Category: Asteroids

Over the past week, several independent efforts were made to search for pre-discovery observations of 2004 mn4. These efforts proved successful today when Jeff Larsen and Anne Descour of the Spacewatch Observatory near Tucson, Arizona, were able to detect and measure very faint images of asteroid 2004mn4 on archival images dating to 15 March 2004. These observations extended the observed time interval for this asteroid by three months allowing an improvement in its orbit so that an Earth impact on 13 April 2029 can now be ruled out.

As is often the case, the possibility of future Earth impacts for some near-Earth objects cannot be entirely ruled out until the uncertainties associated with their trajectories are reduced as a result of either future position observations, or in this case, heretofore unrecognized, pre-discovery observations. When these additional observations were used to update the orbit of 2004 MN4, the uncertainties associated with this object’s future positions in space were reduced to such an extent that none of the object’s possible trajectories can impact the Earth (or Moon) in 2029.

In the accompanying diagram, the most likely position of asteroid 2004 MN4 is shown at the end of the blue line near the Earth on 13 April 2029. However, since the asteroid’s position in space is not perfectly known at that time, the white dots at right angles to the blue line are possible alternate positions of the asteroid. Neither the nominal position of the asteroid, nor any of its possible alternative positions, touches the Earth, indicating that an Earth impact in 2029 is ruled out.

The passage of the asteroid by the Earth in 2029 alters its subsequent trajectory and expands the asteroid’s position uncertainty region (i.e., the line of white dots increases in extent) so the asteroid’s subsequent motion is less certain than it was prior to the 2029 close Earth approach. However, our current risk analysis for 2004 MN4 indicates that no subsequent Earth encounters in the 21st century are of any concern.

Original Source: NASA News Release

The probability that Asteroid 2004 MN4 will strike the Earth on April 13, 2029 has actually been upgraded to a 1-in-43 chance now that more observations have been made. The asteroid has reached an uprecedented 4 on the Torino scale. Of course, this still means that there’s a 98% chance that it’ll completely miss the Earth. The space rock is 400 metres (1,300 feet) across, so a direct impact with our planet would cause a significant amount of damage on a regional level. Update: as of Dec. 28th, the probability has been significantly downgraded thanks to further observations. It’ll definitely miss.

A recently rediscovered 400-meter Near-Earth Asteroid (NEA) is predicted to pass near the Earth on 13 April 2029. The flyby distance is uncertain and an Earth impact cannot yet be ruled out. The odds of impact, presently around 1 in 300, are unusual enough to merit special monitoring by astronomers, but should not be of public concern. These odds are likely to change on a day-to-day basis as new data are received. In all likelihood, the possibility of impact will eventually be eliminated as the asteroid continues to be tracked by astronomers around the world.

This object, 2004 mn4, is the first to reach a level 2 (out of 10) on the Torino Scale. According to the Torino Scale, a rating of 2 indicates “a discovery, which may become routine with expanded searches, of an object making a somewhat close but not highly unusual pass near the Earth. While meriting attention by astronomers, there is no cause for public attention or public concern as an actual collision is very unlikely. New telescopic observations very likely will lead to re-assignment to Level 0 [no hazard].” This asteroid should be easily observable throughout the coming months.

The brightness of 2003 qq47 suggests that its diameter is roughly 400 meters (1300 feet) and our current, but very uncertain, best estimate of the flyby distance in 2029 is about twice the distance of the moon, or about 780,000 km (480,000 miles). On average, an asteroid of this size would be expected to pass within 2 lunar distances of Earth every 5 years or so.

Most of this object’s orbit lies within the Earth’s orbit, and it approaches the sun almost as close as the orbit of Venus. 2004mn4’s orbital period about the sun is 323 days, placing it within the Aten class of NEAs, which have an orbital period less than one year. It has a low inclination with respect to the Earth’s orbit and the asteroid crosses near the Earth’s orbit twice on each of its passages about the sun.

