Asteroid Web Sites
Kuiper Belt homepage
1999 Leonids, European Space Agency page
Near Earth Object Program
NEAR Web site
Planetary Society's descriptions of comet and asteroid missions
Near-Earth Asteroid Tracking (Caltech)
Leonid Meteor Shower/Storm:
On March 1, NASA's Deep Space Network antennas pulled down their last Near Earth Asteroid Rendezvous (NEAR) mission data, bringing to a close the first mission to extensively study an asteroid. NEAR, which was the first mission in NASA's Discovery Program of low-cost, scientifically focused space missions, and the first to land on an asteroid, has delighted astronomy neophytes and scientists alike.
"NEAR has raised the bar," says Dr. Stamatios Krimigis, Space Department head at The Johns Hopkins University Applied Physics Laboratory in Laurel, Md., which built the spacecraft and managed the NEAR mission. "The Laboratory is very proud to have managed such a successful mission and other universities. The team had no weak links and the result was an historic mission that surpassed everyone's expectations."
"This mission has been successful far beyond what was in the original mission plan," says NEAR Mission Director Dr. Robert Farquhar of APL. "We got the first images of a C-class asteroid when we added a flyby of asteroid Mathilde in 1997; we added two low altitude series of passes over the ends of Eros this past October and January that gave us spectacular images from 2.7 kilometers above the surface; and we achieved the first landing of a spacecraft on an asteroid on Feb. 12. All this at no extra cost. When you talk about ' faster, cheaper, better,' this is what 'better' means."
On Feb. 12 at 3:01:52 p.m. (EST), NEAR Shoemaker made a gentle, picture-perfect 3-point landing on the tips of two solar panels and the bottom edge of the spacecraft body. But the mission wasn't finished yet. Much to the amazement of the mission team and millions of observers around the world who were following the descent, the touchdown was so elegant that the craft was still operating and sending a signal back to Earth even after landing.
Jumping at the chance to get "bonus science" from the spacecraft, which had already collected 10 times more data than originally planned, the mission team asked for and got a 10-day extension and then four more days of DSN antenna time, enabling NEAR Shoemaker to send back data through Feb. 28. The extension was granted to allow the gamma-ray spectrometer to collect data from an ideal vantage point about four inches from the surface. The spectrometer team quickly redesigned software and uploaded it to the spacecraft so they could begin collecting elemental composition readings.
The results were spectacular. "This is the first gamma-ray experiment that has ever been done on the surface of a body other than Earth," says Dr. Jacob Trombka, of NASA's Goddard Space Flight Center, in Greenbelt, Md., who heads the gamma-ray spectrometer team. "In fact, we can say it's the first feasibility study of how to design an instrument to be used on a rover that could select samples from the surface, look for the presence of water, or map the surface for the purpose of future mining."
The gamma-ray spectrometer team was able to retrieve data for a period of seven days after the spacecraft landed. "Right now we know we have good data with strong signatures," Trombka says. "But it will take months to scrutinize what we've collected. What we're looking for is information that will help us more precisely classify Eros and determine the relationship between the asteroid and meteorites that have fallen to Earth."
NEAR Shoemaker now rests silently just to the south of the saddle-shaped feature Himeros as the asteroid twists more and more away from the sun with each rotation, moving the southern hemisphere into its winter season and temperatures as low as minus 238 degrees Fahrenheit (minus 150 centigrade).
Project Scientist Dr. Andrew Cheng of APL, says the glamorous part of the mission is over but now scientists can get down to studying the data, including the more than 160,000 detailed images taken by the spacecraft. "We solved mysteries, we unveiled more mysteries. Now we're sharing the amazing amount of data that we collected with scientists all over the world, to sort through and debate and hopefully to help us discover facts about Eros and our solar system that no one knows today."
"This was a bonus," says NEAR (Near Earth Asteroid Rendezvous) Mission Director Dr. Robert Farquhar of the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md., which built the NEAR Shoemaker spacecraft and manages the mission for NASA.
"The NEAR mission and the spacecraft were not designed to touch down on the asteroid, and such a maneuver has never been attempted before," Farquhar says. "But the risk was worth taking. During our yearlong study of Eros we collected 10 times more data than originally planned. And now, at the end of the mission, we had a chance to gather close-up images of Eros' surface - capturing features as small as 4 inches (10 centimeters) across - by executing a controlled descent to the surface of Eros. So we took it."
A successful engine burn at 10:31 a.m. (EST), nudged NEAR Shoemaker toward Eros from about 16 miles (26 kilometers) away. Then four breaking maneuvers brought the spacecraft to rest on asteroid's surface in an area just outside a saddle-shaped depression, Himeros, at approximately 3:05 p.m. (EST).
Since last October, the NEAR imaging team has been puzzling over strange surface features of Eros seen in new, high-resolution images. There is the hope that the close-up images taken in the final few minutes before the spacecraft drops onto the surface will help to answer their questions about the geology of the 22-mile-long asteroid more than 196 million miles (316 million kilometers) from Earth.
"Since last October we have seen details of Eros at 1 meter resolution that we haven't seen anywhere else before and don't understand," says Cornell astronomer Joseph Veverka, who heads the imaging team. "That's why we are so excited about getting close to the surface."
The landing -- what NASA is calling a "controlled descent" -- is a highly risky maneuver, involving four thruster firings over four hours intended to slow the rate of descent to 7 mph from 20 mph. In the final 45 minutes, when the spacecraft is about 3.5 to 4.5 miles (about 6 to 7.5 kilometers) from its landing site at the edge of the crater Himeros, the camera will begin taking a new image about every 30 seconds.
