On Friday 14 June, an asteroid the size of a football pitch made one of the closest ever recorded approaches to Earth. Astronomers working on the LINEAR search programme, near Socorro, New Mexico first detected the giant rock on 17 June, a few days after its close approach.

The Near Earth Object, known to astronomers as '2002MN', was travelling at over 10 km/s (23,000 miles per hour) when it passed Earth at a distance of around 120,000 km (75,000 miles), bringing it well inside the Moon's orbit. The last time a known asteroid passed this close was back in December 1994.

Asteroids are typically too small and distant to measure their size directly from Earth, so scientists use the amount of light they reflect, along with a basic understanding of the materials they are made of, to estimate their size. With a diameter between 50-120 metres, 2002 MN is a lightweight among asteroids and incapable of causing damage on a global scale, such as the object associated with the extinction of the dinosaurs.

However, if it had hit the Earth, 2002MN may have caused local devastation similar to that which occurred in Tunguska, Siberia in 1908, when 2000 square kilometres of forest were flattened. Whilst the vast majority of NEOs discovered do not come this close, such near misses do highlight the importance of detecting these objects. This reminder comes in a week when the UK telescopes on La Palma are being tested to search for NEOs.

Brief Description of Object:

• Object Designation: 2002MN
• Date of First Observation: 17/06/02
• Number of Observations: 14
• Search Team: LINEAR (Lincoln Near Earth Asteroid Research)
• Date of Closest Approach: 14/06/02
• Closest Approach Distance: 0.000797 AU or 119,229 km (0.3 Lunar Distances)
• Asteroids Velocity Relative to Earth at Closest Approach: 10.58 km/s (23,667 miles per hour)
• Estimated Diameter of Asteroid: 50-120 metres
• Orbital Period: 894.9 days

A new study at Southwest Research Institute (SwRI) has identified a recent asteroid breakup event in the main asteroid belt. Computer simulations have shown that the event occurred 5.8 million years ago, when a 15-mile-wide asteroid in the main belt region shattered into numerous fragments following a collision. This observation marks the first time that an asteroid disruption event has been precisely dated. The findings appear in the June 13 issue of the journal Nature.

The main asteroid belt, a population of roaming boulders with sizes ranging from Texas-sized rocks to tiny pebbles, lies between the orbits of Mars and Jupiter. Asteroids in this region frequently collide, possibly explaining why spacecraft and radar images of these bodies show them to have irregular shapes and heavily cratered surfaces. These highly energetic collisions provide critical insights into the physics of the much more massive impacts that helped shape early Earth.

"One problem with studying large-scale asteroid impacts," says lead investigator Dr. David Nesvorny, a researcher at the SwRI Boulder Office, "is that most of these events happened hundreds of millions to billions of years ago, long enough for collisional and dynamical evolution to have eroded most of the telltale features that could shed light on the impact process."

Nesvorny and SwRI team members Dr. William F. Bottke Jr., Dr. Luke Dones, and Dr. Harold P. Levison carefully studied a cluster of asteroid fragments called an "asteroid family," a group of large and small rocks believed to be the leftover pieces produced by a highly energetic collision. Dubbed the "Karin cluster," after the name of its largest member, 11-mile-long asteroid (832) Karin, the orbits of 13 asteroids in the cluster were tracked backwards in time using computer models. The team found that 5.8 million years ago, all 13 bodies shared the same orbital orientation in space, making it possible to identify them as the by-product of a single asteroid disruption event.

"This convergence was not an accident," says Nesvorny. "Tests indicate that the probability of finding such an orbital alignment by chance was less than one part in a million over the lifetime of the solar system."

The relative youth and known age of the Karin cluster could help researchers answer several important questions about asteroid geology and impact physics. The Karin cluster serves as a natural laboratory for the study of asteroid collisions. For example, data from this disruption event could be used to validate computer simulations that show the effects of large bodies colliding at high velocities.

The Karin cluster also could help researchers understand "space weathering." The impacts of highly energetic particles from the sun, along with micrometeorite impacts, over time have changed the optical properties of asteroid surfaces. This makes it difficult for researchers to identify the kinds of asteroids that produce particular types of stony meteorite such as "ordinary chondrites." Because objects in the Karin cluster are young and their formation age is known, further investigation of their surface properties could provide vital clues into the nature and rate at which space weathering modifies their surface features.