2004 MN4 was discovered on 19 June 2004 by Roy Tucker, David Tholen and Fabrizio Bernardi of the NASA-funded University of Hawaii Asteroid Survey (UHAS), from Kitt Peak, Arizona, and observed over two nights. On 18 December, the object was rediscovered from Australia by Gordon Garradd of the Siding Spring Survey, another NASA-funded NEA survey. Further observations from around the globe over the next several days allowed the Minor Planet Center to confirm the connection to the June discovery, at which point the possibility of impact in 2029 was realized by the automatic SENTRY system of NASA’s Near-Earth Object Program Office. NEODyS, a similar automatic system at the University of Pisa and the University of Valladolid, Spain also detected the impact possibility and provided similar predictions.

Original Source: NASA News Release

Image credit: NASA/JPL
University of Arizona scientists have discovered why Eros, the largest near-Earth asteroid, has so few small craters.

When the Near Earth Asteroid Rendezvous (NEAR) mission orbited Eros from February 2000 to February 2001, it revealed an asteroid covered with regolith — a loose layer of rocks, gravel and dust — and embedded with numerous large boulders. The spacecraft also found places where the regolith apparently had slumped, or flowed downhill, exposing fresh surface underneath.

But what NEAR didn’t find were the many small craters that scientists expected would pock Eros’ landscape.

“Either the craters were being erased by something or there are fewer small asteroids than we thought,” James E. Richardson Jr. of UA’s planetary sciences department said.

Richardson concludes from modeling studies that seismic shaking has obliterated about 90 percent of the asteroid’s small impact craters, those less than 100 meters in diameter, or roughly the length of a football field. The seismic vibrations result when Eros collides with space debris.

Richardson, Regents’ Professor H. Jay Melosh and Professor Richard Greenberg, all with UA’s Lunar and Planetary Laboratory, report the analysis in the Nov. 26 issue of Science.

“Eros is only about the size of Lake Tahoe — 20 miles (33 kilometers) long by 8 miles (13 kilometers) wide,” Richardson said. “So it has a very small volume and a very low gravity. When a one-to-two-meter or larger object hits Eros, the impact will set off global seismic vibrations. Our analysis shows how these vibrations easily destabilize regolith overlaying the surface.”

A rock-and-dust layer creeps, rather than crashes, down shaking slopes because of Eros’ weak gravity. The regolith not only slides down horizontally, but also is launched ballistically from the surface and ‘hops’ downslope. Very slowly, over time, impact craters fill up and disappear, Richardson said.

If Eros were still in the main asteroid belt between Mars and Jupiter, a 200-meter crater would fill in about 30 million years. Because Eros is now outside the asteroid belt, that process takes a thousand times longer, he added.

Richardson’s research results match the NEAR spacecraft evidence. Instead of the expected 400 craters as small as 20 meters (about 70 feet) per square kilometer (three-fifths mile) on Eros’ surface, there are on average only about 40 such craters.

The modeling analysis also validates what scientists suspect of Eros’ internal structure.

“The NEAR mission showed Eros to most likely be a fractured monolith, a body that used to be one competent piece of material,” Richardson said. “But Eros has been fractured throughout by large impacts and is held together primarily by gravity. The evidence is seen in a series of grooves and ridges that run across the asteroid’s surface both globally and regionally.”

Large impacts fracture Eros to its core, but many smaller impacts fracture only the upper surface. This gradient of big fractures deep inside and numerous small fractures near the surface is analogous to fractures in the upper lunar crust, Richardson said. “And we understand the lunar crust — we’ve been there. We’ve put seismometers on the moon. We understand how seismic energy propagates through this kind of structure.”

The UA scientists’ analysis of how impact-induced seismic shaking has modified Eros’ surface has a couple of other important implications.

“If we eventually do send spacecraft to mine resources among the near-Earth asteroids or to deflect an asteroid from a potential collision with the Earth, knowing internal asteroid structure will help address some of the strategies we’ll need to use. In the nearer future, sample return missions will encounter successively less porous, more cohesive regolith as they dig farther down into asteroids like Eros, which has been compacted by seismic shaking,” Richardson noted.

“And it also tells us about the small asteroid environment that we’ll encounter when we do send a spacecraft out into the main asteroid belt, where Eros spent most of its lifetime. We know the small asteroids — those between the size of a beachball and a football stadium — are out there. It’s just that their ‘signature’ on asteroids such as Eros is being erased,” Richardson said.

