Experts Estimate Asteroid Risk

At a NASA-sponsored conference, a group of experts have estimated that the Earth is struck once every 1,000 years by an asteroid capable of releasing 10-megatons of energy – not a planet killer, but definitely enough to cause a terrible loss of life. NASA and the astronomical community has been systematically searching for all Near Earth Objects larger than 1 km across, and capable of crossing the Earth’s orbit. So far they’ve found 600 out of an estimated 1,000.

Space Rock Sneaks Past the Earth

Amateur astronomers were given a rare treat on Sunday when asteroid 2002 NY40 passed within 530,000 kilometres of the Earth (a little further than the distance from Earth to the Moon). Over the course of a few hours, the 800 metre space rock passed through several constellations in the night sky, visible in binoculars or a small telescope. 2002 NY40 is of minor concern as astronomers believe there is a 1/500,000 chance it could strike the Earth in 2022 – the flyby will give them an opportunity to better calculate its future path.

Evidence Found for Early Earth Bombardment

Image credit: NASA

Researchers from the University of Queensland have discovered evidence they believe proves that the Earth was bombarded by meteorites 4 billion years ago. Although these impacts are clearly visible on the Moon, movement the Earth’s tectonic plates have obscured the evidence here. The team analyzed 3.8 billion year old rocks from Greenland, and found anomalies in the element tungsten; exactly the same as those found in meteorites.

University of Queensland researchers have for the first time discovered terrestrial evidence of a meteorite bombardment nearly 4 billion years ago.

It is widely accepted that the moon was heavily bombarded at this time, creating huge craters and basins. But although the effect of these impacts is still clearly visible on the moon today, movement of Earth?s dynamic plates over geological time have reshaped the terrestrial surface dramatically, leaving little evidence of these catastrophic events.

In a paper published in the international journal Nature, UQ researchers report evidence of the oldest impact events so far discovered on Earth.

The research team of Dr Ronny Schoenberg, Dr Balz Kamber and Professor Ken Collerson of UQ?s new Advanced Centre for Isotope Research Excellence (ACQUIRE) made the discovery by analysing 3.8 billion year old rocks from West Greenland collected by Oxford University collaborator Professor Stephen Moorbath and from Northern Labrador in Canada collected by Professor Collerson.

The researchers found these very old metamorphosed sedimentary rocks ? derived from the Earth?s early crust ? contain anomalies in the isotope composition of the element tungsten.

?Such anomalies are usually found in meteorites. To our knowledge, this is the first time these anomalies have been shown to exist in terrestrial samples? Professor Collerson said.

?There is no plausible mechanism by which tungsten isotope anomalies could have been preserved in the Earth?s dynamic crust-mantle environment. Therefore, we conclude these rocks must contain a compound derived from meteorites.?

?We have in effect found a chemical fingerprint in the earth?s oldest terrestrial rocks of a heavy meteorite bombardment 3.8 to 4 billion years ago,? he said.

?This finding has implications for the origin of life on earth as these giant impacts would have annihilated any possible existing life forms but also delivered complex molecules from carbonaceous chondrites – a type of meteorite – to the earth?s surface,? Dr. Kamber added.

?Further research on this unique collection of rocks will yield insight into the evolution of life on Earth provided provision of adequate research funding,? Professor Collerson said.

Original Source: UQ News Release

NASA Rules Out Asteroid Collision in 2019

Image credit: NASA

Astronomers announced earlier last week that they had found an asteroid that could strike the Earth in 2019. More astronomers brought their equipment to bear on Asteroid NT7, however, and were able to provide enough trajectory data that NASA has eliminated the possibility of a collision in 2019; although, 2060 could still be a possible impact year (astronomers will probably rule that out eventually too). The asteroid is 2km across, so if it does ever strike the planet, it could cause significant damage.

Asteroid 2002 NT7 currently heads the list on our IMPACT RISKS Page because of a low-probability Earth impact prediction for February 1, 2019. While this prediction is of scientific interest, the probability of impact is not large enough to warrant public concern.