The known age of the Karin-cluster members also could help explain the rate at which asteroids strike one another in the main belt. Because the Karin cluster asteroids could have been given "blank slates" 5.8 million years ago, craters formed since that time by asteroid collisions could be used to estimate the current crater production rate in the main belt. This information could help researchers determine surface ages of asteroids visited by spacecraft.

The team even considers the possibility that some of the meteorites landing on Earth today could be traced back to this breakup event. "If a solid connection can be made between this event and some class of meteorites collected on Earth, we could use laboratory studies of these meteorites to understand the nature of asteroids in the Karin cluster," says Nesvorny. "Results from these studies would be equivalent, in many ways, to a spacecraft sample return mission, thus fulfilling a long-time NASA science objective." Moreover, the SwRI team believes that the Karin cluster may be a source region of the asteroidal dust daily accreted in large amounts by the Earth from outer space.

NASA provided funding for the program. The paper "The Recent Breakup of an Asteroid in the Main-Belt Region," by Nesvorny, Bottke, Dones, and Levison appears in the June 13 issue of Nature.

An ingenious arrangement of three homebuilt 14-inch telescopes on fixed mountings is enabling Tucson-based amateur astronomer Roy Tucker to conduct a backyard hunt for asteroids whose quality is on par with the best professional searches in the world.

Tucker, an instrumentation engineer at the National Optical Astronomy Observatory (NOAO), completed his new telescope last April. The fixed mountings made the whole construction far less expensive than any telescope set up on a normal, steerable mount. Tucker's fixed telescopes scan the sky as the Earth turns and reach a limiting magnitude of 20.5 -- fainter than most professional asteroid searches. And thanks to a cleverly designed steel-and-aluminum framework that automatically compensates for temperature changes, the telescope can run unattended all night with no need for focus adjustments. The three telescopes are arranged so that they produce sequential image triplets that can be compared to reveal any moving objects....

Binary asteroids -- two rocky objects orbiting about one another -- appear to be common in Earth-crossing orbits, astronomers using the world's two most powerful astronomical radar telescopes report. And it is probable, they say, that these double asteroid systems have been formed as a result of gravitational effects during close encounters with at least two of the inner planets, including Earth.

Writing in a report published by the journalScience on its Science Express web site (April 11, 2002), the researchers estimate that about 16 percent of so-called near-Earth asteroids (NEAs) larger than 200 meters (219 yards) in diameter are likely to be binary systems, with about a three-to-one relative size of the two encircling bodies. To date, five such binary systems have been identified by radar, says lead researcher Jean-Luc Margot, an O.K. Earl postdoctoral fellow in the Division of Geological and Planetary Sciences at the California Institute of Technology.

Margot, who at the time of the observations was a research associate in the planetary studies/radar group at the National Science Foundation's (NSF) Arecibo Observatory in Puerto Rico (managed at Cornell University), says that theoretical and modeling results show the binary asteroids appear to be formed extremely close to Earth -- within a distance equal to a few times the planet's radius (6,378 kilometers or 3,963 miles). "The fact that one out of every six large NEAs is a binary and that they typically survive on the order of 10 million years, implies that these close encounters must happen frequently compared to the lifetime of the binary asteroids," says Margot.

The Science article, "Binary Asteroids in the Near-Earth Object Population," is coauthored by Michael Nolan, research associate at Arecibo; Lance Benner, Steven Ostro, Raymond Jurgens, Jon Giorgini and Martin Slade at the Jet Propulsion Laboratory (JPL); and Donald Campbell, professor of astronomy at Cornell. The observations were made at the 70-meter Goldstone NASA tracking telescope in California and at Arecibo Observatory.

NEAs are formed in the asteroid belt, between the orbits of Mars and Jupiter, and nudged by the gravitational attraction of nearby planets, largely Jupiter, into orbits that allow them to enter the Earth's neighborhood. Most of the asteroids are the remnants of the initial agglomeration of the inner planets.

Astronomers have long speculated about the existence of binary NEAs, based in part on impact craters on Earth. Of about 28 known terrestrial impact craters with diameters greater than 20 kilometers, at least three are double craters formed by impacts of objects about the same size as the newly discovered binaries. Astronomers also have noted the changes in brightness of reflected sunlight for some NEAs, indicating a double system was causing an eclipse or occultation of one by the other.