This finding is important because the cratering record on large asteroids provides direct evidence for the size and population of small main-belt asteroids. Earth-based telescopic surveys have catalogued few main-belt asteroids that small. So scientists have to base population estimates for these objects primarily on visible cratering records and asteroid collisional history modeling, Richardson said.

Original Source: UA News Release

Today, September 29, 2004, is undisputedly the Day of Toutatis, the famous “doomsday” asteroid.

Not since the year 1353 did this impressive “space rock” pass so close by the Earth as it does today. Visible as a fast-moving faint point of light in the southern skies, it approaches the Earth to within 1,550,000 km, or just four times the distance of the Moon.

Closely watched by astronomers since its discovery in January 1989, this asteroid has been found to move in an orbit that brings it close to the Earth at regular intervals, about once every four years. This happened in 1992, 1996, 2000 and now again in 2004.

Radar observations during these passages have shown that Toutatis has an elongated shape, measuring about 4.6 x 2.4 x 1.9 km. It tumbles slowly through space, with a rotation period of 5.4 days.

The above images of Toutatis were taken with the ESO Very Large Telescope (during a technical test) in the evening of September 28. They were obtained just over 12 hours before the closest approach that happens today at about 15:40 hrs Central European Summer Time (CEST), or 13:40 hrs Universal Time (UT). At the time of these observations, Toutatis was about 1,640,000 km from the Earth, moving with a speed of about 11 km/sec relative to our planet.

They show the asteroid as a fast-moving object of magnitude 10, about 40 times fainter than what can be perceived with the unaided, dark-adapted eye. They also prove that Toutatis is right on track, following exactly the predicted trajectory in space and passing the Earth at a safe distance, as foreseen.

Detailed calculations, taking into account all available observations of this celestial body, have shown that although Toutatis passes regularly near the Earth, today’s passage is the closest one for quite some time, at least until the year 2562. The ESO observations, obtained at a moment when Toutatis was very close to the Earth, will help to further refine the orbital calculations.

The “parallax effect” demonstrated!
Simultaneous images obtained with telescopes at ESO’s two observatories at La Silla and Paranal demonstrate the closeness of Toutatis to the Earth. As can be seen on the unique ESO PR Photo 28e/04 that combines two of the exposures from the two observatories, the sighting angle to Toutatis from the two observatories, 513 km km apart, is quite different. Astronomers refer to this effect as the “parallax”. The closer the object is, the larger is the effect, i.e., the larger will be the shift of the line-of-sight.

Interestingly, the measured angular distance in the sky of the beginnings (or the ends) of the two trails (about 40 arcsec), together with the known distance between the two observatories and the position of Toutatis in the sky at the moment of the exposures fully define the triangle “Paranal-Toutatis-La Silla” and thus allow to calculate the exact distance to the asteroid.

It is found to be very close to that predicted from the asteroid’s position in its orbit and that of the Earth at the moment of this unique observation, 1,607,900 km. This exceptional, simultaneous set of observations thus provides an independent measurement of Toutatis’ distance in space and, like the measured positions, a confirmation of its computed orbit.

More information about Toutatis is available at the dedicated webpage by the French discoverers and also at the specialised Near-Earth Objects – Dynamic Site.

Original Source: ESO News Release

Early Wednesday morning, a 5,500 million pound asteroid measuring 5 kilometers in length will pass very close to Earth.

An asteroid two to three times that diameter is credited with causing the extinction of 85 percent of the world’s species, including the dinosaurs, when it hit our planet 65 million years ago.

Luckily for us, asteroid Toutatis is only a tourist, and doesn’t plan to stop here. It will come within 1.5 million kilometers (960,000 miles) of Earth, or four times the Earth-moon distance. Toutatis is the largest asteroid to come that close in more than a century.

Many smaller asteroids often pass well inside the moon’s orbit. The Earth is also hit continually with tiny meteors that often become “shooting stars” as they harmlessly burn up in the atmosphere.

