Montage of our solar system

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 as 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.

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