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This page contains recent press releases concerning discoveries and information about minor planets (asteroids) and related issues. The page will updated as and when time permits.
Long-lost asteroid 719 Albert is found -- whereabouts unknown since 1911
(By Lori Stiles)
[ http://cfa-www.harvard.edu/cfa/ps/pressinfo/Albert.html ]
University of Arizona Spacewatch astronomers at Kitt Peak, Ariz., have rediscovered the last "lost" numbered minor planet, the Minor Planet Center located at the Smithsonian Astrophysical Observatory in Cambridge, Mass., demonstrated yesterday (May 9).
Until this month, asteroid 719 Albert has long eluded astronomers. It was last seen by direct observation in 1911, the year it was discovered by astronomer Johann Palisa (1848-1925) at the Imperial Observatory in Vienna, a world-class observatory of the pre-World War I Austro-Hungarian empire.
Palisa was using the observatory's prize 68-cm (27-inch) telescope when he discovered the new minor planet on Oct. 3, 1911. He observed it again on Oct. 4, as did an astronomer at Copenhagen Observatory, which had been notified of Palisa's find.
That was the last direct observation anyone had of 719 Albert, named for a baron who had donated generously to the Vienna Observatory. Until last week.
Jeff A. Larsen first detected asteroid 2000 JW8 -- now known to be 719 Albert -- with the .9-meter (36-inch) Spacewatch telescope early in his May 1 observing run. Larsen, a principal research specialist who joined Spacewatch three years ago, has dedicated most of his time to writing computer software for the telescope.
"This object was very faint, almost at the limits of what Spacewatch can do, and it wasn't moving all that fast," Larsen said. "But it caught my eye because it was moving differently from its neighbors. It moved like a near-Earth object."
Spacewatch director Robert S. McMillan and Spacewatch astronomer James V. Scotti observed the asteroid again on May 3 and May 6. They confirmed that the asteroid, officially named 2000 JW8, was an Earth-approacher. Yesterday, May 9, Michael Hicks and Ron Fevig of the UA Lunar and Planetary Lab observed the asteroid once more, this time using the 2.1-meter (84-inch) Kitt Peak reflector.
Gareth V. Williams, associate director of the Minor Planet Center (MPC), did the orbital calculations that showed 2000 JW8 is long-lost 719 Albert. MPC Director Brian Marsden reviewed the calculations and quickly concurred that, after 89 years, asteroid Albert had been rediscovered.
Astronomers now know the asteroid's orbit precisely. They know that most of the time, asteroid 719 Albert is 300 million miles or more from the sun, and that it makes a complete orbit around the sun every 4.28 years. It makes its closest swings by Earth every 30 years -- in 1911, 1941, 1971 and 2001.
"It's never going to hit Earth," McMillan said. The close approaches are between about 19 million miles and 29 million miles from Earth, or never closer than roughly a fifth the distance from the Earth to the sun.
Astronomers plan to learn more about the asteroid when it comes within 27 million miles of Earth on Sept. 5, 2001.
Scientists don't know for sure how large the asteroid is because they don't know how much light is reflected by the asteroid's surface material, McMillan added. Given its absolute magnitude, or absolute brightness, astronomers estimate that the asteroid is between 2 kilometers (1.24 miles) and 4 kilometers (2.5 miles) in diameter.
Astronomers have remained intrigued by the asteroid "partly because of the intellectual challenge of finding an object that has been lost for so long and comes close to Earth only once every 30 years," McMillan said.
The recovery has been "satisfying for Spacewatch, too, because we found the object when it was quite faint, more than a year before its closest approach to Earth," McMillan said. Spacewatch covers a relatively small area of sky, but it sees very faint objects farther out into the solar system than do other such surveys.
"Other asteroid surveys would have found this eventually, but finding it now gives astronomers time to apply for telescope time on larger telescopes for detailed observations of the asteroid," McMillan said.
"This has been exciting to me, because I've always been interested in classics in astronomy," Larsen said. "I have nothing but the greatest respect for those earlier astronomers. They actually had to look through the telescope lens to find their objects. They didn't have computers or CCDs.
"For me, when I first saw it, this was another unusual asteroid," he said. "It was Gareth Williams of the Minor Planet Center who made the identification. I owe him several beers."
UA Professor Tom Gehrels and McMillan founded the Spacewatch Project in 1980. It is a survey of the whole solar system, from the vicinity of Earth's orbit all the way out to beyond Neptune's orbit. The primary goal is to explore the various populations of small objects in the solar system and to study the statistics of asteroids and comets to better understand the dynamical evolution of the solar system. Spacewatch also finds potential targets for space missions, provides astrometric support for spacecraft mission planning, and finds objects that might present a hazard to the Earth.
