This area will cover relevant news of the threat to the planet from Near Earth Objects (NEOs) including concepts and designs for mitigation. All opinions are those of the author.

09 July 2008

Editorial on Apophis and Russian NEO Efforts

From the article...

Meanwhile, Russia is taking steps to improve its monitoring of the dangerous space intruder. The Russian Space Agency (Roscosmos), together with the Defense Ministry and Academy of Sciences, has launched an anti-asteroid program. The first step will see a special radar mounted on a 70-metre telescope in Ussuriisk. The radar will pick up signals reflected by natural space bodies.

The Lavochkin Research and Production Association (NPO Lavochkin) is working on a space program for 2012-2014 that will gather as much information about Apophis as possible. It is also planning a series of experiments to see if the asteroid's orbit could be changed by sustained energy impacts. An unmanned craft is being developed for the purpose, and different flight scenarios considered.

In mid-June, when examining the NASA budget for the next year, the House of Representatives told the Space Agency to cooperate with Russia in all matters concerning asteroids.

The Americans rightly believe that bodies like Apophis threaten all humanity. Republican Congressman Dan Rohrbacher, who pushed for the inclusion of international asteroid cooperation in NASA's budget, thinks the U.S. should share this responsibility with others.

Russia will not stand aloof. It is willing to contribute its RT-70 high-powered antenna system for deep space communications. Its facilities, located in the Far East and Crimea, will effectively complement American facilities in Puerto Rico and California.

How to deal with "asteroid terrorism"?
09 July 2008

Article: RIA Novosti

07 July 2008

NYTimes Article: "Maybe Chicken Little Wasn’t Paranoid After All"

Selections from the article...

NASA’s Jet Propulsion Laboratory in Pasadena, Calif., has estimated that a Tunguska-size asteroid will enter Earth’s atmosphere once every 300 years and says there may be 375,000 objects of such size out there.

NASA estimates that there are about 940 or so near-Earth space rocks a kilometer in diameter or larger. So far, according to Dr. Yeomans, NASA has identified 743. (Overall, NASA has pinpointed more than 5,500 near-Earth objects.)

Mr. Schweickart said progress so far had come through constant pushing against resistant bureaucracies and politicians focused on whatever is the issue of the moment. And he said there was still no significant effort to devise an international agreement, let alone a deflection technique, for dealing with the inevitable earthbound asteroid or comet, large or small, when it is identified.

“It may be subtle,” he said, “but failure of the international decision process is the most likely reason that we’ll take a hit in the future.”

"Maybe Chicken Little Wasn’t Paranoid After All"
Andrew C. Revkin
06 July 2008

Link: NYTimes Article

Articles: Explosing Asteroid over Canada

From the Astrobiology article...

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. The study could yield clues about how objects from space could affect the future of life on our planet.

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 Allen 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.

During the period known as the Younger Dryas, large mammals like wooly mammoths became extinct.
Credit: USGS
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.

Many scientists hypothesize that mass extinctions our planet’s history may well have been due to comet and asteroid impacts.
Credit: NASA ARC
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 "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 favorite 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."

Link: Astrobiology Press Release

Link: UPI Article

Link: Montreal Gazette Article

06 July 2008

ESA ROSETTA Mission and Flyby of Asteroid Steins

From the article about the ROSETTA mission and its flyby of asteroid Steins...

European Space Agency mission managers woke the Rosetta probe from hibernation to prepare for its encounter with asteroid Steins on Sept. 5, according to a statement this week. The robot's ultimate destination is comet 67/P Churyumov-Gerasimenko.

Launched in March 2004, Rosetta will reach its final destination only in 2014, after traveling a total of about 4 billion miles (6.5 billion kilometers).

Rosetta has swung by Earth twice and Mars once, to get gravitational boosts. The third and last Earth swing-by is scheduled for November 2009. The spacecraft will also fly by asteroid Lutetia in June 2010.

When it reaches asteroid Steins this year, Rosetta will be about twice as far from the sun as is Earth. The encounter will occur in the asteroid belt between Mars and Jupiter. The probe will come within 500 miles (800 kilometers) of the asteroid.