The final picture will be captured at just 550 yards (500 meters) from the surface, enough to capture details as small as perhaps 4 inches (10 centimeters) across. Mission leaders at the Applied Physics Laboratory at Johns Hopkins University, which built the spacecraft and manages the NEAR mission, do not expect images to be transmitted from the surface because Eros's spin and topography will almost certainly prevent communication between Earth and the craft.
Why does Veverka's team want to get such a close look at Eros' surface details? Because, says Veverka, who is professor of astronomy at Cornell, his team is frankly puzzled by what it has seen on Eros over the past few weeks. Last October, with much of NEAR's mission accomplished, the spacecraft was sent into orbit just 4 miles (about 6 kilometers) or so from the asteroid's surface. For the first time the imaging team was seeing details as small as a yard (0.9 meter) across, compared with the approximately 5.5 yards (5 meters) resolution that had been captured by the camera since the spacecraft went into orbit around Eros on Feb. 14, 2000.
"Suddenly, we started seeing things we didn't expect and hadn't seen on other surfaces in the solar system," says Veverka. "It's like another door has opened."
The biggest surprise, says Cornell researcher Peter Thomas, who has been interpreting the geology of the asteroid's surface, "is that some small craters and other small depressions have flat, smooth floors, unlike most craters you see on Eros and other objects. It looks as if fine-grain material has slid down the craters' sides and ponded in the bottoms." Apparently, he says, there is some mechanism "we hadn't anticipated" that moves fine-grain material around on the surface. Although gravity on Eros is only one one-thousandth of that on Earth -- an average person would weigh only an ounce or two -- it is still "very effective in gathering materials in very flat floors on the bottom of depressions."
Another surprise, says Veverka, is the discovery that some small boulders are surrounded by material that appears to have disintegrated from the boulders' surfaces. "There is some process that is very gentle that somehow disintegrates rock. We haven't seen this on the moon, and we haven't seen this before on Eros," he says. "But it seems to be very common."
It is just possible, says Veverka, that the final image will be taken almost at the surface itself. He explains that the camera will remain in focus until about 220 yards (200 meters) from the landing site. If the spacecraft is still on course, it is possible that the camera will take one final image and have time to send a partial image on its way to Earth before the spacecraft touches down. It takes 10 milliseconds for the exposure, 1 second to read the image into the spacecraft recorder and 30 seconds for the data to emerge from the recorder. The data then take 17 to 18 minutes to reach Earth tracking stations.
The imaging team now is getting even higher-resolution images of these features. On Jan. 24 the spacecraft entered a close flyby sequence, including a four-day orbit that produced images from as close as 2 miles (3.2 kilometers) above the surface. These new images are enabling the Cornell imaging team to accumulate data at a resolution of about 1.1 yards (1 meter). "The hope is that during the descent we can improve this resolution by perhaps a factor of 10 so that we can find out more about what is going on there," says Veverka.
o Near Earth Asteroid Rendezvous Mission: < http://near.jhuapl.edu>
The web version of this release, with accompanying photos, may be found at http://www.news.cornell.edu/releases/Feb01/NEAR.landing.deb.html
TUCSON, Ariz. -- The Spacewatch Project at the University of Arizona has discovered a minor planet in the outer reaches of the solar system which appears to be the brightest known such object other than Pluto.
During routine scanning with the Spacewatch 36-inch telescope on 2000 November 28, observer R. S. McMillan was manually blinking the displayed scans in real time and noticed this relatively slow-moving object. Its rate of motion is too slow for the real-time software to detect; normally the slower objects such as this one are found with another computer program that processes the data off-line.
The web link to images of the object is http://www.lpl.arizona.edu/spacewatch/2000wr106.html
The target was subsequently reobserved by J. A. Larsen, whose observing shift followed McMillan's. With their 12 observations spanning three days, The International Astronomical Union's Minor Planet Center at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts determined a preliminary orbit by assuming the orbit is circular. As of 2000 December 1, the MPC's orbit suggests that this object is 43 times farther from the Sun than the Earth is, and is presently 42 times farther from Earth than the Earth is from the Sun. With an apparent magnitude of 20 at those distances, the object would be the brightest of all 346 known Trans-Neptunian Objects other than Pluto.
Further observations of the object's position will be made in the coming weeks and months to improve knowledge of the orbital parameters and thereby the object's absolute magnitude, a parameter describing the object's brightness at a standard distance. Since solar system objects shine by reflected sunlight, this brightness depends on the object's size as well as its reflectivity. Therefore it is not possible to give a definitive diameter for 2000 WR106. However, if it has a reflectivity comparable to other minor planets, its diameter would be between 330 and 750 miles. This can be compared to the diameters of the largest known asteroid (1) Ceres of 570 miles or (4) Vesta of 320 miles. Pluto is at a distance comparable to that of 2000 WR106, and is 1,470 miles in diameter.
Brightness equates with ease of measuring the object's reflection spectrum, so 2000 WR106 will no doubt be the target of spectroscopy and measurements of its thermal infrared flux (heat output). That additional information will allow a determination of its diameter.
The Spacewatch Project is a survey of the whole solar system, from the vicinity of the Earth's orbit all the way out to beyond the orbit of Neptune. It was begun in 1980 by Tom Gehrels and Robert S. McMillan at the Lunar and Planetary Laboratory of The University of Arizona. Spacewatch uses the 36-inch telescope of the University of Arizona's Steward Observatory on Kitt Peak mountain in the Tohono O'odham Nation, Arizona. This project is one of four or five around the world that regularly scan the sky electronically for asteroids and comets. More information about Spacewatch can be found on the web site: http://www.lpl.arizona.edu/spacewatch/
Findings from NASA's Near Earth Asteroid Rendezvous (NEAR) mission - appearing in a special section of the Sept. 22, 2000 issue of the journal Science - confirm that asteroid 433 Eros is a consolidated, primitive sample from the solar system's beginnings.
