A NASA spacecraft made the closest flyby to a solar system object the night of July 28-29 as Deep Space 1 successfully raced about 15 kilometers above the surface of asteroid 9969 Braille at a speed of 15 km per second. The experimental spacecraft had steered near the asteroid under its own autopilot system, one of the dozen new technologies that were tested aboard the spacecraft since its October 1998 launch.

Closest approach to the minor planet occurred at 12:46 a.m. Eastern Daylight Time on Thursday and it took 10 minutes for a frequency change in the spacecraft's radio signal to reach anxious researchers at the Jet Propulsion Laboratory in Pasadena, California. Earlier in the day, DS1 put itself into a "safe mode" when onboard software detected an error. The problem was quickly diagnosed and the spacecraft was reset for the flyby. With 100 percent of the mission objectives met, investigators were understandably satisfied. "This has been by far the most challenging, dramatic and stressful day on the project," said chief mission engineer Marc Rayman. "The last 16 hours before the flyby were really, really exciting. We had the safing event, we recovered from it and we managed to squeeze in a trajectory correction maneuver to update Deep Space 1's flight path." Alas, everything didn't go as planned for the flyby. An error mispointed the camera thus only a few images were obtained.

DS1's target asteroid was recently named in honor of Louis Braille, inventor of the tactile language for the blind. The Planetary Society held a contest to name the space rock -- originally designated 1992 KD. The winner, selected from several hundred submissions, was suggested by Kerry Babcock of Port Orange, Florida.

A Massachusetts Institute of Technology researcher has uncovered new evidence that asteroids circling the sun in similar orbits are in fact "families" that travel together after being broken apart by collisions.

Planetary scientists have made their best measurement yet of the mass and density of the near-Earth asteroid Eros and have concluded it is not a rubble pile, unlike other asteroids.

In a pair of papers published in the July 23 issue of the journal Science, members of two science teams associated with the Near Earth Asteroid Rendezvous (NEAR) mission reported on the size, mass, and density of the asteroid 433 Eros, the second-largest of all know near-Earth asteroids.

Basing their data on a December 23, 1998 flyby of Eros by NEAR, the scientists used images to find that the asteroid is elongated into a shape like that of "a kidney bean or banana", measuring 33 by 13 by 13 km (20 by 8 by 8 mi.). Another team used spacecraft tracking data to measure the perturbation of NEAR by Eros, and found that the asteroid has a mass of 7.2 million billion kilograms (7.2 trillion tonnes).

Combining the two statistics, they computed the mean density of the asteroid to be 2.5 +/- 0.8 grams per cubic centimeter. This density is similar to the density of asteroid 243 Ida, which, like Eros, is an S-class asteroid. The two are the only S-type asteroids for which densities are known.

The density is much higher, though, then the one measured for another asteroid, 253 Mathilde, which NEAR flew by en route to Eros in mid-1997. Eros has a density of only 1.3 +/- 0.2 grams per cubic centimeter, suggesting to scientists that as much as 80 percent of the volume of the asteroid may be empty space, the result of the asteroid being a "rubble pile" of smaller objects held together by self-gravity.

Eros's much higher density, along with the discovery of a long, continuous linear feature seen to extend for at least 20 km (12 mi.) along its surface, suggest to scientists that Eros is not a rubble pile asteroid but instead a structurally coherent body.

The data analyzed and reported in Science were taken as part of a temporary "consolation prize" when NEAR failed to execute a thruster burn as planned on December 20 that was part of a series of maneuvers that would have put the spacecraft in orbit around the asteroid in January. Instead, NEAR collected data during a flyby of the asteroid December 23.

NEAR will get another chance to go into orbit around Eros in February 2000, when the spacecraft again encounters the asteroid.

A Massachusetts Institute of Technology professor has come up with a scale that assigns a number to the likelihood that an asteroid will collide with the Earth. Zero or one means virtually no chance of impact or damage; 10 means certain catastrophe.

Richard P. Binzel, professor of Earth, Atmospheric and Planetary Sciences at MIT, created the scale to help scientists, the media and the public assess the potential danger of asteroids. He hopes that it will assuage concerns about a potential doomsday collision with the Earth.