In 2000, Margot and his co-researchers, using measurements from the Goldstone radar, found that a small, roughly 800-meter-diameter (half-a-mile) asteroid, 2000 DP107 (discovered only months before by a team from the Massachusetts Institute of Technology), was a binary system. Observations over eight days last October with the much more sensitive Arecibo telescope clearly established the physical characteristics of DP107's two asteroids as well as their orbit about each other. The smaller object called the secondary, it was found, is about 300 meters (1,000 feet) in diameter and is orbiting the larger asteroid, the primary, every 42 hours at a distance of 2.6 kilometers (1.6 miles). The two asteroids appear to be locked in synchronous rotation, with the smaller always with the same face oriented to the larger.

Since that observation, says Margot, four more binary NEAs have been discovered, all in Earth-crossing orbits and each with a main asteroid significantly larger than the smaller body. "The primary is rotating much faster than most NEAs in all five binaries that have been discovered," says Cornell's Campbell. The Science Express article speculates that the most likely way the binaries are created is by close encounters of asteroids with the inner planets Earth or Mars. Of the five binary NEAs discovered to date, none has an orbit that brings it as close to the sun as Venus or Mercury.

NEAs, basically piles of rubble held together by gravity, are on trajectories that bring them within a few thousand miles of the planets, where tidal forces ---- essentially the pull of gravity -- can increase the spin rate of the asteroid, causing it to fly apart. The ejected rubble then reforms in orbit around the larger asteroid.

"The asteroid is already rotating very quickly as it approaches the planet. A little extra boost from tidal forces can be enough to exceed its breakup limits, and it sheds mass. This mass can end up forming another object in orbit around the asteroid. Right now this seems the most likely explanation," says Margot.

There is an important reason for studying binary asteroids, says JPL's Ostro: their potential for colliding with Earth. Knowing the density of so-called PHAs (for potentially hazardous asteroids), he observes, "is an extremely important input to any mitigation plans." He says, "Getting NEA densities from radar is dirt cheap compared with getting a density with a spacecraft. Of course, the most important thing to know about any PHA is whether it is two objects or one, and this is why we want to observe these binaries with radar whenever possible."

Margot notes, "Radar gives us very precise measurements of the size of the objects and their shape. The radar measurements of the distance and velocity of each component allows us to obtain precise information on their orbits. From this we can obtain the mass of each of the objects allowing, for the Þrst time, measurements of NEA densities, a very important indicator of their composition and internal structure."

Arecibo Observatory is operated by the National Astronomy and Ionosphere Center at Cornell under a cooperative agreement with the NSF. The research was supported by the NSF, with NASA providing additional support for the planetary radar program at Arecibo.

NASA's Hubble Space Telescope is hot on the trail of an intriguing new class of solar system object that might be called a Pluto "mini-me" -- dim and fleeting objects that travel in pairs in the frigid, mysterious outer realm of the solar system called the Kuiper Belt.

In results published today in the journal Nature, a team of astronomers led by Christian Veillet of the Canada-France- Hawaii Telescope Corporation (CFHT) in Kamuela, Hawaii, is reporting the most detailed observations yet of the Kuiper Belt object (KBO) 1998 WW31, which was discovered four years ago and found to be a binary last year by the CFHT.

Pluto and its moon Charon and countless icy bodies known as KBOs inhabit a vast region of space called the Kuiper Belt. This "junkyard" of material left over from the solar system's formation extends from the orbit of Neptune out to 100 times as far as the Earth is from the Sun (which is about 93 million miles) and is the source of at least half the short-period comets that whiz through our solar system. Only recently have astronomers found that a small percentage of KBOs are actually two objects orbiting around each other, called binaries.

"More than one percent of the approximately 500 known KBOs are indeed binary: a puzzling fact for which many explanations will be proposed in what is going to be a very exciting and rapidly evolving field of research in the coming years," says Veillet.

Hubble was able to measure the total mass of the pair based on their mutual 570-day orbit (a technique Isaac Newton used 400 years ago to estimate the mass of our Moon). Together, the "odd-couple" 1998 WW31 is about 5,000 times less massive than Pluto and Charon.

Like a pair of waltzing skaters, the binary KBOs pivot around a common center of gravity. The orbit of 1998 WW31 is the most eccentric ever measured for any binary solar-system object or planetary satellite. Its orbital distance varies by a factor of ten, from 2,500 to 25,000 miles (4,000 to 40,000 kilometers). It is difficult to determine how KBOs wind up traveling in pairs. They may have formed that way, born like twins, or may be produced by collisions where a single body is split in two.

