22 July 2008

'DUCK!' WON'T SAVE THE DAY.
SCIENTISTS EXPLORE WAYS TO MEET THREAT OF AN ASTEROID STRIKE

By Frank D. Roylance, Baltimore Sun

An asteroid hurtles toward Earth, threatening devastation. A team of attractive young scientists and engineers launches a rocket that crashes into the asteroid and knocks it off course - just in the nick of time.

But wait. The crash pushes the giant space rock toward a "keyhole" in space: a tiny window that guarantees that the asteroid will come back and obliterate some hapless city in the future. What to do?

A scruffy grad student raises his hand. How about a "gravity tractor" to tow it off course?

Real-life scientists are actually exploring the gravity tractor, the keyhole problem and other issues surrounding the very real, if remote, danger that a comet or asteroid will some day cross Earth's orbital path at precisely the wrong time.

A crowd of them met this week in Baltimore as part of the "Asteroids, Comets, Meteors 2008" conference sponsored by the Johns Hopkins University's Applied Physics Laboratory.

It's not an idle concern. On June 30, 1908, astronomers say, a comet exploded over a remote area of Russia with the force of a 10-megaton nuclear bomb. The "Tunguska" impact, as it's known, flattened 80 million trees over 830 square miles - that's enough destructive power to devastate a city of millions, had it struck one.

Since 1950, geologists have identified more than 160 prehistoric meteor impact craters on Earth, and several new ones are found each year, according to the Earth Impact Database maintained by the University of New Brunswick in Canada.

Scientists presume that far more have been obliterated by erosion and plate tectonics. And most objects that strike Earth probably fall into the oceans.

NASA supports four full-time sky surveys to find and track every space rock orbiting the sun that could threaten Earth. So far, scientists have found close to 1,000 at least a kilometer in size -big enough to cause global problems. But they think that may be only 90 percent of the total.

There are also likely to be tens of thousands more rocks big enough to threaten whole cities and millions of human lives. The search programs estimate they've found only a small percentage of those.

One key member of Congress says the U.S. is not doing enough to understand and plan for such threats. Republican Dana Rohrabacher, who represents Southern California's conservative 46th District, is a former chairman of the House Subcommittee on Space and Aeronautics and a senior member of the House Committee on Science.

"It doesn't take a genius to recognize there is a potential threat to our planet that could well cause ... the loss of millions and millions of people," he told the scientists gathered in Baltimore. "We are not prepared, and it's not something that would cost a gazillion dollars to be prepared for."

Vital observatory

He urged the scientists to lobby Congress for funds to help international efforts in the field and to prevent proposed budget cuts in NASA's own search programs.

In particular, he argued for a rescue of the Arecibo Observatory in Puerto Rico. Radar astronomy from Arecibo enhances the accuracy of optical tracking of asteroids. But the National Science Foundation has recommended cuts that could lead to its closure.

"We know what can happen if we just sit by and wait, and pray," Rohrabacher said.

So far, scientists haven't found any dangerous space objects on a path toward Earth. But they are closely watching an asteroid called 99942 Apophis, a rock with a diameter of 700 to 1,000 feet, discovered in 2004.

Apophis caused a brief sensation when initial orbit calculations gave it a 2.7 percent chance of smashing into Earth in 2029 - by far the most serious threat ever identified. Subsequent observations of its orbit eliminated the possibility of a 2029 collision, but scientists still rate the chances of an impact at 1 in 45,000 when the asteroid returns in 2036.

At this week's conference in Baltimore, Jon D. Giorgini, a senior analyst with the Solar System Dynamics group at NASA's Jet Propulsion Lab, said 1 in 45,000 is still regarded as a "historically high" risk, a once-in-800-years event.

The problem, he said, is that precise measurements of the threat from Apophis won't be possible for years. Too many factors have to be understood and evaluated for a precise fix on the asteroid's path.

Solar wind alone can deflect the asteroid by up to 18.6 million miles between now and 2036, Giorgini said. And the tiny gravitational influence of other asteroids could move it by twice the width of Earth, according to a paper Giorgini published this year in the journal Icarus.

Scientists will need new optical observations in 2011 and radar observations from Arecibo in 2012 and 2013 to refine their Apophis calculations.

The Jet Propulsion Lab's Paul W. Chodas reported on work he has done to estimate how much warning we can expect of an asteroid headed for a collision with Earth. The majority of objects miss Earth entirely, of course.

But Chodas simulated 1,000 collisions with asteroids of various sizes and calculated how long after each object's discovery it would take astronomers with optical telescopes to warn of a 50 percent risk of collision.

All of the largest objects would reach that warning point before impact, most within five years of discovery.

"Including radar buys us about nine months more time," he said.