"We can now say that Eros is an undifferentiated asteroid with homogeneous structure, that never separated into a distinct crust, mantle and core," says NEAR Project Scientist Dr. Andrew F. Cheng of the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., which manages the NEAR mission. "We have definitive mass and density measurements plus spectacular images and movies showing ridges, pits, troughs and grooves that provide fascinating clues about its history."
NEAR is the first in-depth study of an asteroid. Now more than halfway through a yearlong orbit mission that began Feb. 14, 2000, the NEAR Shoemaker spacecraft has taken more than 103,300 images and extensive measurements of Eros' composition, structure and landforms, at distances ranging from 22 to 220 miles (35 to 350 kilometers).
NEAR Shoemaker's multispectral imager and now-silent infrared spectrometer have returned a flood of observations revealing heavily cratered expanses abutting relatively smooth areas. The asteroid's largest crater measures 3.4 miles (5.5 kilometers) wide and sits opposite from an even larger 6.2-mile (10-kilometer), saddle-shaped depression. Ejecta blocks - rocks and boulders created by impacts - are abundant and measure up to 330 feet (100 meters) across, although they are not distributed evenly on the asteroid.
Some areas are heavily saturated with craters wider than 660 feet (200 meters). Images taken from lower orbits also reveal "younger" sections where craters have been filled or covered by loose material.
More than 8 million measurements taken by the laser rangefinder to definitively establish Eros' shape have determined the asteroid is a consolidated body rather than an agglomerate of loosely bound, much smaller components. Such a "rubble pile" structure has been inferred for many asteroids, but does not apply to Eros. Its irregular, peanut shape - which a body as large as Earth could not maintain - houses a homogeneous internal structure. Although Eros is consolidated, the ubiquitous fabric of ridges and grooves suggests an extensively fractured interior.
NEAR Shoemaker's X-ray spectrometer has detected low levels of aluminum relative to magnesium and silicon, indicating an undifferentiated composition. Eros, or the parent body it could have broken from, has not experienced the extensive melting process that planets like Earth undergo in their development. This finding leads researchers to believe that Eros may be related to the primitive ordinary chondrites, the most common type of meteorite. NEAR Shoemaker's imager and infrared spectrometer have also found spectral properties consistent with a primitive, chondritic composition.
Using ground-based Doppler and range measurements - and by tracking surface landmarks - scientists have determined the asteroid's mass is 6.687 x 1015 kilograms and density is 2,700 kg/m3, which is about the average density of Earth's crust. The density is relatively uniform throughout the asteroid.
Eros has a stable rotation and an escape velocity that ranges from 3.1 to 17.2 meters per second, which would allow a baseball thrown from its surface to leave forever. The acceleration of gravity on the surface of Eros varies from 2.3 to 5.5 millimeters per second squared, thousands of times smaller than on Earth. A person who weighs 150 pounds on Earth would weigh from 0.56 to 1.3 ounces on Eros - about as much as one or two bags of airline peanuts.
The research detailed in the four Science articles covers the first six months of orbit around Eros. NEAR Shoemaker moves in for a low- altitude flyover of Eros on Oct. 25, 2000, coming within 3.7 miles (6 kilometers), and will end the mission in February 2001 with a slow, controlled descent to the asteroid's surface. The spacecraft is currently 109 million miles (176 million kilometers) from Earth, circling Eros at just under 5 miles per hour.
Visit the NEAR Web site ( near.jhuapl.edu) and http://neo.jpl.nasa.gov for the latest images, movies and mission news.
Johns Hopkins University Press Release, 30 May 2000
New data from NASA's Near Earth Asteroid Rendezvous (NEAR) mission indicate asteroid Eros may be a primordial relic left over from the solar system's formation. Using the X-ray/Gamma-ray Spectrometer (XGRS) on the NEAR Shoemaker spacecraft, scientists will spend the remainder of the mission collecting data from other parts of Eros to determine if their latest findings are consistent across the approximately 21-mile-long asteroid. If they are, studying Eros should help scientists understand how the rocky planets evolved.
The NEAR team gathered the information after a powerful explosion on the sun zapped Eros with X-rays on May 4. The half-hour solar flare caused elements on Eros' surface to emit X-rays, a type of radiation invisible to humans. Instruments on NEAR Shoemaker analyzed the intensity of X-rays produced by the asteroid at different wavelengths - getting a fingerprint of the asteroid's chemical composition.
"Analysis of X-rays from an area roughly 3.7 miles [6 kilometers] across on Eros indicates it has an elemental composition similar to the most primitive rocks in the solar system, the chondritic meteorites," says Dr. Jacob Trombka of NASA's Goddard Space Flight Center, who heads the XGRS instrument team.
"Chondrites are the building blocks of terrestrial planets," says Dr. Tim McCoy of the Smithsonian Institution's National Museum of Natural History, a participating scientist on the XGRS team. "If more data confirm Eros is primordial, Eros will be a link between the chondrite meteorites found on Earth and the history of the solar system's formation. With Eros, we could be looking at the structure of the solar system during a time no longer recorded on Earth."
Astronomers and geologists believe chondrites formed from the nebula of gas and dust that comprised the nascent solar system. Collisions between these dust particles eventually formed larger bodies, called planetesimals, which then collided under the influence of their mutual gravity to form planets. Asteroids are relics of this earliest period in solar system history. The gravitational tugs exerted on asteroids by Jupiter kept a large planet from ever forming in the main asteroid belt. Even today, collisions continue and fragments of asteroids occasionally fall to Earth as meteorites.