But if a rock the size of Toutatis hit, the atmosphere would do little more than slow it down a bit before it slammed to Earth. The impact would create a vast crater, and toss so much dust and vaporized minerals into the air the skies would darken. Seismic waves created by the explosion would generate tsunamis and earthquakes, and red-hot rocks falling back to Earth would ignite forest fires.

Toutatis, also known as asteroid 4179, is 4.6 kilometers (2.9 miles) long and 2.4 kilometers (1.5 miles) wide. Although Toutatis looks like a large peanut, radar images revealed it is actually composed of two rocks that are in close contact. One of the rocks is approximately twice as large as the other.

Toutatis has a strange rotation –instead of the spinning on a single axis, like the planets and most other asteroids do, Toutatis tumbles so erratically that its orientation with respect to the solar system never repeats.

“The vast majority of asteroids, and all the planets, spin about a single axis, like a football thrown in a perfect spiral,” says Scott Hudson of Washington State University, “but Toutatis tumbles like a flubbed pass.”

Toutatis’s four-year orbit around the sun is also eccentric, extending from just inside the Earth’s orbit to the main asteroid belt between Mars and Jupiter.

Astronomer Christian Pollas discovered Toutatis on January 4, 1989. Pollas spotted the asteroid on photographic plates taken by Alain Maury and Derral Mulholland, who had taken the photos while observing Jupiter’s satellites.

Toutatis flew close by Earth in 1992 and 1996, but it hasn’t come this near to us since 1353. The next time it will pass this close again will be in the year 2562. The asteroid’s orbit around the sun is so eccentric that it can’t be predicted with much certainty for more than a few hundred years in the future. Since researchers can’t say Toutatis will never hit Earth, it is currently listed as a Potentially Hazardous Asteroid.

There is a rumor circulating on the Internet that the asteroid will strike Earth during this 2004 flyby. However, astronomers have been tracking the path of Toutatis ever since it was discovered, and they are certain it will pass safely by Earth.

Throughout history, several asteroids have hit Earth. The solar system was cluttered with asteroids while the Earth was young, and the face of the moon and other dead planetary bodies shows how frequent such impacts were. Impacts by large rocks are much less frequent today, but they can still occur.

There are thought to be more than 300,000 nearby small asteroids (asteroids about 100 meters across). Such asteroids should statistically hit Earth once every few thousand years. The most recent such asteroid strike occurred in 1908, when an asteroid measuring about 60 meters in diameter hit Russia. The “Tunguska” bolide exploded in the atmosphere and flattened about 700 square miles of Siberian forest.

Large (1 kilometer or greater) asteroids are far more rare and infrequent. There are only about 1,100 nearby large asteroids, and they are predicted to strike the Earth every half million years or so. But when these asteroids strike, they can cause catastrophic changes in the global climate. Asteroids that cause mass extinctions are thought to be 10 kilometers or greater in diameter.

The Spaceguard Survey was established to track large asteroids and comets that might pose a direct threat to Earth. So far, the Spaceguard Survey has found about half of these NEOs, and they expect to find the majority of them by 2008.

Although Toutatis will be in the far southern sky when it is closest to Earth, the asteroid is expected to brighten a few days prior to a 10th magnitude point of light visible from the Northern Hemisphere. Sky-watchers should look for it near the bright star Delta Capricorni.

Toutatis won’t be visible to the naked eye, but binoculars should suffice for spotting it in the night sky. A telescope would provide the best viewing, because it would allow the viewer to detect the slow motion of Toutatis against the background stars.

Original Source: Astrobiology Magazine Article

Image credit: Josh O?Conner and wildlandfire.com
Scientists conclude that, 65 million years ago, a 10-kilometer-wide asteroid or comet slammed into what is now the Yucat?n peninsula, excavating the Chicxulub impact crater and setting into motion a chain of catastrophic events thought to precipitate the extinction of the dinosaurs and 75 percent of animal and plant life that existed in the late Cretaceous period.

“The impact of an asteroid or comet several kilometers across heaps environmental insult after insult on the world,” said Dr. Daniel Durda, a senior research scientist at Southwest Research Institute? (SwRI?). “One aspect of the devastation wrought by large impacts is the potential for global wildfires ignited by material ejected from the crater reentering the atmosphere in the hours after the impact.”