But for objects as small as 460 feet across, it would be 18 years before 75 percent reached the point at which scientists could determine a 50 percent risk of a strike. Radar cuts that to 15 years.

But 20 percent of those objects would strike Earth before they were discovered.

With enough warning, humans could come up with a scheme to deflect a dangerous comet or asteroid before it strikes. The first tactic would likely require smacking it with "impacters" or explosives to shove it off course.

The 'keyhole' factor

But the JPL's Donald K. Yeomans noted that an asteroid like Apophis might be deflected by an impacter onto a course that sends it (or fragments of it) through a "keyhole" in space that makes collision with Earth on a later orbit all but inevitable.

He and several colleagues proposed a "gravity tractor" - a spacecraft equipped with thrusters and a radar transponder to report its precise position.

Launched simultaneously with the asteroid impacter, the unmanned tractor would take a position just above the asteroid in, say 2028. Scientists on the ground would use its radar transponder to determine the asteroid's precise course.

All of this, according to Yeomans, is possible with current technology, and "it's not that hard."

If the impact sends the asteroid toward the keyhole for a collision with Earth in 2049, the tractor would go into action. Using only its thrusters and the weak gravitational attraction between itself and the asteroid - no cables or chains - the tractor would begin to tug the big rock onto another course.

The gravitational pull is "very tiny," Yeomans conceded, barely 0.16 inch per second for a 460-foot asteroid.

"But it's acceleration, and over 21 years it can amount to quite a bit."

The tractor doesn't have to move the asteroid a distance equal to the diameter of Earth to avoid a collision, only the width of the keyhole in space it's trying to avoid - about 2,000 feet wide in the case of Apophis.

The bottom line, said Yeomans: "In 200 days of tractoring the asteroid ... it could be moved completely off the 2049 keyhole, thus saving the world from imminent disaster."

17 July 2008

CERES MAY BE AN ASTEROID IMPERSONATOR
By Ron Cowen, Science News

The largest member of the asteroid belt could have emigrated from the solar system's fringe

If planetary scientist Bill McKinnon's hunch is right, the largest asteroid in the solar system isn't an asteroid at all. Ceres, as the 470-kilometer-wide object is called, may be a relative of Pluto that formed at the solar system's fringes but came in from the cold several billion years ago.

McKinnon, based at Washington University in St. Louis, said he was first struck by Ceres' unusually low density - more similar to icy comets from the outer solar system than the rocky bodies found in the asteroid belt that lies between the orbits of Mars and Jupiter. The density of Ceres, referred to as a dwarf planet, is only slightly higher than that of Pluto. Models suggest Ceres "looks remarkably Pluto-like," McKinnon says.

But it was a recently developed model of the early solar system that prompted McKinnon to formally propose that Ceres might be an escapee from the Kuiper belt, an outer solar system reservoir of frozen bodies that includes Pluto. He presented his proposal July 15 in Baltimore at the Asteroids, Comets, Meteors conference.

According to the model, developed by researchers including Hal Levison and Bill Bottke of the Southwest Research Institute in Boulder, Colo., and Alessandro Morbidelli of Observatory of the Cote d'Azur in Nice, France, the orbits of the outer four planets - Jupiter, Saturn, Uranus and Neptune - were initially packed much closer together than they are today.

Beyond these planets resided a band of dust, ice and gas particles. Over time, as some of these particles leaked inward, their gravitational tug lengthened the distance between the orbs. For instance, Jupiter migrated inward, while Saturn moved outward.

At some point, according to the theory, Saturn reached a gravitational sweet spot: The time it took to go around the sun became exactly twice that of Jupiter's. That interplay strengthened the planets' mutual tug, and ultimately hurled Uranus and Neptune into the outlying band of dust, ice and gas. The entry of Uranus and Neptune scattered debris from the chilly band, sending some of its denizens into the inner solar system.

That's how Ceres might have migrated from the outer solar system into the asteroid belt, McKinnon suggests.

"We are saying that many objects from the outer solar system - what we call the primordial disk of comets that went on to produce the Kuiper belt - are captured in the outer part of the asteroid belt as a byproduct of the model," Bottke says. He and Levison presented updated versions of the theory at the meeting just before McKinnon's presentation.

"I consider McKinnon's idea as something of a thought balloon to stimulate thinking," Bottke says. "It is indeed possible that he is correct, but I would not bet for it at this point."

Additional information on Ceres' composition, to be gathered by NASA's Dawn spacecraft when it visits Ceres in 2015, could clarify the body's origin. But proof may require measuring the ratio of hydrogen to its heavier isotope, deuterium, in the ices or water vapor venting from the body, which would require a mission beyond Dawn, McKinnon says. If the ratio matches that observed in comets, "the case is closed" for Ceres being an emigre to the asteroid belt, he says.