The most primitive meteorites, called chondrites, are a homogeneous mixture of heavy and light materials - a sign that the asteroid from which they originated was never subjected to melting. The process of heating and cooling that changes the nature and distribution of these materials is referred to as differentiation. If Eros is undifferentiated, like the chondritic meteorites that fall to Earth, it probably never was exposed to intense heating. In contrast, asteroids that were extensively melted have a layered structure - like that of the Earth - resulting from the separation of materials of different density. Heavy materials, like nickel-iron metal, tend to sink to the center while lighter materials, composed mostly of silicon and oxygen, rise to the surface. If Eros were composed mostly of light materials, it would probably be a fragment from near the surface of a larger body, while an abundance of heavy materials would point to the origin of Eros as the core of a larger, broken-up asteroid.
The latest XGRS data show the surface of Eros does not have an excess of heavy or light materials, and that the relative abundance of elements is similar to that in chondrites. This indicates that the particular region observed during the solar flare on May 4 is undifferentiated and likely primordial.
"It's possible that other areas on Eros were exposed to some melting and differentiation, or that the surface composition has been altered by the constant hail of micrometeorites in space," says Trombka. "As we get closer and take more data, we will gradually unveil Eros' character. July will be particularly exciting, because we plan to descend to within 12 to 18 miles [19 to 29 kilometers] from the surface, allowing us to take higher-quality data, and our orbit will take us directly over a few large craters. Since the XGRS instrument can only identify surface composition, looking inside these large craters should reveal much about the asteroid's interior."
More than 94 million miles (152 million kilometers) from Earth, NEAR Shoemaker is currently observing Eros from an orbital altitude of 31 miles (50 kilometers). On July 7, the spacecraft will start a gradual descent into orbit 22 miles (35 kilometers) from the asteroid's center - its closest look at Eros so far. Since beginning its historic yearlong orbit around Eros on February 14, 2000, NEAR Shoemaker has circled the rotating space rock 37 times at various speeds, angles and altitudes. The first in NASA's Discovery Program of low-cost planetary missions, NEAR was launched from Cape Canaveral Air Station, Fla., on Feb. 17, 1996. The car-sized spacecraft is observing Eros from various distances and will come within several miles of its surface before the mission ends in February 2001. The Johns Hopkins University Applied Physics Laboratory in Laurel, Md., designed and built the NEAR spacecraft and manages the mission for NASA.
The NEAR XGRS team includes members from Goddard Space Flight Center, the University of Arizona, Cornell University, and Max Planck Institute for Chemistry, and participating scientists from the Smithsonian Institution National Museum of Natural History, Harvard- Smithsonian Center for Astrophysics, Los Alamos National Laboratory, University of California at Berkeley, University of California at San Diego, and Computer Sciences Corporation.
For the latest images and mission information, visit the NEAR Web site at http://near.jhuapl.edu
In this Web site from Douglas Hamilton at the University of Maryland,
you can choose the size, velocity, and type of asteroid that will
crash into an object of your choice, and then see a typical picture of
of a similar crash.
Other innovative astronomy labs are linked to the site.
Also, see impact sites around the Earth at
The NASA satellite conducting the first-ever close-up study of an asteroid will be renamed to honor Dr. Eugene M. Shoemaker, a legendary geologist who influenced decades of research on the role of asteroids and comets in shaping the planets. The Near Earth Asteroid Rendezvous (NEAR) spacecraft, currently orbiting asteroid 433 Eros more than 145 million miles from Earth, will now be known as NEAR Shoemaker.
"Gene Shoemaker was an inspirational, charismatic pioneer in the field of interplanetary science," said Dr. Carl B. Pilcher, Director of Solar System Exploration at NASA Headquarters, Washington, DC. Pilcher announced the new name today during the Lunar and Planetary Science Conference in Houston. "It is a fitting tribute that we place his name on the spacecraft whose mission will expand on all he taught us about asteroids, comets and the origins of our solar system. "
Shoemaker died in a 1997 car accident in the Australian outback while on an annual study of asteroid impact craters. With his wife and research partner, Carolyn, Shoemaker was part of the leading comet discovery team of the past century, perhaps most famous for finding the comet (Shoemaker-Levy 9) that broke up and collided with Jupiter in 1994.
He was an expert on craters and the impacts that caused them. Shoemaker's work on the nature and origin of Meteor Crater in Arizona in the 1960s laid the foundation for research on craters throughout the solar system. He also established the lunar geological time scale that allowed researchers to date the features on the moon's surface.
Though he never realized his dream of tapping a rock hammer on the moon, Shoemaker taught Apollo astronauts about craters and lunar geology before they left Earth. Last year, when NASA's Lunar Prospector spacecraft crashed on the Moon in an experiment at the end of its mission, a small vial of Shoemaker's ashes, carried aboard the spacecraft, was scattered on the lunar surface.
Shoemaker was a key member of the 1985 working group that first studied the NEAR mission, defining its science objectives and designing a conceptual payload. Many of the group's recommended instruments were included in the actual spacecraft, which only a month into its yearlong orbit of Eros is already returning fascinating data on the asteroid's surface and geology.
See images at
See discussions at http://near.jhuapl.edu/
On behalf of the Cassini Imaging Team, I am pleased to announce the successful
imaging of asteroid 2685 Masursky with the Cassini cameras. You can enjoy the
two soon-to-be-released images, and our preliminary results, at the Imaging
Team web site:
It may be only a dot, but it's a very special dot to us!