Discovered on July 9, 2002 by the LINEAR team, asteroid 2002 nt7 is in an orbit, which is highly inclined with respect to the Earth’s orbit about the sun and in fact nearly intersects the orbit of the Earth. While the orbits of Earth and 2002NT7 are close to one another at one point in their respective orbits, that does not mean that the asteroid and Earth themselves will get close to one another. Just after an asteroid like 2003 qq47 is discovered, the limited number of observations available do not allow its trajectory to be tightly constrained and the object’s very uncertain future motion often allows a very low probability of an Earth impact at some future date. Just such a low probability impact has been identified for February 1, 2019 and a few subsequent dates. As additional observations of the asteroid are made in the coming months, and perhaps pre-discovery archival observations of this object are identified, the asteroid’s orbit will become more tightly constrained and the future motion of the asteroid will become better defined. By far the most likely scenario is that, with additional data, the possibility of an Earth impact will be eliminated.

This is an example of the type of scenario that we can expect as some types of near-Earth objects are discovered. For some objects, their uncertain initial orbits cannot be used to immediately rule out future very low-probability Earth impacts, but when additional observations are used to refine the initial orbit, these low-probability Earth impact possibilities will go away. Other recently discovered near-Earth asteroids will be added to the Risk page until their orbits are refined and they are then dropped off the list of closely watched objects. This is how the system is expected to work and any initial indication of a low-probability Earth impact followed by a removal of that event from our IMPACT RISKS tables should not be considered a mistake. It is a natural result of the on-going process of monitoring the motions of near-Earth objects.

Original Source: NASA News Release

Asteroid Narrowly Misses the Earth

An asteroid the size of a football field came extremely close to the Earth last week, and scientists didn’t even know until after it passed us by. Asteroid 2002 MN is 100 metres in diameter and passed by us at a distance of only 120,000km, less than one-third the distance from the Earth to the Moon. This is one of the closest near-misses ever recorded of a spacerock this size; had it actually struck the planet it would have cause considerable destruction and loss of life.

Write Your Name on an Asteroid?

A Japanese spacecraft will give people an opportunity to land their name on an asteroid. Due for launch near the end of this year, the MUSES-C spacecraft will meet up with asteroid 1998 SF36 when it crosses the Earth’s orbit in 2005. It will land on the asteroid, collect samples which it will later return to Earth. As part of its mission, the spacecraft will deposit an aluminium foil sheet containing the names of hundreds of thousands of space enthusiasts. If you want to get your name on board, you have until July 5th.

Looking for Asteroids in the Earth’s Blindspot

Lately it seems that astronomers are discovering potentially harmful asteroids after they nearly miss our planet. This is because they’re coming from our planet’s “blind spot” – the space directly between the Earth and the Sun – and we can only see them after they pass. The European Space Agency is developing a new space-based observatory called Gaia, which will be able to see close to the Sun’s disk and detect many more of these previously unseen asteroids. Gaia is expected to launch in 2010.

Astronomers Find Five Double Asteroid Systems

Image credit: Cornell

According to researchers from Cornell University, binary asteroids – where a small asteroid orbits a larger one – are actually pretty common in Earth crossing orbits. In fact, they think that gravitational interactions with the Earth might actually help to cause the arrangement. The researchers estimate that 16% of asteroids larger than 200 metres in diameter have a companion – so far they’ve found five using two of the world’s largest radio telescopes.

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 ?rst 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.

Original Source: Cornell News Release

Asteroids Could Be Double Previous Estimates

Asteroids in our solar system could be more numerous than previously thought according to a new survey done using the European Space Agency’s Infrared Space Observatory (ISO). The ISO’s survey indicates that there are between 1.1 million and 1.9 million asteroids larger than 1km in the main asteroid belt, located between Mars and Jupiter. This result is roughly double the number of asteroids estimated by previous surveys using other observatories.

New Strategy for Deflecting Smaller Asteroids

Image credit: NASA

A planetary scientist from the University of Arizona believes that smaller asteroids could be moved out of collision paths with the Earth by changing how much sunlight they reflect. Given many years of lead time (decades or even centuries), a smaller asteroid could be covered in dirt, painted white, or covered with a solar collector. This would change the amount of heat the asteroid radiates into space, and cause its orbit to drift away from a killer trajectory.