16 July 2008

FORMER ASTRONAUT CALLS FOR INTERNATIONAL EFFORT TO IDENTIFY POSSIBLE DANGER

So, a big nasty rock is heading for Earth. Who you gonna call?

Nineteen regions or countries, including Canada, have space programs, but - Hollywood epics notwithstanding -- there is no cooperative process for deflecting a killer asteroid.

The math means humans sooner or later will have to take a shot at bumping an asteroid off-course before knowing for sure whether it will hit us.

We could nudge it with a small explosion or tow it so it misses the planet. A nuclear explosion would be a last resort, and might actually cause more, smaller problems by blowing it into bits.

With new telescopes, including one to be launched by Canada in 2010, expected to increase asteroid detection by 100 times in the next 10 to 15 years, paranoia is likely to soar to new heights.

The actual risk of an impact won't increase, but many more of us will be asking, "Do we duck or do we take action?" former astronaut Rusty Schweickart told a conference of 2,000 of the world's top space scientists in Montreal Monday.

About 3,000 asteroids are known to be capable of destroying a major city on impact. That number will soon leap to 300,000, he said.

Between five and 10 of those will require a concerted decision on deflection by 2020, Schweickart predicted.

The coming asteroid awareness boom could bring on "a reign of crying wolf," he added, since the data probably will be interpreted by crackpots and experts alike.

"If only one in 10,000 is a real threat, then 9,999 aren't threats," he said. "We need to avoid misinformation.

"We can't have the Italians or the Venezuelans issuing warnings only to have the United States saying that's a bad interpretation."

Speaking to the 37th International Scientific Assembly, gathered in Montreal for a week-long meeting, Schweickart said the United Nations is the best organization to ensure a level playing field to deal with the mounting awareness of potential cataclysms.

Canada will play a leading role in the asteroid awareness boom when it launches the Near Earth Object Surveillance Satellite in 2010.

The $12-million, suitcase-size NEOSSat will be the world's first space telescope devoted to tracking threatening space rocks.

The 72-year-old Schweickart, who founded the Association of Space Explorers, a 300-member group of former astronauts from 32 countries, said existing technology would have to be 99 per cent effective to divert an asteroid off its doomsday path.

Schweickart, who flew on the Apollo 9 mission to low Earth orbit in 1969, emphasized to about 150 scientists that packed a room for his speech that he was not calling for another layer of bureaucracy or much more additional spending.

All that is required is better networking and an authoritative body to sort through potential threats, he said.

"We need to have a process in place now that will decide the criteria" for what is the quickest, cheapest and safest way to shove off an asteroid, he said.

At the conference, which runs through Sunday, scientists will discuss a range of topics, from the Earth's changing environment to how solar flares affect the price of wheat, to the likelihood of other habitable planets.

6 July 2008

ASTEROID-HUNTING SATELLITE A WORLD FIRST

Canada's NEOSSat space telescope to discover near-Earth objects and track high-altitude satellites

Canada is building the world's first space telescope designed to detect and track asteroids as well as satellites. Called NEOSSat (Near Earth Object Surveillance Satellite), this spacecraft will provide a significant improvement in surveillance of asteroids that pose a collision hazard with Earth and innovative technologies for tracking satellites in orbit high above our planet.

Weighing in at a mere 65-kilograms, this dual-use $12-million mission builds upon Canada's expertise in compact "microsatellite" design. NEOSSat will be the size of a large suitcase, and is cost-effective because of its small size and ability to "piggyback" on the launch of other spacecraft. The mission is funded by Defence Research Development Canada (DRDC) and the Canadian Space Agency (CSA). Together CSA and DRDC formed a Joint Project Office to manage the NEOSSat design, construction and launch phases. NEOSS at is expected to be launched into space in 2010. The two projects that will use NEOSSat are HEOSS (High Earth Orbit Space Surveillance) and the NESS (Near Earth Space Surveillance) asteroid search program.

"Canada continues to innovate and demonstrate its technological expertise by developing small satellites that can peer into near and far space for natural and man-made debris," said Guy Bujold, President, Canadian Space Agency. "We are on the cutting edge, building the world's first space-based telescope designed to search for near-Earth asteroids."

NEOSSat is the first follow up mission to the groundbreaking MOST (Microvariability and Oscillation of STars) spacecraft, a 60-kilogram satellite designed to measure the age of stars in our galaxy. NEOSSat also marks the first project using Canada's Multi-Mission Microsatellite Bus. CSA's Space Technology branch launched the Multi-Mission Bus project to capitalize on technology developed for the MOST project by making it adaptable to future satellite missions.