My own observations were from El Saler near Valencia, Spain, where I saw a dozen meteors per minute at the peak of the shower at 2:08 UT on November 17, 1999. But most of the meteors were the brightness of the various stars in Orion; few were fireballs. Still, it was obvious that the meteors seemed to originate in the constellation Leo, which was high in the eastern sky. 12 meteors per minute = 720 meteors per hour in the direction I was looking, so to correct for the fraction of the sky I was observing, the total rate of meteors (known as the Zenith Hourly Rate, ZHR) must have been over 2000. Still, we didn't see hundreds of meteors in the sky at the same time, the way pictures of the meteor storm of 1833 show.
The shower was sharply peaked, with the substantial activity lasting less than 2 hours. It was over by the time Leo rose in the US, and few Leonids were seen here.
Daniel Fischer's report at
http://www.astro.uni-bonn.de/~dfischer/leo99/story.html includes links to many other sites.
Other reports and links are available at the European Southern Observatory reports and links page.
The Deep Space 1 asteroid flew by Asteroid Braille in late July, but the images were blank. Apparently the pointing was wrong.
Though the closeup photos of asteroid 9969 Braille didn't work: The team has received good data from an instrument that studies the three-dimensional distribution of ions and electrons, or plasma, and from an ion engine diagnostic instrument.
The best black-and-white image was at about 70 minutes prior to closest approach, with the asteroid appearing as just four pixels across. An apparent target-tracking problem prevented further photo return in the visible-light camera. Several infrared-camera images were obtained.
The ion propulsion engine was turned on again in order to place the spacecraft on track for a possible flyby in January 2001 of Comet Wilson-Harrington as part of an extended mission after the primary mission concludes September 18, 1999. Comet Wilson-Harrington is believed to be either a dormant comet or a "transition object" that is in the process of changing from a comet to an asteroid. The object has not been observed to behave like a comet -- spewing gas with a coma and tail -- since 1949; it is very unusual for a comet to exhibit this type of change in behavior. The second possible target of an extended mission, Comet Borrelly, is one of the most active comets that regularly visit the inner solar system. Deep Space 1 could fly by Borrelly in September 2001.
Extensive information on Deep Space 1, including a detailed press kit, is available on the Internet at: http://www.jpl.nasa.gov/ds1news/
ROYAL ASTRONOMICAL SOCIETY PRESS NOTICE
In the early hours of 17th November last year (1998), meteor watchers awaiting the Leonid shower were taken by surprise when a spectacular display of bright meteors occurred 16 hours before the predicted time for the maximum of the shower. The explanation has now been uncovered as a result of research by Dr David Asher and Professor Mark Bailey, of Armagh Observatory, and Professor Vacheslav Emel'yanenko, of South Ural University, Chelyabinsk, Russia. They have shown that the bright meteors were seen when Earth passed through a dense arc-shaped cloud of particles shed from Comet Tempel-Tuttle in the year 1333. By matching theory and observation, Dr Asher and colleagues have proved for the first time that meteoroid streams associated with Halley-like comets have complex braid-like structures within them. This work points the way to more precise predictions of the timing and intensity of meteor showers in the future. These results are reported in the 21st April 1999 issue of the Monthly Notices of the Royal Astronomical Society.
The Leonid meteor shower occurs between 15 and 21 November each year, with peak activity on the night of the 17/18 November. These meteors are produced when small dust particles ejected from Comet Tempel-Tuttle enter the Earth's atmosphere at high speed and burn up. Comet Tempel-Tuttle moves around the Sun in an elliptical orbit taking approximately 33 years for a complete revolution. Its orbit is similar to that of Halley's Comet, and so Comet Tempel-Tuttle is classified as a Halley-type short-period comet. Owing to the large angle between the Earth's orbit and the comet's (162 degrees), the dust grains collide almost head-on with the Earth, and hit the atmosphere at about 71 kilometres per second. At this speed, a one-centimetre particle carries the same amount of energy as a speeding truck on a motorway.
Every 33 years or so, when Comet Tempel-Tuttle passes near to the Earth, the intensity of the Leonid display is greatly enhanced because the stream of dust grains is more densely packed close to the comet. Meteor 'storms' have been seen many times during the past thousand years, notable events being those of 1799, 1833, 1866 and 1966. The earliest record of Leonid meteors dates back to the year 899.
November 1998 saw astronomers preparing for a possible meteor storm during the night of 17/18 November. Although a moderately strong peak was observed as predicted, the meteor shower as a whole was dominated by the appearance of hundreds of exceptionally bright meteors, known as fireballs, more than 16 hours ahead of the predicted peak.
The intensity and duration of this exceptional event indicated that the Earth must have passed through an extremely dense, narrow stream of large dust grains and particles, having sizes ranging up to several centimetres. The timing suggested that these particles occupied an orbit somewhat different from the main stream of small grains, and that they left the comet's nucleus many hundreds of years ago. But in that case, it is necessary to explain why the stream has held together so tightly for so long.
To solve the problem, Dr David Asher and his co-workers calculated the motion of large dust grains ejected from the comet at each of the last 42 occasions when it made its closest approach to the Sun. (Comets release very little dust, if any, when they are far from the Sun's heat.) They checked each case to see whether any of the particles could explain the fireballs seen in 1998, and identified September 1333 as the time when most of the observed particles were released. These particles did not spread out in space because of a dynamical process known as a resonance. (A similar process gives rise to the fine structure seen in Saturn's rings.)
Many comets and asteroids swing around the Sun in orbits that are simple multiples of the orbital period of Jupiter, the most massive planet in the solar system and the biggest disturbing influence on cometary orbits. Comet Tempel-Tuttle is no exception to this rule, having entered one of these 'resonant' orbits as long ago as the seventh century AD. For every fourteen revolutions of Jupiter, Comet Tempel-Tuttle makes five, and the same relation holds true for the largest dust particles gently released by the comet.