Humans could deflect small but dangerous asteroids from Earth by changing how much sunlight the asteroids reflect, a University of Arizona planetary scientist suggests in the current issue (April 5) of Science.

Possible schemes might include covering the upper few centimeters of the asteroid with dirt, or painting its surface white, or fusing part of its surface with a spaceborne solar collector ? all technically feasible and civically preferable to launching a nuclear warhead to blast an incoming asteroid off course.

Changing how much heat a space rock radiates will change how it drifts in its orbit because of the Yarkovsky effect, said Joseph N. Spitale of the UA Lunar and Planetary Laboratory in his article, “Asteroid Hazard Mitigation Using the Yarkovsky Effect.”

The Yarkovsky effect is a long-known but long-obscure phenomenon named for the Polish engineer who first described it around 1900. The effect is caused because when an unevenly heated body re-radiates heat, hotter spots are subjected to a stronger recoil force than are cooler spots. As I.O. Yarkovsky noted, the differences in momentum nudge the object so that it drifts slightly in its orbit, Spitale said. The effect is a relatively small force, but it accumulates through time.

Not until the mid-1990s did planetary scientists begin to realize how important the Yarkovsky effect is in calculating motions of asteroid fragments in the belt between Mars and Jupiter. These include Cornell University’s William F. Bottke Jr., David P. Rubincam of NASA Goddard Space Flight Center, Paolo Farinella of the University of Pisa in Italy, David Vokrouhlicky of Charles University in the Czech Republic, and William Hartmann of the Planetary Science Institute in Tucson.

The mechanism explains why more asteroid fragments than otherwise can be predicted are launched from the main asteroid belt toward Earth, hitting as meteorites, according to their papers. And it explains how space rocks can drift for millions of years before arriving at main belt asteroid “resonance” zones from which they’re flung to the inner solar system, they conclude.

“It’s pretty clear that this is an important effect when it comes to getting material from the asteroid belt to the inner planets,” Spitale said in an interview.

He’s working to develop a sophisticated thermal model to use to precisely calculate Yarkovsky drift for specific asteroids. Asteroid shape, spin, composition and surface details all must be factored in to get a precise orbit for a specific asteroid.

In his Science article, Spitale describes his calculations of Yarkovsky drift for three stony near-Earth asteroids, 6489 Golevka (300 meters diameter), 1566 Icarus (one kilometer diameter) and 1620 Geographos (2.5 kilometers diameter).

The idea then is to change a threatening asteroid’s surface temperatures so that, over decades or centuries, its orbit veers away from Earth.

“You might take one of the smaller bare-rock bodies and put a lot of dirt on it, for a dramatic change in thermal conductivity,” Spitale said. “Blanketing the asteroid with a centimeter of dirt is technically feasible, but it would be expensive.

“Another way you could do it would be to paint it. If you could cover the surface with a millimeter of white material, you could ‘turn off’ the Yarkovsky effect altogether. That could produce a fairly big change in where the body would be in another century or so.

“This would be effective in another approach, suggested by Jay Melosh (UA professor of planetary sciences). It is to use a solar collector – basically just a big dish that focuses sunlight on a body ? to fuse a region of the surface and blast off mass, so the object changes course because of its different mass. But in the process of this, you’d also change the thermal conductivity of the asteroid, giving it a new orbit also because of the Yarkovsky effect.”

Spitale said the proposed technique would be useless for a large asteroid or an asteroid less than decades away from Earth..

“This technique will work best on objects the size of Golevka or smaller (300 meters, about 1,000 feet, or smaller). An object that size could do damage to the better part of a country. Even a 100-meter or 50-meter object can take out a good part of a city.”

“The biggest technical problem right now with this approach is just doing the calculations to understand how we’d actually be affecting the orbit by doing something to an asteroid surface,” Spitale said.

If the orbit is miscalculated, an object on course to deliver Earth a glancing blow may be “mitigated” into an object on course to deliver a direct hit.

The flip side of that is, you need a good model to compute Yarkovsky effect perturbations even to know which asteroids pose real hazards, Spitale added. “That may be the most important use of all for this model, to predict which are going to hit in the first place,” he said.

Original Source: UA News Release