"Spacecraft Woken for Asteroid Encounter" Staff
05 July 2008

From Paolo and IM4 on the forums ( Other Missions/ Cometary and Asteroid Missions/ Rosetta flyby of Asteroid Steins, 5th September 2008)...

Steins is a small body less than 10 km across, discovered on 4 November 1969 by Soviet astronomer N. Chernykh at the Nauchnyj Observatory in Crimea and named after Karlis Steins, former director of the Latvian University Astronomical Observatory. While the properties of Lutetia, the second asteroid target are relatively well known, being a largish and bright object discovered more than a century ago, almost nothing was known about Steins, and observational campaigns were started in 2004 to characterize it. Steins was observed by the largest astronomical observatory on Earth, including the European Very Large Telescope (VLT) in Chile, JPL's Table Mountain Observatory, the Spitzer infrared astronomy satellite and Rosetta itself.

The Rosetta orbiter observed continuously Steins (1.06 AU away) for 24 hours with its science camera on 11 March 2006 in order to collect a “light curve” at phase angles larger than ever achievable from Earth, due to observational geometry constrains and for a longer time span, uninterrupted by day and night cycles. A total of 238 images were taken, covering four rotations.

Although researchers initially catalogued Steins in the S class (like most previously-visited bodies), the observational campaigns found that its spectral and polarimetric properties placed it in the E taxonomic class of reddish bodies with high albedo believed to be thermally evolved and of igneous origin, which underwent at least a partial melting and differentiation early in their history. E asteroid spectra probably makes them related to some rare enstatite chondrite or aubrite meteorites and therefore these bodies are believed to have a surface consisting of iron-free or iron-poor silicates. Observations pinpointed Steins' rotation period at about 6.05 hours, while the measured diameter could vary between 2 and 5 km, depending on its albedo. Asymmetries in the light curve confirmed that the small body has an irregular shape, with a ratio between the main axes of about 1.3. Other researchers pointed out that some of the characteristics of Steins could hint at a young and very rough surface at most a few million years old.

While fewer than 30 members of the E class of asteroids were known, including (44) Nysa, the largest, and two near-Earth objects, (3103) Eger and (4660) Nereus (a recurrent space mission target), little is known about the evolutionary history of the type. Steins was initially placed in the same family as (64) Angelina, but other studies showed that it shared most of the same spectral characteristics as Eger, both bodies being believed to be members of an old eroded family which formed in the inner asteroid belt close to the present position of Steins, which appears to be the largest member of the family. The presence of Eger in an Earth-intersecting orbit of course provides a path for enstatite and aubrite meteorites to hit our planet.
The encounter on 5 September 2008 will be at a distance of 1745 km and a relative speed of 8.6 km/

Flyby in a nutshell:

1. Closest approach on 5 Sept. 2008 18:33:57 ± 30 sec
2. Heliocentric distance 2.14 AU, geocentric distance 2.41 AU
3. Low phase angle during approach, high phase angle after closest approach, coverage 0-140°
4. Minimum flyby distance of 800 km
5. Zero phase angle will be reached at a distance of 1280 km
6. One way light travel time will be 20 min

Flyby strategy

1. Default pointing
2. Spacecraft flip (T-40 min -> T-20 min)
3. Inverted pointing (> T-20 min)
4. +Z axis points at Steins at all times

Flyby Science:

1. Alice: point at Steins to obtain FUV spectra search for exosphere/coma around Steins
2. COSIMA: execute normal dust collection cycle
3. GIADA: Impact sensor operational, but cover closed
4. MIRO: Steins observations during approach and recession. Special asteroid mode sequence at closest approach (CA)
5. ROSINA: Pressure monitoring, single mass measurement sequence
6. RPC: Measurements of plasma environment
7. RSI: Attempt for mass determination (probably not feasible)
8. OSIRIS: Light curve for 2 weeks before CA for shape reconstruction. At CA: spectrophotometry, mapping and surface properties, satellite/dust search
9. VIRTIS: Steins light curve determination starting 7 hours before CA. At CA: mineralogy mapping