Captain Tony Morris of DRDC Ottawa, and Deputy Program Manager of the NEOSSat Joint Project Office, says, "NEOSSat is a technological pathfinder for us to demonstrate the potential of microsatellite technologies to satisfy operational requirements of the Canadian Forces. NEOSSat will demonstrate the ability of a microsatellite to enhance the CF's contribution to the NORAD mission -- providing accurate knowledge of the traffic orbiting our planet. This would contribute to the safety of critical Canadian assets, military and civilian, in an increasingly congested space environment."

Dr. Brad Wallace leads the science team at DRDC for HEOSS, which will use NEOSSat for traffic control of Earth's high orbit satellites. Dr. Wallace says, "We have already done satellite tracking tests using MOST, so we know that a microsatellite can track satellites. The challenge now is to demonstrate that it can be done efficiently, reliably, and to the standards required to maximize the safety of the spacecraft that everyone uses daily, like weather and communication satellites."

The HEOSS project will demonstrate how a microsatellite could contribute to the Space Surveillance Network (SSN), a network of ground based telescopes and radars located around the world. Until the 1980s, Canada contributed to the SSN with two ground-based telescopes in eastern and western Canada. The fact that HEOSS will be a space-based capability on a microsatellite represents an exciting enhancement to the contribution and offers significant advantages to the SSN. Ground-based sensors' tracking opportunities are constrained by their geographic location and the day-night cycle. In Sun-synchronous orbit around our planet, NEOSSat will offer continuous tracking opportunities and the ability to track satellites in a wide variety of orbit locations.

"NEOSSat requires remarkable agility and pointing stability that has never before been achieved by a microsatellite," says David Cooper, General Manager of Mississauga-based Dynacon Inc., the prime contractor for the NEOSSat spacecraft and the manufacturer and operator of the MOST satellite. "It must rapidly spin to point at new locations hundreds of times per day, each time screeching to a halt to hold rock steady on a distant target, or precisely track a satellite along its orbit, and image-on-the-run." Cooper says. "Dynacon is the world leader in this microsatellite attitude-control-system technology."

Dr. Alan Hildebrand, holder of a Canada Research Chair in Planetary Science in the University of Calgary's Department of Geoscience, leads an international science team for the NESS asteroid search project and is excited by its prospects.

"NEOSSat being on-orbit will give us terrific skies for observing 24-hours a day, guaranteed," Hildebrand says. "Keeping up with the amount of data streaming back to us will be a challenge, but it will provide us with an unprecedented view of space encompassing Earth's orbit."

Although NEOSSat's 15-centimetre telescope is smaller than most amateur astronomers', its location approximately 700 kilometres above Earth's atmosphere will give it a huge advantage in searching the blackness of space for faint signs of moving asteroids. Twisting and turning hundreds of times each day, orbiting from pole to pole every 50 minutes, and generating power from the Sun, NEOSSat will send dozens of images to the ground each time it passes over Canada. Due to the ultra-low sky background provided by the vacuum of space, NEOSSat will be able to detect asteroids delivering as few as 50 photons of light in a 100-second exposure.

Hildebrand, who oversees the U of C's ground-based asteroid observation program using the Rothney Astrophysical Observatory's wide-field Baker Nunn telescope, said NEOSSat will greatly enhance the study of asteroids and comets as they approach Earth. "NEOSSat will discover many asteroids much faster than can be done from the ground alone. Its most exciting result, however, will probably be discovering new targets for exploration by both manned and unmanned space missions," he observes. "By looking along Earth's orbit, NEOSSat will find 'low and slow' asteroids before they pass by our planet and sprint missions could be launched to explore them when they are in the vicinity of the Earth."

3 July 2008

ROSETTA AWAKES FROM HIBERNATION FOR ASTEROID ENCOUNTER

Spacecraft controllers have just awoken Rosetta from hibernation to prepare for its encounter with asteroid (2867) Steins on 5 September. ESA's comet chaser will study the relatively rare asteroid as it flies by on its way to comet 67/P Churyumov-Gerasimenko.

Launched in March 2004, Rosetta will reach its final destination only in 2014, after travelling a total of about 6500 million km. The distance between the spacecraft and the Sun as it approaches the comet will be about 600 million or 4 AU (1 AU or 1 Astronomical Unit is equal to 150 million km, the mean distance between Earth and the Sun).

Rosetta has swung by Earth twice and Mars once, performing gravity-assist manoeuvres, that gave it the necessary boost to continue on its journey. The third and last Earth swing-by is scheduled for November 2009. The spacecraft will also fly by two asteroids and study them on the way: (2867) Steins in September this year and (21) Lutetia in June 2010. As it closes in on (2867) Steins in September, Rosetta will have travelled about 3700 million km and will be 2.1 AU from the Sun.

After its last planetary swing-by on 13 November last year, Rosetta headed towards the asteroid belt located between the orbits of Mars and Jupiter. On 27 March 2008, the spacecraft switched to its near-Sun hibernation mode for a period of three months. During this phase, a few subsystems were put into a dormant state to optimise their lifetime (as this is only the beginning of the mission's science phase).