Binzel's risk-assessment system is similar to the Richter scale used for earthquakes. It is named the Torino Impact Hazard Scale for the Italian city in which it was adopted at a workshop of the International Astronomical Union (IAU) in June.

The IAU will announce today (July 22) at the third United Nations conference on the exploration and peaceful uses of outer space (UNISPACE III in Vienna, Austria) that it has officially endorsed the Torino scale to gauge potential impacts with asteroids and comets, collectively referred to as near-Earth objects (NEOs).

"What I find especially important about the Torino impact scale is that it comes in time to meet future needs as the rate of discoveries of near-Earth objects continues to increase," said Hans Rickman, IAU assistant general secretary.

"The Torino scale is a major advance in our ability to explain the hazard posed by a particular NEO," said Carl Pilcher, science director for solar system exploration in the NASA Office of Space Science in Washington, D.C. "If we ever find an object with a greater value than one, the scale will be an effective way to communicate the resulting risk."

"Naming the newly proposed hazard scale after Torino is a highly appreciated recognition of the Torino Astronomical Observatory's great deal of work over the past two decades," said Alberto Cellino, astronomer at the Torino Astronomical Observatory.

DEEP IMPACT?

Based on the orbit trajectory for a given NEO, the scale takes into account the object's size and speed as well as the probability that it will come into contact with the Earth. The scale can be used at different levels of complexity by scientists, science journalists and the general public.

The scale assigns a number from zero through 10 to a predicted close encounter by an NEO. A zero, in the white zone, means that the object has virtually no chance of colliding with the Earth or that the object is so small it would disintegrate into harmless bits if it passed through the Earth's atmosphere. A red 10 means that the object will definitely hit the Earth and have the capability to cause a "global climatic catastrophe."

Close encounters in the green, yellow and orange zones with "scores" from one to seven are categorized as "events meriting careful monitoring" to "threatening events." Certain collisions fall in the red zone, with values of eight, nine or 10, depending on whether the impact energy is large enough to be capable of causing local, regional or global devastation.

No asteroid identified to date has ever made it out of the green zone by having a scale value greater than one. Several asteroids that had initial hazard scale values of one have been reclassified into category zero after additional orbit measurements showed that the chances of impact with the Earth became zero. All currently known asteroids have scale values of zero.

A COSMIC SHOOTING GALLERY

Binzel, who has been developing the scale for five years, aims to give scientists a consistent way to communicate about the growing number of close-encounter asteroids being spotted. Increasingly sophisticated equipment, partially funded by NASA, such as the Lincoln Near Earth Asteroid Research (LINEAR) project at MIT's Lincoln Laboratory in Lexington, Mass., is being used to detect and track a growing number of the estimated 2,000 NEOs larger than about a half-mile (1 kilometer) in diameter.

The LINEAR project uses technology originally developed for the surveillance of Earth-orbiting satellites to detect and catalog NEOs. It has detected almost 250,000 asteroids to date, more than any other source. Of these, 228 are newly discovered NEOs.

While more asteroids than ever are being identified in the cosmic shooting gallery inhabited by our planet, Binzel points out that there is no increase in the number of asteroids out there -- only in our awareness of them. Because we know about more asteroids, there is an increasing awareness that many of them can make close passes by the Earth. "This doesn't mean that the Earth is in any greater danger," he said. "Fortunately, the odds favor that newly discovered objects will miss."

On the other hand, space-borne objects do hit the Earth. Tiny fragments as big as grains of sand bombard us constantly, and objects the size of a small car hit a few times a year. An asteroid bigger than a mile across might hit once every 100,000 to 1 million years. The planet bears scars from these encounters.

In the 1960s, Eugene Shoemaker of the U.S. Geological Survey proved that a big dent in the Arizona desert is a meteor crater. Most scientists believe that the dinosaurs were wiped out by a massive object 65 million years ago. The well-documented collision of the Shoemaker-Levy comet with Jupiter demonstrates that impacts are still a reality in the solar system today, but, Binzel points out, "No one has clearly documented deaths from a meteorite impact."