Expected data volume:

1. 600 MiB before closest approach
2. 2400 MiB after closest approach

Link: article

Link: LPSI Abstract on Asteroid Steins (PDF)

Link: Forums

Link: Wikipedia Entry for Asteroid Steins

NYTimes DotEarth Blog Picks Up on Planetary Defense

The New York Times DotEarth Blog picks up on the planetary defense story. They include both the SGAC Move An Asteroid 2008 competition and The Planetary Society's Apophis Mission Design Competition (both of which I was involved with).

"Solar System Roulette"
Andrew C. Revkin
06 July 2008

Link: NYTimes DotEarth Blog

04 July 2008

Article: Strange Asteroid Shapes Explained

Selections from the article...

Researchers have been using a vast database to study a staggering 11,735 asteroids. They have discovered that asteroids change shape over time, and they think they know the reason why.

Gyula Szabó from the University of Szeged [Hungary] is the lead author of the study, which was published in the July edition of Icarus. He explains, "There are several hundred thousand asteroids in our solar system. They orbit the sun, but because they are small their surface gravity is low. This means that many have strange, irregular shapes."

Scientists like Gyula think that about one third of known asteroids belong to groups called "families." These clusters probably formed from piles of debris after larger objects collided.

But what changes the asteroids' shape? Gyula and his team have shown that asteroids change shape from elongated to roughly spherical due to being impacted during their lifetimes. They are like pebbles on the beach that become worn smooth over many years -- only in space, erosion is caused by small impacts as rocks knock into each other and chip pieces off.

"Strange Asteroid Shapes Explained"
Lee Pullen
Astrobiology Magazine
03 July 2008

Link: article


"The shape distribution of asteroid families: Evidence for evolution driven by small impacts"

Gyula M. Szabóa, b, Corresponding Author Contact Information, E-mail The Corresponding Author and László L. Kissc

aMagyary Fellow, School of Physics A28, University of Sydney, NSW 2006, Australia

bDepartment of Experimental Physics, University of Szeged, 6720 Szeged, Hungary

cSchool of Physics A28, University of Sydney, NSW 2006, Australia

Received 17 July 2007;
revised 22 November 2007.
Available online 29 February 2008.


A statistical analysis of brightness variability of asteroids reveals how their shapes evolve from elongated to rough spheroidal forms, presumably driven by impact-related phenomena. Based on the Sloan Digital Sky Survey Moving Object Catalog, we determined the shape distribution of 11,735 asteroids, with special emphasis on eight prominent asteroid families. In young families, asteroids have a wide range of shape elongations, implying fragmentation–formation. In older families we see an increasing number of rough spheroids, in agreement with the predictions of an impact-driven evolution. Old families also contain a group of moderately elongated members, which we suggest correspond to higher-density, more impact-resistant cores of former fragmented asteroids that have undergone slow shape erosion. A few percent of asteroids have very elongated shapes, and can either be young fragments or tidally reshaped bodies. Our results confirm that the majority of asteroids are gravitationally bound “rubble piles.”

Link: ICARUS Abstract

Secure World Foundation and NEOs

The Secure World Foundation has some activities on NEOs. Selection from an article on their activities...

Speaking recently at a Secure World Foundation luncheon at the University of Colorado - Boulder, Schweickart [Apollo astronaut, Russell L. (Rusty) Schweickart] emphasized that what is needed is an international protocol – “mission rules” -- that deal with asteroids that are menacing to Earth. Such a plan could calls upon nations around the globe to consider and embrace steps that can help mitigate the destructive nature stemming from an asteroid striking our planet.

Working with the Association of Space Explorers and the talents of the B612 Foundation – and supported in part by the Secure World Foundation -- the former astronaut has organized a series of workshops on the NEO threat, geared to creating a protocol for submission to the United Nations for its deliberation. These are to be spelled out in a mission rules document for protecting the Earth from asteroid impacts.