Next stop, Steins

Rosetta will be closest to (2867) Steins at 20:37 CEST on 5 September, at a distance of 800 km. The spacecraft will zoom past at a relative speed of 8.6 km/s.

In preparation for the fly-by, all the instruments will be checked and tested through the month of July. Between 4 August and 4 September, spacecraft operators will conduct an optical navigation campaign: Steins will be tracked by the on-board cameras and the observations will be used to refine the knowledge of its orbit which has been derived only from ground-based measurements so far.

Asteroids are samples of the Solar System's material at different stages of evolution, and studying them helps scientists understand the origin and evolution of Earth and of our planetary neighbourhood.

(2867) Steins is a relatively rare type of asteroid. Based on ground-based observations it has been classified as an E-type asteroid, composed mainly of silicates and basalts, but its properties are not known in detail. For these reasons, it has been selected as one of the two asteroids that Rosetta will study, from among those that were within reach of the mission. The knowledge gained from the measurements will add to our knowledge of the composition and evolution of E-type asteroids and will also supplement and help interpret future ground-based data on asteroids.

Making most of the fly-by

The observations will be used to characterise the asteroid and its environment and to test Rosetta's instruments, most of which will be active during the fly-by.

The science objectives of the fly-by observations are as follows:

* To characterise the asteroid by studying its physical and chemical properties
* To study its kinematic properties (its rotation, for example)
* To study the asteroid's surface and to perform comparative studies with surfaces of other asteroids in order to understand differences between asteroid types
* To study the interaction between the solar wind and the asteroid
* To study the asteroid's environment, including the presence of natural satellites, the magnetic and electrical properties of the immediate environment, and gas or dust orbiting the asteroid

The fly-by will push Rosetta to its design limits, especially owing to the fast rotation of the spacecraft around the time of closest approach. The manoeuvre is necessary to ensure that the asteroid will stay in the field of view of the instruments. In view of this, a full in-flight simulation of the fly-by was performed on 24 March 2008. The tests were successful, confirming the spacecraft's robustness.

Major journey milestones

Launch: 2 March 2004
First Earth swing-by: 4 March 2005
Mars swing-by: 25 February 2007
Second Earth swing-by: 13 November 2007
Third Earth swing-by: 13 November 2009
Comet 67/P Churyumov-Gerasimenko rendezvous: May 2014
Landing on the comet: November 2014
Escorting the comet: until end 2015

2 July 2008

EXPLODING ASTEROID THEORY STRENGTHENED BY NEW EVIDENCE LOCATED IN OHIO, INDIANA

Geological evidence found in Ohio and Indiana in recent weeks is strengthening the case to attribute what happened 12,900 years ago in North America -- when the end of the last Ice Age unexpectedly turned into a phase of extinction for animals and humans -- to a cataclysmic comet or asteroid explosion over top of Canada.

A comet/asteroid theory advanced by Arizona-based geophysicist Allen West in the past two years says that an object from space exploded just above the earth's surface at that time over modern-day Canada, sparking a massive shock wave and heat-generating event that set large parts of the northern hemisphere ablaze, setting the stage for the extinctions.

Now University of Cincinnati Assistant Professor of Anthropology Ken Tankersley, working in conjunction with West and Indiana Geological Society Research Scientist Nelson R. Schaffer, has verified evidence from sites in Ohio and Indiana -- including, locally, Hamilton and Clermont counties in Ohio and Brown County in Indiana -- that offers the strongest support yet for the exploding comet/asteroid theory.

Samples of diamonds, gold and silver that have been found in the region have been conclusively sourced through X-ray diffractometry in the lab of UC Professor of Geology Warren Huff back to the diamond fields region of Canada.

The only plausible scenario available now for explaining their presence this far south is the kind of cataclysmic explosive event described by West's theory. "We believe this is the strongest evidence yet indicating a comet impact in that time period," says Tankersley.

Ironically, Tankersley had gone into the field with West believing he might be able to disprove West's theory.

Tankersley was familiar through years of work in this area with the diamonds, gold and silver deposits, which at one point could be found in such abundance in this region that the Hopewell Indians who lived here about 2,000 years ago engaged in trade in these items.

Prevailing thought said that these deposits, which are found at a soil depth consistent with the time frame of the comet/asteroid event, had been brought south from the Great Lakes region by glaciers.

"My smoking gun to disprove (West) was going to be the gold, silver and diamonds," Tankersley says. "But what I didn't know at that point was a conclusion he had reached that he had not yet made public -- that the likely point of impact for the comet wasn't just anywhere over Canada, but located over Canada's diamond-bearing fields. Instead of becoming the basis for rejecting his hypothesis, these items became the very best evidence to support it."