The large grains therefore have average orbital periods very close to that of the comet, and are kept in step by the influence of Jupiter. Instead of spreading around the whole orbit, they occupy a rather short arc, leading to the formation of a dense strand of large particles, distinct from the 'normal' storm strands of small particles, ahead of and behind the comet. The structure of the meteoroid stream close to the comet can be visualized as rather like a telephone wire, made up of many separate, narrow strands. These form a complex, braided structure of material within the broader, ribbon-like meteoroid stream.
The calculations by David Asher and co-workers showed that in November 1998 most of the resonant arcs missed the Earth by a wide margin, but the arc of particles released in 1333 cut right through the Earth's orbit, and the calculated time for when this happened matched the observed fireball maximum to the hour.
This remarkable result is the first observational demonstration of one of the most important dynamical features of meteoroid streams associated with Halley-type short-period comets. The work highlights the presence of fine structure *within* meteoroid streams, and suggests important new avenues for research. For example, by observing the variations in meteor rates close to the peak of a shower it may be possible to infer the precise distribution in space of the meteor-producing strands. Variations in meteor rates may be correlated with changes in the meteor brightness distribution to infer the history of mass loss by a comet over many revolutions around the Sun.
The researchers are not expecting a repeat performance of bright fireballs in November this year (1999). All the resonant strands in the meteoroid stream will be well past Earth in space. However, a strong 'normal' display is likely, peaking at about 2 a.m. on November 18th, due to meteoroids ejected from Comet Tempel-Tuttle in the years 1866, 1899 and 1932, which have not yet had time to disperse around the comet's orbit.
Asteroid 433 Eros is slightly smaller than predicted, with at least two medium-sized craters, a long surface ridge, and a density comparable to the Earth's crust, according to measurements from NASA's Near Earth Asteroid Rendezvous (NEAR) spacecraft.
NEAR's science instruments observed about two-thirds of Eros on Dec. 23, 1998, as the spacecraft flew by the asteroid following an unsuccessful firing of its main engine a few days earlier. A subsequent successful firing of the engine put NEAR on course to rendezvous with Eros to begin its planned yearlong orbital mission starting in mid-February 2000.
Scientists and engineers at The Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, MD, which manages the mission, and science team members from affiliated institutions quickly planned the valuable flyby observations in the wake of the unsuccessful engine burn on Dec. 20.
During the flyby, 222 photos and supporting spectral observations were taken from as close as 2,375 miles (3,830 kilometers) from the asteroid by the spacecraft's multispectral imager, infrared spectrometer, and radio science experiment. "The flyby of Eros has given us fundamental information that will help us plan a better orbital mission at Eros," said Dr. Andrew F. Cheng, NEAR project scientist at APL. "It has taken some of the risk out of our orbit insertion maneuver and early operations."
First observed from the Earth more than 100 years ago, Eros was known to be an S-type asteroid with high concentrations of silicate minerals and metal. However, few details about its structure or composition are observable from the ground. The NEAR flyby produced evidence of variations in surface color and reflected light (or albedo) that suggest the asteroid has a diverse surface makeup. Closer observations during the comprehensive yearlong orbital study of Eros will be needed to determine its precise composition.
The science team has determined that Eros is slightly smaller than originally estimated from ground-based radar observations, with a size of 21 by 8 by 8 miles (33 by 13 by 13 kilometers), versus an estimate of 25.3 by 9 by 8 miles (40.5 by 14.5 by 14 km). The asteroid rotates once every 5.27 hours and has no discernible moons.
The asteroid's density is approximately 1.55 ounces per cubic inch (2.7 grams per cubic centimeter), close to the average density of Earth's crust. This makes Eros about twice as dense as asteroid 253 Mathilde, a C-type, carbon-rich asteroid that NEAR flew past in June 1997, and about the same density as S-type asteroid 243 Ida, which NASA's Galileo spacecraft flew past in 1993. Eros and Ida are the only S-type asteroids for which a mass and density have been determined.
Flyby imaging of the asteroid's surface revealed a prominent elongated ridge that extends along its length for as much as 12 miles (20 km). "This ridge-like feature, combined with the measurements of high density, suggests that Eros is a homogeneous body rather than a collection of rubble" such as Mathilde appears to be, said Dr. Joseph Veverka, of Cornell University, Ithaca, NY, who heads the mission's imaging team. "It might even be a remnant of a larger body that was shattered by an impact."
The surface of Eros is pocked with craters. The two largest craters are four miles and 5.3 miles (8.5 and 6.5 km) in diameter, less than half the size of asteroid Mathilde's largest craters. The existence of fewer, smaller craters could be an indication that Eros has a relatively young surface when compared to Ida.
NEAR and Eros will cross paths again in February 2000. The spacecraft will then be inserted into orbit around the asteroid and begin its yearlong study. Images taken during orbit are expected to have more than 200 times better resolution than those obtained during the flyby and will be taken from as close as nine miles (15 km) from the asteroid's surface.
Flyby images of Eros and a related movie, a shape model of the asteroid, and a chart of spectral observations are available on the NEAR mission Web site at:
Despite an aborted engine burn that postponed the January 1999, rendezvous of the NEAR spacecraft with asteroid Eros, mission operations team members were able to quickly upload new commands to the spacecraft, making it possible to obtain valuable information during a December 23, 1998, flyby of the asteroid.
Science data, including multicolor images, spectral data, and magnetic field measurements taken during the flyby, are now coming into the Applied Physics Laboratory's NEAR Science Data Center. Doppler navigation data and real-time telemetry were collected that will help determine the mass of the asteroid.