More information about Spacewatch can be found on the web site Spacewatch. [Return to Index]
NEAR Shoemaker Science Update
(by Andrew Cheng, NEAR Project Scientist)
As of the first week in May, NEAR Shoemaker has reached its prime mission orbit at about 50 km from the center of Eros. In this orbit, NEAR Shoemaker is close enough to the asteroid to measure composition, search for a magnetic field, and study internal structure. The spacecraft is also close enough now for the orbit to be affected strongly by the irregularity of the Eros gravity field. It is by studying the orbit that we will learn about the mass distribution within Eros. In some ways the orbit of NEAR Shoemaker is not very different from what is familiar, but in other ways it is quite strange.
The small size of Eros, and its correspondingly weak gravity compared to that of Earth (for example), mean that the spacecraft orbital velocity is much lower than we are accustomed to. In the 50 km orbit, the orbital velocity is about 3 m/s [7 mph] whereas in low Earth orbit the circular velocity is about 7700 m/s [18000 mph]. However, this is not a fair comparison, because the low Earth orbit is just skimming the surface of the almost spherical Earth, whereas the 50 km orbit around Eros is far above the elongated asteroid surface. If we found the radius of a sphere with the same volume as Eros, the radius of that sphere would be 8.5 km. That is, if Eros were fluid or a strengthless gravel pile, and it were not spinning, it would collapse into a sphere of radius 8.5 km because of its own gravity. Incidentally, we can infer from the shape of Eros that it must have some strength, but the required strength is very low even compared to that of ordinary soils on Earth, let alone rocks - more on that another time. Returning to our orbit around Eros, we should ask how fast would the orbital speed be for a circular orbit just skimming the surface of our hypothetical) 8.5 km radius asteroid: the orbital speed would now be 7 m/s.
Why did we insist on comparing the orbital speeds this way? Because if we now compare the orbital periods of the low Earth orbit and the 8.5 km orbit around the hypothetical asteroid, we find they are no longer very different - the orbital period is 89 minutes around Earth and 120 minutes around the asteroid. It turns out that if the mean densities of the Earth and the hypothetical asteroid were the same, then the periods of the Earth orbit and the asteroid orbit would be equal. The period of an orbit skimming the surface of a spherical mass is inversely proportional to the square root of the mean density. Actually, the mean density of Eros is about the same as that of Earth's crust, which is only about half the overall mean density of Earth, because Earth's iron-rich core is somewhat denser than its crust. Earth's higher density than Eros means that the period of a surface-skimming orbit is smaller, but not by very much, because the square-root-of-density is never very different when comparing any ordinary materials, whether rocky or icy . At most this square-root-of-density factor differs by something like a factor of two, whereas the size, for example, differs by many orders of magnitude between Earth and asteroids. Hence for any asteroid, the orbital period of a low, surface-skimming circular orbit will always be about the same, regardless of the size of the asteroid. That means the speed in such an orbit is directly proportional to the radius of the asteroid - a spacecraft would orbit at about twice the speed around an asteroid of twice the radius, so as to complete an orbit in about the same time.
What about orbits that are not surface-skimming? We must now scale the orbital radius in terms of the body radius to make a fair comparison. That is, we compare the orbital period at a distance measured in Earth radii from the center, with the orbital period at the same number of asteroid radii, and we find that the orbital periods are the same (except for the inverse dependence on square root of mean density). The orbital period at six Earth radii, or 38000 km from the center of Earth, is about 22 hours, not very different from the 30 hour period at six asteroid radii, or 6 x 8.5 = 51 km from the center of Eros. This is remarkable considering the great disparities in size, mass, and strength of gravity between Earth and an asteroid.
Hence, this aspect of orbiting an asteroid is not terribly different from orbiting a planet like Earth, but that is because we have considered so far only the effect of small size. We have not yet discussed effects of the irregular shape. We know from Kepler's laws that orbits around a spherical body are conic sections, or in our case ellipses (with circles included as a special type of ellipse). However, as soon as we introduce a nonspherical gravity field, such as formed by a mass with a quadrupole moment, the orbits are no longer conic sections. In fact, they are no longer closed curves of any kind at all, but trace out a fantastic three-dimensional filigree, without ever returning to where they started and without even remaining in any fixed plane. We say that such orbits precess. At present, NEAR Shoemaker is trying to stay in an orbit that is close to circular at 50 km radius, but because of Eros's irregular shape, the distance from the center actually varies from 48 to 52 km - an incredibly bumpy ride. NEAR Shoemaker is navigating the most irregular, non-spherical gravity field that any spacecraft has ever experienced.