Large impacts can blast thousands of cubic kilometers of vaporized impactor and target sediments into the atmosphere and above, expanding into space and enveloping the entire planet. These high-energy, vapor-rich materials reenter the atmosphere and heat up air temperatures to the point that vegetation on the ground below can spontaneously burst into flame.

“In 2002, we investigated the Chicxulub impact event to examine the extent and distribution of fires it caused,” said Durda. This cosmic collision carved out a crater some 40 kilometers (25 miles) deep and 180 kilometers (112 miles) across at the boundary between two geologic periods, the Cretaceous, when the dinosaurs ruled the planet, and the Tertiary, when mammals took supremacy.

“We noted that fires appeared to be global, covering multiple continents, but did not cover the entire Earth,” Durda continued. “That suggested to us that the Chicxulub impact was probably near the threshold size event necessary for igniting global fires, and prompted us to ask ‘What scale of impact is necessary for igniting widespread fires?'”

In a new study, Durda and Dr. David Kring, an associate professor at the University of Arizona Lunar and Planetary Laboratory, published a theory for the ignition threshold for impact-generated fires in the August 20, 2004, issue of the Journal of Geophysical Research. Their research indicates that impacts resulting in craters at least 85 kilometers wide can produce continental-scale fires, while impact craters more than 135 kilometers wide are needed to cause global-scale fires.

To calculate the threshold size impact required for global ignition of various types of vegetation, Durda and Kring used two separate, but linked, numerical codes to calculate the global distribution of debris reentering the atmosphere and the kinetic energy deposited in the atmosphere by the material. The distribution of fires depends on projectile trajectories, the position of the impact relative to the geographic distribution of forested continents and the mass of crater and projectile debris ejected into the atmosphere.

They also examined the threshold temperatures and durations required to spontaneously ignite green wood, to ignite wood in the presence of an ignition source (such as lightning, which would be prevalent in the dust-laden energetic skies following an impact event) and to ignite rotting wood, leaves and other common forest litter.

“The Chicxulub impact event may have been the only known impact event to have caused wildfires around the globe,” Kring noted. “The Manicouagan (Canada) and Popigai (Russia) impact events, however, may have caused continental-scale fires. The Manicouagan impact occurred in the late Triassic, and the Popigai impact event occurred in the late Eocene, but neither has been firmly linked yet to the mass extinction events that occurred at those times.”

Kring is currently at the International Geological Congress in Florence, Italy, giving a keynote address on the Chicxulub impact event and its relationship to the mass extinctions at the Cretaceous-Tertiary boundary period. Durda is available for comment at the SwRI offices in Boulder, Colo.

Original Source: SWRI News Release

On 9 July 2004, the Near-Earth Object Mission Advisory Panel recommended that ESA place a high priority on developing a mission to actually move an asteroid. The conclusion was based on the panel?s consideration of six near-Earth object mission studies submitted to the Agency in February 2003.

Of the six studies, three were space-based observatories for detecting NEOs and three were rendezvous missions. All addressed the growing realisation of the threat posed by Near-Earth Objects (NEOs) and proposed ways of detecting NEOs or discovering more about them from a close distance.

A panel of six experts, known as the Near-Earth Object Mission Advisory Panel (NEOMAP) assessed the proposals. Alan Harris, German Aerospace Centre (DLR), Berlin, and Chairman of NEOMAP, says, ?The task has been very difficult because the goalposts have changed. When the studies were commissioned, the discovery business was in no way as advanced as it is now. Today, a number of organisations are building large telescopes on Earth that promise to find a very large percentage of the NEO population at even smaller sizes than visible today.?

As a result, the panel decided that ESA should leave detection to ground-based telescopes for the time being, until the share of the remaining population not visible from the ground becomes better known. The need for a space-based observatory will then be re-assessed. The panel placed its highest priority on rendezvous missions, and in particular, the Don Quijote mission concept. ?If you think about the chain of events between detecting a hazardous object and doing something about it, there is one area in which we have no experience at all and that is in directly interacting with an asteroid, trying to alter its orbit,? explains Harris.

