a 100-kilometer body located in the Kuiper belt, this extends to about a million kilometers. Large bodies can accidentally pass through one another's Hill spheres. Such encounters last a couple of centuries and, if no additional process is involved, the "transient binary" dissolves, and the two objects continue on separate orbits around the sun. The transient binary must lose energy to become bound. The researchers estimate that in about 1 in 300 encounters, a third large body would have absorbed some of the energy and left a bound binary. An additional mechanism for energy loss is gravitational interaction with the sea of small bodies from which the large bodies were accreting. This interaction slows down the large bodies. Once in every 30 encounters, they slowed down sufficiently to become bound.

Starting with a binary of large separation a million kilometers apart, continued interaction with the sea of small objects would have led to additional loss of energy, tightening the binary. The time required for the formation of individual objects is sufficient for a binary orbit to shrink all the way to contact. Indeed, the research predicts that most binaries coalesced in this manner or at least became very tight. But if the binary system was formed relatively late, close to the time that accretion in the Kuiper belt ceased, a widely separated binary would survive. These are the objects we observe today. By this mechanism it can be predicted that about 5 percent of objects remain with large enough separation to be observed as a binary. The prediction is in agreement with recent surveys conducted by Caltech associate professor of planetary astronomy Mike Brown. The majority of objects ended up as tighter binaries. Their images cannot be distinguished from those of isolated objects when observed from Earth using existing instruments.

These ideas will be more thoroughly tested as additional objects are discovered and further data is collected. Further theoretical work could predict how the inclination of a binary orbit, relative to the plane of the solar system, evolves as the orbit shrinks. If it increases, this would suggest that the Pluto/Charon system, although tight, was also formed by the 'outside-in' mechanism, since it is known to have large inclination.

An improved estimate of the number of nearby asteroids still capable of causing local destruction suggests these pesky rocks are likely to hit Earth about once every 1,000 years. Astronomers had thought such minor catastrophes occurred about once per century.

The new calculations, from Alan Harris of the Space Science Institute in Boulder, CO, show there are about 500,000 relatively small asteroids that inhabit roughly the same region of space through which Earth orbits. The asteroids are in the 50-75 meter (165-245 foot) size range.

Rocks this size can flatten a forest and would cause tremendous damage and even death if it targeted a city.

A notorious example occurred in 1908 when an asteroid in this size range is believed to have exploded above the uninhabited Tunguska region of Siberia, leveling trees for some 800 square miles (2,000 square kilometers) around. Astronomers have for a decade or so said so-called Tunguska events probably occur about once every hundred years, leading some to speculate that we're about due for another.

Harris stressed that impacts are random events; the idea that another could be "about due" is incorrect.

"The fact that one occurred only a century ago makes it no less (or more) likely that one will happen tomorrow instead of 1,000 years from now," he told SPACE.com. "My lower estimate of collision frequency makes it less likely on any given day (year, decade, century) that an impact will occur, but does not allow one to make any statement as to when specifically the next one might occur."

Harris, along with several NASA scientists and many asteroid hunters, worries far more about larger Near Earth Objects (NEOs); those bigger than 1 kilometer (0.62 miles) that could cause global destruction and even threaten civilization.

About 1,100 large NEOs are thought to exist. More than 600 have been found by programs stemming from a NASA effort to locate 90 percent of them by 2008.

None are known to be on a collision course with Earth.

Smaller asteroids are harder to detect, however. Now and then, one passes relatively close to the planet and some are spotted only after such flybys. A few vocal scientists have long called for stepped-up funding to find these smaller NEOs, because they are more likely to strike and are a more immediate threat.

NASA's line has been to find the big ones first, then consider going after the smaller ones. Only recently, however, have discussions on how to do this become serious. New telescopes would be needed, and telescopes generally take a few years to design and build.

Coincidentally on Tuesday, the University of Hawaii announced a $3.4 million Air Force grant to design an array of telescopes that would find faint objects, including smaller asteroids. The planned observatory, called the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS), is expected to begin operation in 2006.

Rolf Kudritzki, who leads the university's Institute for Astronomy, said Pan-STARRS would help complete the search for large NEOs "and will extend the search to much smaller objects."

Alan Harris said that because smaller, Tunguska-sized impacts are regarded by most asteroid experts as "a very minor component of the overall risk," this overall risk is not changed much, in practical terms, by his new calculations.

There are smaller object out there, too.

Asteroids smaller than 165 feet (50 meters), which might in fact hit the Earth about once per century, would not be expected to cause significant damage "beyond a few broken windows and such," Harris said, "though I'm sure such an event would be spectacular from under it."

The smallest rocks, car-sized or smaller, routinely enter the atmosphere but generally vaporize or break apart before reaching the ground.

Harris' new computer-generated estimate was made possible because about 30 asteroids in the Tunguska size range have now been discovered by MIT's Lincoln Near Earth Asteroid Research (LINEAR) survey, which is designed primarily to find large NEOs.

Harris' estimate is uncertain by a factor of about three.

Robert Roy Britt

For the results of an extremely well-observed asteroidal occultation (seen in Europe on Sept 17; 60 observers so far reported), go to: