Astounding Asteroid--It’s Actually Two : Astronomy: By a stroke of good luck, scientists have captured the first detailed images of such a small object in space.
In an extraordinary stroke of scientific serendipity, scientists who happened to have the right instrument at the right time in the right place have captured the first detailed images of an asteroid. And to their amazement, the small object turned out to be two bodies--not one--dancing cheek to cheek across the solar system.
Because most asteroids are very small and far away from Earth, the only photographic record of asteroids consisted of faint streaks captured during long exposures of the nighttime sky.
“This is the first two-dimensional image of an asteroid,” said Steven J. Ostro of NASA’s Jet Propulsion Laboratory in Pasadena, lead author of a report in today’s issue of the journal Science. Moreover, the twin bodies could explain impact craters on the moon that seem to have occurred simultaneously.
The fact that Ostro and his colleagues were able to capture 64 images of the asteroid last Aug. 22 is nearly miraculous. The asteroid, discovered only 13 days earlier by Caltech astronomer Eleanor Helin, just happened to be in exactly the right place at the right time, while Ostro had four days of viewing time already scheduled on the world’s largest radio telescope in Arecibo, Puerto Rico.
Asteroids are rocky objects, sometimes called minor planets, that dwell mainly in a belt between the orbits of Mars and Jupiter. Some, however, travel into the inner solar system like comets, but even then they have been too far away and too small to be imaged.
But last year two asteroids passed unusually close to Earth. Ostro and his team were able to capture radar images of one of the asteroids just before it made its closest approach. Equally remarkable, the asteroid is only a little more than a mile in diameter, making it the smallest object ever photographed in the solar system, Ostro said.
Halley’s Comet, which was photographed by five spacecraft as it made its rare approach to the sun in 1986, is 10 times bigger than the asteroid captured by Ostro’s team.
“We’ve never seen anything this small,” Ostro said.
But what is particularly astonishing to Ostro and his colleagues is that the images reveal that the asteroid is two small bodies, each less than a mile in diameter, and they appear to be touching as they rotate around each other during their long journey through the solar system.
Does that mean that “paired asteroids” are common in the solar system? Ostro said he doesn’t know.
“Is this typical?” he asked. “Or did we just with a stroke of luck see something that is extremely rare?”
It will take several years, and improvements that are now under way at the Arecibo observatory, for scientists to answer that question. But for now, Ostro is just savoring a moment that comes rarely to an astronomer.
Arecibo can bounce a radar beam off a target and then capture the echo when it returns to Earth. That in turn provides an image of the object much the same as light from an electronic flash returns from a photographic target to create an image on light-sensitive film.
But Arecibo’s 1,000-foot-diameter dish is mounted in a bowl-shaped valley and cannot be moved to track objects across the sky. So in order to study anything, the object must pass through its field of view.
Quick calculations told Ostro that the asteroid would pass over Arecibo at exactly the time that he was scheduled to be studying another distant asteroid. Along with K.D. Rosema and D.K. Yeomans of JPL, J.F. Chandler and I.I. Shapiro of the Harvard-Smithsonian Center for Astrophysics and A.A. Hine of Arecibo, Ostro decided to abandon his earlier target and shoot instead for the approaching asteroid, named 1989 PB.
The project was extremely difficult because asteroids traveling through the inner solar system follow orbits that are hard to predict due to the gravitational influence of various planets. And since radar images depend on bouncing the signal off the object as it speeds across the sky, the position had to be known exactly.
Ostro sent out an urgent message to other astronomers around the world, asking for help in pinpointing the position of the asteroid. But astronomers were hampered by the fact that the asteroid was in the same part of the sky as the moon, and the moon was full, making observations of the dim object extremely difficult.
Then, something most extraordinary happened. On Aug. 17, a total lunar eclipse occurred, as the moon passed through the shadow of the Earth. This made it possible for two scientists in Great Britain to zero in on the asteroid. Meanwhile, other astronomers in Australia were able to make precise measurements, permitting the Arecibo team to fine-tune the great telescope.
On Aug. 22, the last day that Ostro had reserved at Arecibo, Ostro captured the historic images. The asteroid was 3.4 million miles away, 14 times as far away as the moon, a tiny object racing across the sky as it was nailed by a radar beam from Earth.
By studying the images, Ostro has determined that the twin asteroids have a rough, irregular surface. They are rotating counterclockwise in the images, and it takes 4.1 hours for them to complete one rotation.
How did these two cold, hard bodies come to be fellow travelers?
No one is sure, but the leading theory is that they were remnants from a past collision. Two larger bodies probably collided at some time in the distant past, sending debris flying off in all directions. In the case of Asteroid 1989 PB, two small chunks of rock were probably blasted off in about the same direction.
Over time, perhaps even billions of years, they were drawn together by their small gravitational fields and left to fly together, undetected, until Ostro and his colleagues came along at just the right moment.
