It is known since last October that 1999 TY2 spins once every 7(!) minutes - see http://sunkl.asu.cas.cz/~ppravec/99ty2.htm and http://www.geocities.com/skyweek/mirror/152.html story #7.
From Fall 1999/Number 87 issue of Lunar and Planetary Information
Asteroid 1998 KY26, where the Sun rises or sets every five minutes, was observed June 28, 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 a recent issue of Science magazine, Dr. Steven J. Ostro of NASA's Jet Propulsion Laboratory, Pasadena, California, and an international team of astronomers used a radar telescope in California and optical telescopes in the Czech Republic, Hawai'i, Arizona, and California to image the 30-meter, water-rich ball as it twirled through space. It is the smallest solar system object ever studied in detail.
"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," Ostro said.
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 1000 asteroids measured to date. In addition, 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 Goldstone, California, antenna of NASA's Deep Space Network to transmit radar signals continuously to the asteroid and turned a 34-meter-diameter 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 that formed during the origin of the solar system and are unlike any rocks formed on Earth. They contain complex organic compounds as well as 1020% 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.