Moreover, the document is to be briefed to stakeholders around the globe – from spacefaring nations and agencies, as well as to key decision-makers in leadership positions.

Link: ScienceDaily Article

Link: Secure World Foundation - NEO Page

Link: Secure World Foundation - Press Release on NEO Talk

02 July 2008

Article: Tunguska at 100

"Tunguska meteoroid turns 100 years old: KP journalist visits the region where the 20th Century's most mysterious cosmic visitor exploded"
Andrey Moiseenko
02 July 2008

Link: Part 1 of Article

Link: Part 2 of Article

01 July 2008

NEO News (07/01/08) Population and impact frequency

From Dave Morrison.

NEO News (07/01/08) Population and impact frequency

This edition of NEO News contains one of our periodic updates on the progress of the Spaceguard survey, a topic that is intertwined with estimates of the population of NEAs as a function of size. As in the past, the most detailed such estimates come from Alan Harris. Reproduced below is his recent article on this subject, published in Nature last week. I have not included any of the graphics (two figures) or the references; for these, please reference the original article in Nature.

All estimates of NEA populations and impact rates are plagued by uncertainties concerning the albedos of NEAs and hence the conversions from the observed quantities (magnitudes) to size and energy. The are also uncertainties in the magnitude scales used by different observing groups. Models include different assumptions, and it is thus difficult to compare detailed numerical results. The results from Harris are internally self-consistent, but other self-consistent models yield slightly different conclusions, even using the same observational data. Harris reports (for early June) that 742 near-Earth asteroids of diameter greater than 1 km have been discovered. He estimated a total of 940, concluding that the Spaceguard Survey has identified about 79% of NEAs larger than 1 km. Lindley Johnson of NASA Headquarters, using the same data, uses the discovery of 744 (as of late June) but adopts a different total population of NEAs >1 km to yield a completeness of 86%. Either way, the Spaceguard Survey has been a great success.

The most important innovation in the Harris models discussed below is in the use of new astronomical data from the surveys to conclude that the size frequency distribution of NEAs is not a straight-line power law (his Figure 1 in the Nature paper). The dip in population of sub-km NEAs results in lower risks, as he discusses below.

In spite of the differences between models, it seems to me that with all the new data from Spaceguard, the population and impact frequency numbers are now pretty solid. But how we interpret them is an individual matter. Some feel that the risk from sub-km NEAs is unacceptable, others that it is negligible. It is in this area of interpretation that we see the strongest divergence of opinion about the magnitude of the remaining NEA impact risk.

David Morrison


Nature: Vol 453 pg 1178 June 2008

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 108 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.


NEO News (now in its fourteenth year of distribution) is an informal compilation of news and opinion dealing with Near Earth Objects (NEOs) and their impacts. These opinions are the responsibility of the individual authors and do not represent the positions of NASA, Ames Research Center, the International Astronomical Union, or any other organization. To subscribe (or unsubscribe) contact For additional information, please see the website If anyone wishes to copy or redistribute original material from these notes, fully or in part, please include this disclaimer.

The Planetary Society's Planetary Radio Show On Tunguska

Airdate: Monday, June 30, 2008
Running Time: 00:28:52
Listen: Windows Media | MP3

Early on the morning of June 30, 1908, a peaceful Siberian forest was suddenly leveled by the explosion of a meteor. Scientist, author and artist Bill Hartmann returns to Planetary Radio to talk about this event. Emily Lakdawalla provides another Phoenix Mars mission update. Bill Nye reports on his visit to Congress. Catch the winner of the Random Space Fact recording contest on this week's What's Up with Bruce Betts and Mat Kaplan, which includes a chance to win an Explorer's Guide to Mars poster.

Link: Planetary Radio Show (Monday, June 30, 2008)

Link: Planetary Radio (Windows Media)

Link: Planetary Radio (.mp3)

The Plantaery Society's NEO Fund

From an email solicitation from The Planetary Society to donate to their NEO fund.