Additional sourcing work is being done at the sites looking for iridium, micro-meteorites and nano-diamonds that bear the markers of the diamond-field region, which also should have been blasted by the impact into this region.

Much of the work is being done in Sheriden Cave in north-central Ohio's Wyandot County, a rich repository of material dating back to the Ice Age.

Tankersley first came into contact with West and Schaffer when they were invited guests for interdisciplinary colloquia presented by UC's Department of Geology this spring.

West presented on his theory that a large comet or asteroid, believed to be more than a mile in diameter, exploded just above the earth at a time when the last Ice Age appeared to be drawing to a close.

The timing attached to this theory of about 12,900 years ago is consistent with the known disappearances in North America of the wooly mammoth population and the first distinct human society to inhabit the continent, known as the Clovis civilization. At that time, climatic history suggests the Ice Age should have been drawing to a close, but a rapid change known as the Younger Dryas event, instead ushered in another 1,300 years of glacial conditions. A cataclysmic explosion consistent with West's theory would have the potential to create the kind of atmospheric turmoil necessary to produce such conditions.

"The kind of evidence we are finding does suggest that climate change at the end of the last Ice Age was the result of a catastrophic event," Tankersley says.

Currently, Tankersley can be seen in a new documentary airing on the National Geographic channel. The film "Ancient Asteroids" is part of that network's "Naked Science" series.

The new discoveries made working with West and Schaffer will be incorporated into two more specials that Tankersley is currently involved with -- one for the PBS series "Nova" and a second for the History Channel that will be filming Tankersley and his UC students in the field this summer. Another documentary, this one being produced by the Discovery Channel and the British public television network Channel 4, will also be following Tankersley and his students later this summer.

As more data continues to be compiled, Tankersley, West and Schaffer will be publishing about this newest twist in the search to explain the history of our planet and its climate.

Climate change is a favourite topic for Tankersley. "The ultimate importance of this kind of work is showing that we can't control everything," he says. "Our planet has been hit by asteroids many times throughout its history, and when that happens, it does produce climate change."

1 July 2008

COMMENTARY: WHAT SPACEGUARD DID
by Alan Harris

The sky isn't falling, but there are still good reasons for keeping an eye on it. In 1991, a NASA-sponsored international working group convened to develop a thorough survey of near-Earth objects (NEOs) - predominately asteroids with an orbit that brings them within 1.3 astronomical units of the Sun. The objective of the survey would not be a mere sampling of the large asteroids that might constitute a risk to Earth, but rather a census. The report defined the 'Spaceguard Survey'. Spaceguard's goal was to identify most NEOs larger than 1 kilometre in diameter within a decade. The impact of an asteroid larger than 1 kilometre in diameter has the potential to cause a global climatic perturbation, similar to a 'nuclear winter', and could lead to billions of deaths worldwide. Such events, although less frequent than smaller 'impacts' such as the Tunguska event in Russia (see page 1157), nevertheless present a greater risk of death, even to individuals. Moreover, they carry the additional risk of ending civilization. So, it is clear to most why a survey might be important.

The idea was slow to catch on within NASA, but by May 1998, Carl Pilcher, the Science Director of Solar System Exploration in the NASA Office of Space Science, testified before the Subcommittee on Space and Aeronautics of US Congress that "NASA is committed to achieving the goal of detecting and cataloguing 90% of NEOs larger than 1 kilometre in diameter within ten years". This for many was the formal start of Spaceguard, so it is appropriate, a decade later, to ask whether its goals have been met. The pedantic answer is no, but in terms of risk reduction - or more precisely, knowing whether an impact will, or will not, occur in our lifetimes - Spaceguard identified a fraction of NEOs responsible for more than 90% of the potential impact risk, and found that impacts from that fraction pose a negligible risk in the next 50-100 years. The remaining short-term risk is almost entirely from any remaining undiscovered NEOs. In that sense, the Spaceguard Survey has been a remarkable success.

Two years ago, I was commissioned by NASA, through the NEO Program Office at the Jet Propulsion Laboratory in Pasadena, California, to assess the progress of Spaceguard. I filed my final report with NASA in March 2007 and have presented a brief summary of the main results. It is easy enough to keep count of the number of discovered objects larger than a given size, but to know when 90% have been found, one must estimate the total population. This is a bit of a bootstrap process, using the survey itself to estimate everything out there. In the simplest terms, if we scan the sky tonight, the number of detections of already-known objects compared to the total number of objects detected during a test interval gives us a measure of completeness. In detail, it is not so simple because not all NEOs are equally detectable.