Ever since the first KBO was discovered in 1992, astronomers have wondered how many KBOs may be binaries, but it was generally assumed that the observations would be too difficult for most telescopes. However, the insights to be gained from study of binary KBOs would be significant: measuring binary orbits provides estimates of KBO masses, and mutual eclipses of the binary allow astronomers to determine individual sizes and densities. Assuming some fraction of KBOs should be binary -- just as has been discovered in the asteroid belt -- astronomers eventually began to search for gravitationally entwined pairs of KBOs.

Then, finally, exactly a year ago on April 16, 2001, Veillet and collaborators announced the first discovery of a binary KBO: 1998 WW31. Since then, astronomers have reported the discoveries of six more binary KBOs. "It's amazing that something that seems so hard to do and takes many years to accomplish can then trigger an avalanche of discoveries," says Veillet. Four of those discoveries were made with the Hubble Space Telescope: two were discovered with a program led by Michael Brown of the California Institute of Technology in Pasadena, Calif., and two more with a program led by Keith Noll of the Space Telescope Science Institute in Baltimore. The sensitivity and resolution of Hubble is ideal for studying binary KBOs because the objects are so faint and so close together.

The Kuiper Belt is one of the last big missing puzzle pieces to understanding the origin and evolution of our solar system and planetary systems around other stars. Dust disks seen around other stars could be replenished by collisions among Kuiper Belt-type objects, which seems to be common among stars. These collisions offer fundamental clues to the birth of planetary systems.

Observations made in 1996-97 by the European Space Agency's Infrared Space Observatory show that the asteroid belt contains about twice as many objects as previously thought. The new census involved tallying up the main-belt asteroids spotted in selected locations, then extrapolating those counts to include the entire sky. The result, says Edward Tedesco (TerraSystems), suggests that the main belt (between Mars and Jupiter) contains 1.1 to 1.9 million minor planets at least 1 kilometer across. Previous studies in 1998 and 2001 had estimated the count of 1-km or larger objects at 860,000 and 740,000, respectively.

A kilometer-size asteroid, whose whereabouts have been unknown since just after its discovery 52 years ago, has suddenly reemerged as an object that may pose a significant threat to Earth in the distant future. Astronomers at Lowell Observatory rediscovered the wayward object, known as 1950 DA, by accident on New Year's Eve 2000, and three months later teams of radar astronomers pinged it from Goldstone, California, and Arecibo, Puerto Rico. When orbital dynamicists combined the high-precision radar tracks with the half-century-long photographic record, they realized that 1950 DA is likely to make three close brushes with Earth in the centuries ahead. One of those, on March 16, 2880, could result in a direct hit....

However, notes coauthor Steven R. Chesley (JPL), "The impact risk is not the story here, because we can say almost unequivocally that it's not going to hit Earth." The real story, he says, is how having such a precise orbit has allowed dynamicists to push the realm of impact prediction so far into the future....

Billed as the "blind-spot" asteroid, a building-size space rock passed the Earth unnoticed two weeks ago. An automated sky survey detected minor planet 2002 EM7 on March 12th. Subsequent orbital calculations determined that the asteroid had come closest to the Earth four days earlier at a distance of about 464,000 kilometers (288,000 miles), slightly more than the distance from the Earth to the Moon. Prior to the flyby, 2002 EM7 was too close to the Sun, hence the "blind-spot" moniker.

When the close call was made public, it raised considerable concern. Researchers estimated the object to be about 50 to 70 meters across, thought to be a little smaller than the object that exploded over the Tunguska River region of Siberia in 1908 and flattened thousands of square kilometers of forest. A Tunguska explosion over a populated area would undoubtedly cause incredible damage.

Despite the media attention in the wake of 2002 EM7's passage, such "close" flybys are not uncommon. According to Jim Scotti (University of Arizona), "Simply put, objects the size of the Tunguska impactor pass within the distance that 2002 EM7 did about 25 times every year." Rocks the size of 2002 EM7 come by nearly 100 times a year. This particular instance grabbed headlines because the minor planet was actually observed. Scotti explains that astronomers cannot fully tally asteroids about 50 m in diameter using today's survey techniques, regardless of whether the objects are moving from the direction of the Sun or not. Alas, this is of little solace to people worried about space-borne threats.