JUDGING HAZARDS

"If you tell a Californian that an earthquake registering one on the Richter scale was going to hit tomorrow, he would say, 'So what?' " Binzel said. "If you were talking about a six, that would be different."

So Binzel hopes it will be with asteroids. Nobody should lose sleep, he said, over an asteroid in the zero or one category, which accounts for the vast majority of them. He hopes to avoid sensationalism such as that surrounding the 1997 XF11 asteroid that led to the New York Post headline of March, 13, 1998, "Kiss your asteroid goodbye," or embarrassments such as astronomers' announcement -- and quick retraction -- regarding the 1999 AN10 asteroid's potential impact. AN10 is now known to be a sure miss, a zero on the Torino scale.

"Scientists haven't done a very good job of communicating to the public the relative danger of collision with an asteroid," said Binzel, who is a specialist on planetary astronomy. "Scientist-astronomers who are going to be confronted with this should have some means of clearly communicating about it so as to clearly inform but not confuse or unnecessarily alarm the public."

Once an asteroid is detected, scientists try to use information that shows a tiny section of its orbit to calculate where it will be in 10, 15 or 100 years. There is some uncertainty in this prediction because the orbit measurements are not perfect and the NEO may be altered by gravity if it passes close to the Earth or another planet, but "orbits generally behave like clockwork," Binzel said.

As more information is gathered about a particular asteroid, its placement on the scale can be adjusted accordingly, he points out. "It is hoped that in all cases the placement will go to zero.

"What I hope the scale will accomplish is to put in perspective whether an object merits concern," he said. "This is a case of a high-consequence but low-probability event. It's difficult in human nature to figure out what level of anxiety we should assign to an approaching asteroid."

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A unique near-Earth asteroid discovered last year by Spacewatch at the Univerity of Arizona in Tucson is the fastest-spinning solar system object yet found, scientists report in tomorrow's issue (July 23) of Science.

Only 30 meters (100 feet) across, asteroid 1998 KY26 spins once every 10.7 minutes. That's 10 times faster than the spin rate of any other object and almost 60 times faster than the average of all known asteroid rotation periods, the scientists say.

Whirling at that speed and given its size, 1998 KY26 has to be a strong, single chunk of rock that was sent reeling from its parent asteroid in some space collision, said James V. Scotti , a senior research specialist at the UA Lunar and Planetary Laboratory (LPL) and a co-author of the Science paper.

LPL Professor Tom Gehrels, Spacewatch co-founder, discovered asteroid 1998 KY26 on May 28, 1998, using the 0.9 meter (36-inch) Spacewatch telescope at Kitt Peak, Ariz. Six nights later Scotti, joined at the Spacewatch telescope by Dan Durda, took 111 images of the asteroid, measuring its minimum to maximum changes in brightness. Durda of the Southwest Research Institute in Boulder, Co., was formerly with LPL.

Astronomers at telescopes in the Czech Republic, Hawaii and California also made the same kind of photometric measurements from June 2 to 8. This was when the asteroid made its closest swing by Earth at a half million miles, or twice the distance between the Earth and the moon. Between June 6 and 8, Steven J. Ostro headed a team from the NASA Jet Propulsion Laboratory in Pasadena, Calif., that used the Goldstone X- band radar of NASA's Deep Space Network to track the asteroid. Radar echoes revealed the asteroid's rapid spin rate. Petr Pravec of Ondrejov Astronomy Institute in the Czech Republic combined data gathered by the different optical observing groups and constructed a light curve to determine the precise rotation rate.

The astronomers discovered the size and shape of 1998 KY26 from the radar echoes. This asteroid is unusual in that it is almost spherical, with a bare-rock surface pocked at least in part by meteoroid bombardment, they report. Their optical and radar observations show this asteroid is similar to carbonaceous chondritic meteorites, objects that formed early in solar system history. These meteorites are rich in primordial complex organic compounds and water.

Asteroids in the 30-meter-diameter range survive between 10 million and 100 million years before being destroyed in space collisions. Carbonaceous chondrites are weaker meteorites, so this asteroid will be smashed sooner than later, they add.