The Don Quijote mission concept will do this by using two spacecraft, Sancho and Hidalgo. Both are launched at the same time but Sancho takes a faster route. When it arrives at the target asteroid it will begin a seven-month campaign of observation and physical characterisation during which it will land penetrators and seismometers on the asteroid?s surface to understand its internal structure.

Sancho will then watch as Hidalgo arrives and smashes into the asteroid at very high speed. This will provide information about the behaviour of the internal structure of the asteroid during an impact event as well as excavating some of the interior for Sancho to observe. After the impact, Sancho and telescopes from Earth will monitor the asteroid to see how its orbit and rotation have been affected.

Harris says, ?When we do actually find a hazardous asteroid, you could imagine a Don Quijote-type mission as a precursor to a mitigation mission. It will tell us how the target responds to an impact and will help us to develop a much more effective mitigation mission.?

On 9 July, the findings were presented to the scientific and industrial community. Representatives of other national space agencies were also invited in the hope that they will be interested in developing a joint mission, based around this concept.

Andr?s Galvez, ESA?s Advanced Concepts Team and technical officer for the NEOMAP report says, ?This report gives us a solid foundation to define programmatic priorities and an implementation strategy, in which I also hope we are joined by international partners?.

With international cooperation, a mission could be launched as early as 2010-2015.

The six mission concepts studied were:

* Earthguard-1 ? a small space telescope for NEO discovery, especially the Atens and ?inner-Earth objects? (IEOs) that are difficult to detect from the ground.
* European Near-Earth Object Survey (EUNEOS) ? a space telescope for NEO discovery
* NEO Remote Observations (NERO) ? an optical/infrared space telescope for NEO discovery and physical characterisation.
* Smallsat Intercept Missions to Objects Near Earth (SIMONE) ? a flotilla of low-cost microsatellites for near-Earth asteroid rendezvous and in-situ remote sensing
* Internal Structure High-resolution Tomography by Asteroid Rendezvous (ISHTAR) ? uses radar tomography for an in-situ study of internal structure
* Don Quijote ? uses explosive charges, an impactor, seismic detectors and accelerometers for an in-situ study of internal structure and momentum transfer

Original Source: ESA News Release

Image credit: NASA
According to new research led by a University of Colorado at Boulder geophysicist, a giant asteroid that hit the coast of Mexico 65 million years ago probably incinerated all the large dinosaurs that were alive at the time in only a few hours, and only those organisms already sheltered in burrows or in water were left alive.

The six-mile-in-diameter asteroid is thought to have hit Chicxulub in the Yucatan, striking with the energy of 100 million megatons of TNT, said chief author and Researcher Doug Robertson of the department of geological sciences and the Cooperative Institute for Research in Environmental Sciences. The “heat pulse” caused by re-entering ejected matter would have reached around the globe, igniting fires and burning up all terrestrial organisms not sheltered in burrows or in water, he said.

A paper on the subject was published by Robertson in the May-June issue of the Bulletin of the Geological Society of America. Co-authors include CU-Boulder Professor Owen Toon, University of Wyoming Professors Malcolm McKenna and Jason Lillegraven and California Academy of Sciences Researcher Sylvia Hope.

“The kinetic energy of the ejected matter would have dissipated as heat in the upper atmosphere during re-entry, enough heat to make the normally blue sky turn red-hot for hours,” said Robertson. Scientists have speculated for more than a decade that the entire surface of the Earth below would have been baked by the equivalent of a global oven set on broil.

The evidence of terrestrial ruin is compelling, said Robertson, noting that tiny spheres of melted rock are found in the Cretaceous-Tertiary, or KT, boundary around the globe. The spheres in the clay are remnants of the rocky masses that were vaporized and ejected into sub-orbital trajectories by the impact.

A nearly worldwide clay layer laced with soot and extra-terrestrial iridium also records the impact and global firestorm that followed the impact.

The spheres, the heat pulse and the soot all have been known for some time, but their implications for survival of organisms on land have not been explained well, said Robertson. Many scientists have been curious about how any animal species such as primitive birds, mammals and amphibians managed to survive the global disaster that killed off all the existing dinosaurs.

Robertson and colleagues have provided a new hypothesis for the differential pattern of survival among land vertebrates at the end of the Cretaceous. They have focused on the question of which groups of vertebrates were likely to have been sheltered underground or underwater at the time of the impact.