How is Spaceguard doing? As of 10 June 2008, 742 near-Earth asteroids of diameter greater than 1 kilometre had been discovered. In my report I estimated a total of 940, and so the Spaceguard Survey has identified about 79%; not quite 90%, but not bad considering the uncertainties and the efforts required to reach 90%. Meanwhile the estimated risk of impact is dwindling. In the very largest size range, asteroids about 10 kilometres in diameter, the three already discovered are almost certainly all that exist. These would produce an impact similar to that which killed the dinosaurs 65 million years ago, with an estimated impact interval of around 10^8 years - roughly the last time dinosaurs walked on Earth. Oddly, an object that might cause a Tunguska-like event - roughly 50 metres in diameter - should collide with Earth only about every 1,500 years, and the last event we saw was only 100 years ago.

Recently, Mark Boslough at Sandia National Laboratories, in Albuquerque, New Mexico, suggested that the energy of the Tunguska event may have been as low as 3 megatonnes. That adjustment reduces the expected time between similar events to perhaps about once in 500 years, still leaving the chances of an event within a century as unlikely. 'Statistics of one' cannot be held too rigorously to formal probability estimates, but our view of the skies has produced a strong predictor for the frequency of impacts. It is so strong, in fact, that it could and should rule out some suggestions of past impacts such as the multiple kilometre-sized objects claimed by some to have pelted Earth during the Holocene period. Such an event is inconsistent with what we see in the skies, by about two orders of magnitude.

Another NASA study in 2003, estimated the expected damage from impacts of various sizes. Using those values of expected damage, and the impact frequency from the newly derived population, I estimated the 'risk spectrum' of impacts over the entire size range of those that can penetrate the atmosphere. Figure 2 shows that 'spectrum', first for the entire population, that is, the 'intrinsic risk' before any NEOs had been discovered, and secondly the 'residual risk' from the fraction of the NEO population that remains undiscovered. Since the objects that have been discovered have been found to have no, or a vanishingly small, probability of hitting Earth in the next 50 or more years, we can think of that fraction of the intrinsic risk as 'retired' for the short term over which we can predict impact trajectories, about a human lifetime.

Figure 2 shows that the risk from large impacts -- the kind that would cause global climatic disaster and potentially bring down our civilization - has been dramatically reduced, by more than an order of magnitude. In the smaller size range, from several-hundred-metre-diameter objects that could cause massive tsunamis if they crashed into an ocean, down to sub-hundred-metre objects the size of that in the Tunguska event - which could cause ground damage from airbursts - current surveys have done little to retire the risk. But the intrinsic risk from these events is very small, and in fact resembles that of other natural disasters such as tsunamis, earthquakes and volcanic eruptions in that they do not pose a global threat to life as we know it.

In the 2003 NASA report, the recommendation was made for a new survey to reduce the assessed residual impact risk from objects less than 1 kilometre in diameter by a further order of magnitude. It was estimated at that time that to achieve this goal would require discovering 90% of NEOs larger than 140 metres in diameter. This has become the new mantra of survey plans, but perhaps this should be reconsidered. Because of the steep dip in the population curve in the size range between about 50 metres and about 500 metres, the intrinsic impact frequency, and hence the impact risk, is about three times lower than was estimated in the 2003 report. So, in a way, two-thirds of the risk assumed to exist in those reports is gone already, without even looking at the sky. In the earlier reports, the 'residual risk' to be addressed by a next-generation survey was assumed to be approximately 300 fatalities per year, but using my new population estimate that figure drops to around 80 per year. In comparison to other risks in life, this is negligible.

What is the risk that your death will come from the sky? Before the Spaceguard Survey, it was thought to be comparable to the risk of dying in a commercial aeroplane accident. Currently, however, the residual risk from the remaining undiscovered NEOs is more comparable to the risk of death from a fireworks accident. At some point one has to ask how far down we need to drive the residual risk, especially because the cost of doing so increases steeply as the size of impactors decreases.

Be that as it may, plans are continuing for next-generation surveys and they may serve another purpose. The Large Synoptic Survey Telescope (LSST), a ground-based, wide-field instrument with an 8.4 metre aperture, is planned to enter service by about 2012. Recognizing the diminishing value of driving our assessement of the impact risk so low, the LSST project has adopted the NEO survey as only one of many scientific goals for the telescope, and in particular has emphasized the scientific value of a Solar System survey.

Indeed, the Spaceguard Survey itself has yielded scientific results aside from the value of impact risk reduction. Recently William Bottke and his colleagues at the Southwest Research Institute in Boulder, Colorado, used orbital statistics of asteroids discovered by the surveys to propose that the event that killed off the dinosaurs came from an 'impact shower' resulting from the collisional breakup that produced the Baptistina asteroid family. The size frequency distribution of impactors is itself interesting. The drop in numbers from those of a few hundred metres in diameter to those of a few tens of metres is not yet explained, but is perhaps due to the transition, at around 200 metres diameter, from 'rubble pile' structure among larger asteroids, which are less resistant to disruption by collisions, to monolithic bodies in the smaller size range, which are more resistant to further collisional breakup. Thus, although continuing surveys for the sole purpose of risk reduction may be of diminishing value, the scientific rewards will remain high, and we can hope that ever more powerful surveys will continue in the future.