Link: The Planetary Society NEO Fund

Dear Member,

On the 100th anniversary of the Tunguska asteroid explosion, how simply can I state Earth's situation without hyperbole? Let me give this a try:

* We know of 4,700 asteroids that could strike Earth.
* 700 of them could cause global disaster.
* We'll need to know within a few years if we need to deflect the closest NEO, Apophis, from its current orbit, or there may not be time to stop it.
* Congress is tying NASA's hands, or "blinding us" if you prefer, because of budgetary constraints.
* We have to determine the threat level without their help.

In case you were wondering, Apophis is the size of Yankee Stadium, and it's traveling at 45,000 kilometers per hour.

How likely are we to be hit and significantly damaged? The Tunguska Event occurred 100 years ago today. On that day, an asteroid exploded over Siberia, killing all life and flattening trees over 2000 square kilometers.

The Planetary Society is now working in two ways to prevent this from happening again. We are:

1. Working to keep the Arecibo Observatory in operation. It's 20 times more sensitive than any other radio-telescope instrument in the world. But it's scheduled to be closed because of budget problems.

2. Funding research through the Shoemaker NEO Grant fund. One of our recipients, Roy Tucker, is a co-discoverer of Apophis. He used his grant to buy the equipment which helped him discover Apophis.

Your support for our NEO Fund will help Shoemaker recipients, and facilities like the Arecibo Observatory, stay on watch for inbound NEOs. You'll also help develop strategies to protect us from a catastrophic hit.

Donate now and your support will help our best minds take on these all-too-real threats to our only home: Our planet Earth.


Louis Friedman
Dr. Louis D. Friedman
Executive Director

P.S. As far as we now know, there are 700 NEOs heading our way that could cause global disaster. Are there more? Please donate today, so we can continue to fund NEO research.

Planetary Soceity Press Conference on Arecibo

From the Press Release..

The Planetary Society today joined with US Congressman Dana Rohrabacher, 46th District of California, to call for greater support for near-Earth object research and continued funding of the Arecibo radiotelescope to track potentially threatening objects in space. They were joined at the press conference by Alan W. Harris, Senior Research Scientist of the Space Science Institute, and Don Yeomans, Manager of NASA’s Near-Earth Object Program Office at the Jet Propulsion Laboratory.

June 30 marks the 100th anniversary of the Tunguska cataclysm when a fireball from space exploded over a Siberian forest, flattening more than 2000 square kilometers. If such an impact occurred over Los Angeles or Moscow today, the death toll would be catastrophic. The dangers our planet faces from near-Earth objects (NEOs) are the focus of The Planetary Society's year-long program, Target Earth.

Astronomers have already discovered that one asteroid, named Apophis, will pass closer to Earth in 21 years than our geosynchronous communications satellites, and its trajectory has a small probability of taking it on a collision course with Earth seven years after that. The Planetary Society awarded $50,000 in an Apophis Mission Design contest for the best idea for tagging a potentially hazardous asteroid, marking it in some way that it could be better tracked.

If an object is known to be dangerous decades before it will impact Earth, humanity will have the chance to alter the object's course and the planet's destiny.

"We can't afford to remain ignorant of a dangerous object that might be headed our way in 20, 30, or even 100 years," said Louis Friedman, Executive Director of The Planetary Society. “The lives of millions may be at stake.”

Earth orbits the solar system in a swarm of NEOs. Tracking these objects by radar is an ideal way to precisely know the probability of impact, and the Arecibo radiotelescope is the most powerful instrument for the job. However, Arecibo currently faces closure by the National Science Foundation due to lack of funds, and The Planetary Society joins its voice to many from the science community, urging Congress to reinstate the telescope's budget.

Over the past few years, our knowledge of NEOs has greatly increased as researchers have discovered and tracked many comets and asteroids that travel through our neighborhood of space. The Planetary Society has funded several NEO researchers through its Gene Shoemaker grant program. Still, many more objects remain to be found, and the orbits of many known objects need to be better refined.

Link: The Planetary Society Press Release
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