Their answer closely matches the observed patterns of survival. Pterosaurs and non-avian dinosaurs had no obvious adaptations for burrowing or swimming and became extinct. In contrast, the vertebrates that could burrow in holes or shelter in water — mammals, birds, crocodilians, snakes, lizards, turtles and amphibians — for the most part survived.

Terrestrial vertebrates that survived also were exposed to the secondary effects of a radically altered, inhospitable environment. “Future studies of early Paleocene events on land may be illuminated by this new view of the KT catastrophe,” said Robertson.

Original Source: CU-Boulder News Release

Image credit: NASA/JPL
The ongoing search for near-Earth asteroids at Lowell Observatory has yielded another interesting object. Designated 2004 JG6, this asteroid was found in the course of LONEOS (the Lowell Observatory Near-Earth Object Search) on the evening of May 10 by observer Brian Skiff.

“I immediately noticed the unusual motion,” said Skiff, “so it was certain that it was of more than ordinary interest.” He quickly reported it to the Minor Planet Center (MPC) in Cambridge MA, which acts as an international clearinghouse for asteroid and comet discoveries. The MPC then posted it on a Web page for verification by astronomers worldwide. It happened that all the initial follow up observations, however, were obtained by amateur and professional observers in the Southwest US. The additional sky positions measured in the ensuing few days allowed an orbit to be calculated.

The official discovery announcement and preliminary orbit were published by the MPC on May 13. This showed that the object was located between Earth and Venus (presently the very bright “evening star” in the western sky). In addition, 2004 JG6 goes around the Sun in just six months, making it the asteroid with the shortest known orbital period. Ordinary asteroids are located between the orbits of Mars and Jupiter, roughly two to four times farther from the Sun than Earth, taking several years to go around the Sun.

Instead, 2004 JG6 orbits entirely within Earth’s orbit, only the second object so far found to do so. “What makes this asteroid unique is that, on average, it is the second closest solar system object orbiting the Sun,” said Edward Bowell, LONEOS Director. Only planet Mercury orbits closer to the Sun.

As shown in the included diagram, JG6 crosses the orbits of Venus and Mercury, passing less than 30 million miles from the Sun every six months. The approximate average orbital speed of this asteroid is more than 30 km/sec, or 67,000 miles per hour. Depending on their locations, the asteroid may pass as close as 3.5 million miles from Earth and about 2 million miles from planet Mercury. In the coming weeks 2004 JG6 will pass between Earth and the Sun, just inside Earth’s orbit. It will move through the constellations Cancer and Canis Minor low in the western sky at dusk. Because of the near-exact six-month period, the asteroid should be observable again in nearly the same spot in the sky next May, having gone around the Sun twice while Earth will have made only one circuit.

From present estimates, 2004 JG6 is probably between 500 meters and 1 km in diameter. Despite its proximity, the object poses no danger of colliding with Earth.

Asteroids with orbits entirely within the Earth?s orbit have been informally called “Apoheles,” from the Hawaiian word for orbit. Apohele also has Greek roots: “apo” for outside, and “heli” for Sun. Objects orbiting entirely within Earth?s orbit are thought by dynamicist William F. Bottke of Southwest Research Institute and colleagues to comprise just two percent of the total near-Earth object population, making them rare as well as difficult to discover. This is because they stay in the daylight sky almost all of the time. There may exist about 50 Apoheles of comparable size to or larger than 2004 JG6, but many of them are certain to be unobservable from the ground.

The first asteroid found entirely inside Earth?s orbit was 2003 CP20, found just over a year ago by the NASA-funded Lincoln Laboratory Near-Earth Asteroid Research project, which observes near Socorro, New Mexico. Although larger than 2004 JG6, 2003 CP20 is a little more distant from the Sun.

LONEOS is one of five programs funded by NASA to search for asteroids and comets that may approach our planet closely. The NASA program?s current goal is to discover 90 percent of near-Earth asteroids larger than 1 km in diameter by 2008. There are thought to be about 1,100 such asteroids.

Original Source: Lowell Observatory News Release