Alan Harris is a senior research scientist with the Space Science Institute, 4603 Orange Knoll Avenue, La Canada, California 91011-3364, USA.

28 June 2008

100 YEARS OF SPACE ROCK: THE TUNGUSKA IMPACT

Jet Propulsion Laboratory

At around 7:17 on the morning of June 30, 1908, a man based at the trading post at Vanavara in Siberia is sitting on his front porch. In a moment, 40 miles from the center of an immense blast of unknown origin, he will be hurled from his chair and the heat will be so intense he will feel as though his shirt is on fire. The man at the trading post, and others in a largely uninhabited region of Siberia, near the Podkamennaya Tunguska River, are to be accidental eyewitnesses to cosmological history.

"If you want to start a conversation with anyone in the asteroid business all you have to say is Tunguska," said Don Yeomans, manager of the Near-Earth Object Office at NASA's Jet Propulsion Laboratory. "It is the only entry of a large meteoroid we have in the modern era with first-hand accounts."

While the impact occurred in '08, the first scientific expedition to the area would have to wait for 19 years. In 1921, Leonid Kulik, the chief curator for the meteorite collection of the St. Petersburg museum led an expedition to Tunguska. But the harsh conditions of the Siberian outback thwarted his team's attempt to reach the area of the blast. In 1927, a new expedition, again lead by Kulik, reached its goal.

"At first, the locals were reluctant to tell Kulik about the event," said Yeomans. "They believed the blast was a visitation by the god Ogdy, who had cursed the area by smashing trees and killing animals."

While testimonials may have at first been difficult to obtain, there was plenty of evidence lying around. Eight hundred square miles of remote forest had been ripped asunder. Eighty million trees were on their sides, lying in a radial pattern.

"Those trees acted as markers, pointing directly away from the blast's epicenter," said Yeomans. "Later, when the team arrived at ground zero, they found the trees there standing upright -- but their limbs and bark had been stripped away. They looked like a forest of telephone poles."

Such debranching requires fast moving shock waves that break off a tree's branches before the branches can transfer the impact momentum to the tree's stem. Thirty seven years after the Tunguska blast, branchless trees would be found at the site of another massive explosion -- Hiroshima, Japan.

Kulik's expeditions (he traveled to Tunguska on three separate occasions) did finally get some of the locals to talk. One was the man based at the Vanara trading post who witnessed the heat blast as he was launched a few yards. His account:

Suddenly in the north sky...the sky was split in two, and high above the forest the whole northern part of the sky appeared covered with fire...At that moment there was a bang in the sky and a mighty crash...The crash was followed by a noise like stones falling from the sky, or of guns firing. The earth trembled.

The massive explosion packed a wallop. The resulting seismic shockwave registered with sensitive barometers as far away as England. Dense clouds formed over the region at high altitudes which reflected sunlight from beyond the horizon. Night skies glowed, and reports came in that people who lived as far away as Asia could read newspapers outdoors as late as midnight. Locally, hundreds of reindeer, the livelihood of local herders, were killed, but there was no direct evidence that any person perished in the blast.

"A century later some still debate the cause and come up with different scenarios that could have caused the explosion," said Yeomans. "But the generally agreed upon theory is that on the morning of June 30, 1908, a large space rock, about 120 feet across, entered the atmosphere of Siberia and then detonated in the sky."

It is estimated the asteroid entered Earth's atmosphere traveling at a speed of about 33,500 miles per hour. During its quick plunge, the 220-million-pound space rock heated the air surrounding it to 44,500 degrees Fahrenheit. At 7:17 a.m. (local Siberia time), at a height of about 28,000 feet, the combination of pressure and heat caused the asteroid to fragment and annihilate itself, producing a fireball and releasing energy equivalent to about 185 Hiroshima bombs.

"That is why there is no impact crater," said Yeomans. "The great majority of the asteroid is consumed in the explosion."

Yeomans and his colleagues at JPL's Near-Earth Object Office are tasked with plotting the orbits of present-day comets and asteroids that cross Earth's path, and could be potentially hazardous to our planet.

Yeomans estimates that, on average, a Tunguska-sized asteroid will enter Earth's atmosphere once every 300 years. On this 100th anniversary of the Tunguska event, does that mean we have 200 years of largely meteor-free skies?

"Not necessarily," said Yeomans. "The 300 years between Tunguska-sized events is an average based on our best science. I think about Tunguska all the time from a scientific point of view, but the thought of a another Tunguska does not keep me up at night."