Information from recent asteroid flybys suggests that large asteroids are less dense than the meteorites recovered and measured on Earth. Scientists theorize that most larger asteroids are porous "rubble piles" rather than monolithic bodies, Scotti said. Current theory says that "these rubble piles are conglomerates of debris broken apart by multiple collisions and held together by their mutual gravity, spinning slowly enough so that they don't fall apart," he added.

Studying the detailed structure of these asteroids involves more than just scientific curiosity, Scotti said. There are two practical reasons for learning more about them: Asteroid minerals can provide raw materials for future space construction, and knowing how asteroids are put together provides critical knowledge for deflecting large ones headed for Earth.

Each month, Spacewatch - the world's first telescope dedicated to searching for near-Earth asteroids - finds an average of two-to-three asteroids in our vicinity, and another 2,000 new ones in the asteroid belt. Spacewatch is funded by NASA, the University of Arizona and private donors.

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Spinning faster than any object ever observed in the solar system, a lumpy, water-rich sphere known as 1998 KY26, about the diameter of a baseball diamond, is rotating so swiftly that its day ends almost soon as it begins, NASA scientists report.

Asteroid 1998 KY26, where the Sun rises or sets every five minutes, was observed June 2-8, 1998, shortly after it was discovered and as it passed 800,000 kilometers (half a million miles) from Earth, or about twice the distance between Earth and the moon. Publishing their findings in tomorrow's issue of Science magazine, Dr. Steven J. Ostro of NASA’s Jet Propulsion Laboratory, Pasadena, CA, and an international team of astronomers used a radar telescope in California and optical telescopes in the Czech Republic, Hawaii, Arizona and California to image the 30-meter (100-foot), water-rich ball as it twirled through space. It is the smallest solar system object ever studied in detail.

"These observations are a breakthrough for asteroid science and a milestone in our exploration of the small bodies of the solar system," Ostro said. "Enormous numbers of objects this small are thought to exist very close to Earth, but this is the first time we've been able to study one in detail. Ironically, this asteroid is smaller than the radar instruments we used to observe it."

The asteroid's rotation period was calculated at just 10.7 minutes, compared to 24 hours for Earth and at least several hours for the approximately 1,000 asteroids measured to date. In addition to these findings, the minerals in 1998 KY26 probably contain about a million gallons of water, enough to fill two or three olympic-sized swimming pools, Ostro said.

"This asteroid is quite literally an oasis for future space explorers," he said. "Its optical and radar properties suggest a composition like carbonaceous chondrite meteorites, which contain complex organic compounds that have been shown to have nutrient value. These could be used as soil to grow food for future human outposts. And among the 25,000 or so asteroids with very reliably known orbits, 1998 KY26 is in an orbit that makes it the most accessible to a spacecraft."

The solar system is thought to contain about 10 million asteroids this small in orbits that cross Earth's, and about 1 billion in the main asteroid belt between Mars and Jupiter. However, only a few dozen of these tiny asteroids have ever been found and, until now, hardly anything was known about the nature of these objects.

Ostro and his colleagues used the 70-meter-diameter (230- foot) Goldstone, CA, antenna of NASA's Deep Space Network to transmit radar signals continuously to the asteroid and turned a 34-meter-diameter (112-foot) antenna on it to collect echoes bouncing back from the object.

1998 KY26's color and radar reflectivity showed similarities to carbonaceous chondrites, primordial meteorites which formed during the origin of the solar system, and unlike any rocks formed on Earth. They contain complex organic compounds as well as 10 percent to 20 percent water. Some carbonaceous chondrites contain amino acids and nucleic acids, which are the building blocks of proteins and DNA, and hence, are of interest to scientists trying to unravel the origins of life.

A second team of astronomers used optical telescopes to track 1998 KY26, which was discovered by the University of Arizona's Spacewatch telescope, the world's first instrument dedicated to searching for near-Earth asteroids. Dr. Petr Pravec of the Czech Republic’s Academy of Sciences said collisions likely gave 1998 KY26 its rapid spin.

But one way or another, Pravec said, this object's 10.7- minute "day" is the shortest of any known object in the solar system. "The motion of the sky would be 135 times faster than it is on Earth," he said. "Sunrises and sunsets take about two minutes on Earth, but on 1998 KY26, they would take less than one second. You'd see a sunrise or sunset every five minutes."