The flyby gave NEAR Mission Operations an opportunity to test tracking and instrument sequences in preparation of a rendezvous event, says Mission Operations Center Manager Mark Holdridge. "The flight recorders, full of Eros science data, are being played back at the present time. The spacecraft is healthy and doing just fine and it has been confirmed that the flyby pointing and instrument command sequence executed flawlessly to completion."
In addition to flyby data processing, team members are also studying data sent by the spacecraft soon after contact with it was reestablished on Dec. 22, following 27 hours of communication blackout to determine the exact nature of the software anomaly that led to the rendezvous burn failure. NEAR and Eros are now traveling in separate orbits around the sun as plans are being made for a rendezvous sometime between August 1999 and April 2000.
Images of Eros taken during the flyby are being posted on the NEAR Web site as they are processed.
The orbit of XF11 will bring it close to the Earth many times. In October 2002 it will be an excellent target for detailed radar observations, and in 2028 it may even be bright enough to be seen without telescopic aid.
The special interest in this object began when International Astronomical Union minor planet notice #6837 released by Brian Marsden on March 11 estimated a miss distance of only 50,000 km in its passage near Earth on October 26 2028. Marsden wrote in a press release reproduced below: "Recent orbit computations indicate it is virtually certain that it will pass within the Moon's distance of the Earth a little more than 30 years from now. The chance of an actual collision is small, but one is not entirely out of the question."
The story was widely reported, with the following AP coverage typical:
WASHINGTON (AP) - An asteroid large enough to cause widespread destruction may be heading toward a 2028 collision with the Earth and will certainly pass closer to the planet than any such space object in modern times, astronomers said Wednesday. ``The chance of an actual collision is small, but one is not entirely out of the question,'' according to a notice filed by the International Astronomical Union. ``It has enormous destructive potential,'' said Steven Maran of the American Astronomical Society, but he added it will take several more years of observations before experts are certain of its path. ``It scares me. It really does,'' said Jack G. Hills, an asteroid specialist at the Los Alamos National Laboratory. ``An object this big hitting the Earth has the potential of killing many, many people.''
The London Times reported in a lead article by Nigel Hawes: Apocalypse could be just 30 years away, astronomers said yesterday. They have identified an asteroid a mile across on a near-collision course with Earth. It is by far the most alarming object yet identified in the search for asteroids and comets with Armageddon potential.
Following this announcement several astronomers searched older photographic records to try to locate previously unrecognized observations of 1997XF11. Eleanor Helin and her colleagues from the Jet Propulsion Lab soon found images taken in 1990 that permitted calculation of an improved orbit for the asteroid. Based on this expanded observation set, Don Yeomans and Paul Chodas at JPL recalculated the orbit and found that the 2028 close approach circumstances are: Time: 2028 Oct 26.26732 (06:24 UT) +/- 63 minutes; closest approach distance = 0.00638 AU = 954340 km +/- 0.00058 AU; relative velocity at closest approach = 13.914 km/s. This means that the asteroid will pass by the Earth at about twice the distance of the Moon, and that the probability of impact with Earth is effectively zero. The JPL press release providing this information is included below.
The discovery of 1997XF11 illustrates several aspects of the asteroid impact hazard. (1) When astronomers carry out searches, they typically find even threatening asteroids decades to centuries before their actual impact with the Earth. If it had turned out that XF11 posed a threat to Earth in 2028, we would have had three decades to deal with this threat. (2) With a diameter of about a mile, XF11 is near the threshold for global disaster. The impact of an object this size with the Earth would release a million megatons of energy and would probably lead to the death of hundreds of millions of people. (3) Most of the asteroids that could strike the Earth and cause a global catastrophe have not yet been found. For the year 2028 (or any other year) the chances of an unknown asteroid hitting the Earth are much greater than the chances of this particular asteroid hitting. (4) if an unknown asteroid should hit us, we would likely have no warning at all. The first we would know of the danger is when we saw the flash of light and felt the ground shake. (5) At the current rate of discovery, it will take more than a century to find 90% or more of the objects this large with Earth-crossing orbits. (6) For better or for worse, the astronomers who carry out these searches and orbit calculations work in the public eye. The idea that a threatening asteroid could be kept secret (or that anyone would want to keep it secret) is ludicrous
For further information see the NASA asteroid and comet impact hazard website at:
View of the Earth from the NEAR spacecraft as it passed the Earth in January 1998 along with earlier views of the asteroid Mathilde are available.
NASA has announced the selection of another NEO mission, to provide multiple comet flybys. The Discovery mission called Contour is led by PI Joseph Veverka of Cornell University. This is the third NASA Discovery mission to study small bodies. Currently the first Discovery mission, Near Earth Asteroid Rendezvous (NEAR), is en route to Eros, while under development is the mission Stardust to return samples of the coma of comet Wild-2. More details of this Discovery selection are given in the NASA press release below, taken from the NASA homepage at http://www.nasa.gov.
MISSIONS TO GATHER SOLAR WIND SAMPLES AND TOUR THREE COMETS SELECTED AS NEXT DISCOVERY PROGRAM FLIGHTS
A mission to gather samples of the wind flowing from the Sun and a mission to fly by three near-Earth comets have been selected as the next flights in NASA's Discovery Program of lower-cost, highly focused scientific spacecraft.
The Genesis mission is designed to collect samples of the charged particles in the solar wind and return them to Earth laboratories for detailed analysis. It is led by Dr. Donald Burnett from the California Institute of Technology, Pasadena, CA, at a total cost to NASA of $216 million. Due for launch in January 2001, it will return the samples of isotopes of oxygen, nitrogen, the noble gases, and other elements to an airborne capture in the Utah desert in August 2003. Such data are crucial for improving theories about the origin of the Sun and the planets, which formed from the same primordial dust cloud.