[Return to Index]Astronomers Catch Images of Giant Metal Dog-Bone Asteroid
NASA astronomers have collected the first-ever radar images of a "main belt" asteroid, a metallic, dog bone-shaped rock the size of New Jersey, an apparent leftover from an ancient, violent cosmic collision.
The asteroid, named 216 Kleopatra, is a large object in the main asteroid belt between Mars and Jupiter; it measures about 217 kilometers (135 miles) long and about 94 kilometers (58 miles) wide. Kleopatra was discovered in 1880, but until now, its shape was unknown.
"With its dog bone shape, Kleopatra is one of the most unusual asteroids we've seen in the solar system," said Dr. Steven Ostro of NASA's Jet Propulsion Laboratory, Pasadena, Calif., who led a team of astronomers observing Kleopatra with the 305-meter (1,000-foot) telescope of the Arecibo Observatory in Puerto Rico. "Kleopatra could be the remnant of an incredibly violent collision between two asteroids that did not completely shatter and disperse all the fragments."
The astronomers used the telescope to bounce radar signals off Kleopatra. With sophisticated computer-analysis techniques, they decoded the echoes, transformed them into images, and assembled a computer model of the asteroid's shape. The Arecibo telescope underwent major upgrades in the 1990s, which dramatically improved its sensitivity and made it feasible to image more distant objects.
These new radar images were obtained when Kleopatra was about 171 million kilometers (106 million miles) from Earth. Traveling at the speed of light, the transmitted signal took about 19 minutes to make the round trip to Kleopatra and back.
"Getting images of Kleopatra from Arecibo was like using a Los Angeles telescope the size of the human eye's lens to image a car in New York," Ostro said.
Kleopatra is one of several dozen asteroids whose coloring suggests they contain metal. Kleopatra's strong reflection of radar signals indicates it is mostly metal, possibly a nickel-iron alloy. These objects were once heated, melted and differentiated into structures containing a core, mantle and crust, much as the Earth was formed. Unlike Earth, those asteroids cooled and solidified throughout, and many underwent massive collisions that exposed their metallic cores. In some cases, those collisions launched fragments that eventually collided with Earth, becoming iron meteorites like the one that created Meteor Crater in Arizona.
"But we don't need to worry about Kleopatra -- it will never hit Earth," Ostro said.
"The radar-based reconstruction of Kleopatra's shape shows the object's two lobes connected by a handle, forming a shape that resembles a distorted dumbbell, or dog bone," said Dr. R. Scott Hudson of Washington State University, Pullman, WA. "The shape may have been produced by the collision of two objects that had previously been thoroughly fractured and ground into piles of loosely consolidated rubble. Or, Kleopatra may once have been two separate lobes in orbit around each other with empty space between them, with subsequent impacts filling in the area between the lobes with debris."
"The radar observations indicated the surface of Kleopatra is porous and loosely consolidated, much like surface of the Moon, although the composition is different," said Dr. Michael Nolan of the Arecibo Observatory. "Kleopatra's interior arrangement of solid metal fragments and loose metallic rubble, and the geometry of fractures within any solid components, are unknown. What is clear is that this object's collision history is extremely unusual."
"It is amazing that nature has produced a giant metallic object with such a peculiar shape," said Ostro. "We can think of some possible scenarios, but at this point none is very satisfying. The object's existence is a perplexing mystery that tells us how far we have to go to understand more about asteroid shapes and collisions."
The team's findings will appear in the May 5 issue of the journal Science. Ostro's team includes Hudson; Nolan and Jean-Luc Margot of the Arecibo Observatory; Dr. Daniel Scheeres of the University of Michigan, Ann Arbor; Dr. Donald Campbell of Cornell University, Ithaca, N.Y; Dr. Christopher Magri of the University of Maine at Farmington; and Jon Giorgini and Dr. Donald Yeomans of the Jet Propulsion Laboratory.
The Kleopatra images are available at: http://www.jpl.nasa.gov/pictures/kleopatra
The Arecibo Observatory is part of the National Astronomy and Ionosphere Center, operated by Cornell University for the National Science Foundation. The Kleopatra radar observations were supported by NASA's Office of Space Science, Washington, DC. JPL is managed for NASA by the California Institute of Technology in Pasadena.
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