Dr. Scott Hudson of Washington State University in Pullman found the asteroid's shape particularly surprising. Asteroids thousands of times larger have spherical shapes as a result of their large masses and strong gravitational fields, he said. 1998 KY26 is very unusual, however, because gravity and mass play no significant role in its shape. Instead, the spheroid shape is the result of collisions with other asteroids.

While much larger near-Earth asteroids could pose a long- term collision hazard, 1998 KY26's size makes it harmless if it were on a collision course. The asteroid would most likely explode in the upper atmosphere and its fragments would fall harmlessly to Earth. Moreover, 1998 KY26 is in an orbit whose shape and low inclination with respect to the ecliptic plane make it unusually easy to intercept.

Tracking of 1998 KY26 by Ostro and his colleagues in the international scientific community was supported by NASA's Office of Space Science, Washington, DC, and by the Czech Republic's Academy of Sciences in Prague. JPL is a division of the California Institute of Technology, Pasadena, CA.

ITHACA, N.Y. -- The possibility of the Earth being struck by comets or asteroids is being given more and more attention by researchers, according to Paul Chodas of NASA's Jet Propulsion Laboratory (JPL). Chodas will discuss this potential threat to the planet when he moderates one of the daily press conferences that will provide important new insights into the latest space research at the seventh International Asteroids, Comets and Meteors Conference at Cornell University July 26-July 30.

In recent movies, asteroids and comets are shown threatening to collide with the Earth, only to be destroyed at the last minute by astronaut heroics. But Chodas believes that the hazards of collisions with comets or asteroids are more than a topic for fiction. Chodas and Milani will discuss their independent efforts to predict close-Earth approaches and impact probabilities further into the future than has been previously possible.

It has been estimated that only 15 to 20 percent of NEO's larger than one kilometer have been detected to date. Several telescopic discovery programs are actively searching for these large NEO's, and the discovery rate is increasing. However, Chodas believes, it may be at least 10 years before 90 percent of the total population is revealed.

A new analysis by Rabinowitz shows that there may be only half as many large hazardous objects as previously estimated. Harris will present new evidence to refute a controversial theory that the Earth is continually bombarded by a population of house-sized comets.

What is the nature of these seemingly "loose cannons" that might be posing a threat? A new view has evolved, suggesting that many large asteroids are "rubble-piles," according to William Bottke, a research associate in Cornell's Department of Astronomy, who will moderate the press session (Friday, July 30, 10 a.m., Princeton-Yale Room, Statler Hotel) on asteroid moons and spins. Other participants will include Petr Pravec of Ondrejov Astronomy Institute, Czech Republic, and JPL's Harris.

Until recently, most planetary scientists considered asteroids to be little more than beat-up rocks in space, with solid interiors and lunar-like surfaces. It is difficult to prove or disprove this theory since remote sensing techniques can only probe the top layers of an asteroid's surface, says Bottke. But data taken from spacecraft fly-bys, ground-based observations and computer modeling indicate that large asteroids are no more than piles of rubble held together not by physical strength but by the gravitational attraction of the pieces of rubble.

Although Pravec's research, based on an extensive study of near-Earth asteroid (NEA) rotation rates, is consistent with the "rubble-pile" scenario, his new results suggest that some small asteroids are solid. At the conference he will discuss two asteroids, 1996 KY26 and 1995 HM, both smaller than 100 meters, that are spinning so fast that they have to be solid objects -- weakly bound rubble piles fly apart if spun too fast. These two bodies are small enough that they provide the "missing link" to the rubble-pile theory that astronomers have been looking for -- filling the gap between between kilometer-size asteroids that cannot spin overly fast and fast-spinning meter-sized meteorites that are known to be solid.

Computer modeling suggests the transition size for solid objects to turn into rubble piles is roughly a few hundred meters in diameter. More support for the rubble-pile theory also comes from Pravec: He will report that many NEO's have small moons. These moons are most likely to be a by-product of close encounters between rubble-pile asteroids and planets. When a rubble-pile asteroid passes too close to a planet like Earth, tidal forces can pull it apart, leaving some of the fragments to orbit one another.