The Comet Nucleus Tour (CONTOUR) will take images and comparative spectral maps of at least three comet nuclei and analyze the dust flowing from them. CONTOUR is led by Dr. Joseph Veverka of Cornell University, Ithaca, NY, at a total cost to NASA of $154 million. It is scheduled for launch in July 2002, with its first comet flyby to occur in November 2003. This flyby of Comet Encke at a distance of about 60 miles (100 kilometers) will be followed by similar encounters with Comet Schwassmann-Wachmann- 3 in June 2006 and Comet d'Arrest in August 2008.
The Galileo spacecraft discovered a satellite, Dactyl, for the asteroid 243 Ida when it flew by in 1993. Dactyl is only 1.5 km across, 1/4o the diameter of Ida.
Now a second asteroid satellite has been discovered, this one from ground-based observations at the European Southern Observatory's site in Chile. The brightness of asteroid 3671 Dionysius was being monitored when a substantial drop in brightness showed up. The most reasonable interpretation was that the scientists were seeing a partial eclipse of Dionysius by a satellite--a moon or it, if asteroids can be said to have moons. This moon, though only 0.5 km as far as can be estimated, is half the diameter of Dionysius, and orbits only a few kilometers from it.
Astronomers have used NASA's Hubble Space Telescope to discover a giant impact crater on the asteroid Vesta. The crater is a link in a chain of events thought responsible for forming a distinctive class of tiny asteroids as well as some meteorites that have reached the Earth.
The giant crater is 285 miles across, which is nearly equal to Vesta's 330 mile diameter. If Earth had a crater of proportional size, it would fill the Pacific Ocean basin. Astronomers had predicted the existence of one or more large craters, reasoning that if Vesta is the true "parent body" of some smaller asteroids then it should have the wound of a major impact that was catastrophic enough to knock off big chunks. The observations are described in the September 5 issue of Science Magazine.
"In hindsight we should have expected finding such a large crater on Vesta," says Peter Thomas of Cornell University, Ithaca, NY. "But it's still a surprise when it's staring you in the face." Another surprising finding is that such a large crater, relative to Vesta's size, might have been expected to cause more damage to the rest of the minor planet. "This is a unique opportunity to study the effects of a large impact on a small object," says Michael Gaffey of Rensselaer Polytechnic Institute, Troy, New York. "This suggests that more asteroids from the early days of the solar system may still be intact."
The collision gouged out one percent of the asteroid's volume, blasting over one-half million cubic miles of rock into space. This tore out an eight-mile deep hole that may go almost all the way through the crust to expose the asteroid's mantle (Vesta is large enough to be differentiated like Earth - with a volcanic crust, core and mantle, making it a sort of "mini-planet".)
Because of the asteroid's small diameter and low gravity, the crater resembles smaller craters on the Moon that have a distinctive central peak. Towering eight miles, this cone-shaped feature formed when molten rock "sloshed" back to the bull's-eye center after the impact.
One clue for a giant crater came in 1994 when Hubble pictures showed that one side of Vesta's football shape appeared flattened. "We knew then there was something on Vesta that was unusual," says Thomas.
The astronomers had to wait for a better view from Hubble when Vesta made it closest approach to Earth in a decade, in May 1996, when the asteroid was 110 million miles away.
A total of 78 Wide Field Planetary Camera 2 pictures were taken. The team then created a topographic model of the asteroid's surface by noting surface irregularities along the limb and at the terminator (day/night boundary) where shadows are enhanced by the low Sun angle.
The immense crater lies near the asteroid's south pole. This is probably more than coincidental, say researchers. The excavation of so much material from one side of the asteroid would have shifted its rotation axis so that it settled with the crater near one pole.
Unlike some other large asteroids that have jumbled surfaces due to the asteroids breakup and recollapse, the rest of Vesta's surface is largely intact, despite the cataclysm. This is based on previous measurements showing it has a surface of basaltic rock - frozen lava - which oozed out of the asteroid's presumably hot interior shortly after its formation 4.5 billion years ago, and has remained largely intact ever since.
Approximately six percent of the meteorites that fall to Earth are similar to Vesta's mineralogical signature, as indicated by their spectral characteristics. Vesta's spectrum is unique among all the larger asteroids. The crater may be the ultimate source of many of these meteorites.
Most meteorites are believed to come from other asteroids, but their specific objects of origin cannot be determined in most cases. Thus the distinctive mineralogical makeup of these meteorites means that Vesta is the only world other than the Earth, the Moon and Mars for which scientists have samples of specifically known origin.
A mystery has been that the meteorites could not have traveled directly from Vesta because at Vesta's location in the asteroid belt, there are no perturbing gravitational forces which would cause pieces to fall into orbits intersecting the inner planets like apples shaken out of a tree. However, Vesta's "daughter" asteroids -- literally "chips off the block" which have color characteristics similar to Vesta, are near a "chaotic zone" in the asteroid belt where Jupiter's gravitational tug can redirect fragments into orbits which intersect Earth's orbit.
A good determination of the shape of Vesta was necessary for the next step in interpretation, which will use multi-color images of Vesta obtained with HST to study the detailed mineralogy of surface regions including the region of the giant crater. Also, a team led by Don McCarthy of The University of Arizona plans to obtain additional images of Vesta at longer wavelengths this fall using the new Near Infrared and Multi-Object Spectrometer (NICMOS) science instrument on board Hubble.
Members of the Vesta research team are Principal Investigator Ben Zellner of Georgia Southern University; the Co-Investigators are Richard Binzel, MIT, Michael Gaffey, Rensselaer Polytechnic Institute, Alex Storrs, Space Telescope Science Institute, Peter Thomas, Cornell University and Dr. Ed Wells, Computer Sciences Corporation.