Monday, June 3, 2013

“The Size of Nine Ocean Liners”: Asteroid 1998 QE2


The sky is falling!  And it’s full of ocean liners!

On 30 May 2013 the press reverberated with the news that the Near-Earth Asteroid 1998 QE2, a monster “the size of nine ocean liners”, was going to sail by Earth (without docking).  This is a terrifying image: picture nine ocean liners falling from the sky!  What a splash that would make!  And some sources say it is long as nine ocean liners, and some say the width of nine ocean liners, and some say as big as nine ocean liners…well, whatever.

Of course the comparison of an asteroid with an ocean liner was motivated by the asteroid’s moniker (QE2), and should be understood as poetic license, or at least as unlicensed poeticism.  But is an asteroid of this size really a big threat?  True, it missed us by 5.8 million kilometers THIS time, but its orbit will bring it back across Earth’s orbit over and over again.  We can’t guarantee we will always be so lucky! 

The realists among us will appreciate that the news media routinely push the envelope of truth in order to generate scary headlines.  This is usually done by means of liberal use of adjectives (huge, gigantic, etc.; strangely I haven’t seen anyone refer to it as Titanic) rather than numerical facts.  So, assuming we know about the existence and usefulness of numbers, does QE2 represent a real threat?  Good question… and thanks for asking!

1998 QE2 is in fact about 1.7 miles in diameter, a pretty decent piece of rock, and has a 2000-foot satellite in orbit around it.  Now, let’s see: 1.7 miles is about 2700 meters in diameter, or a radius of 1350 meters and a volume of 10.3 billion cubic meters.  At an average meteorite or asteroid density of 3 tonnes per cubic meter, this is 31 billion tonnes.  Now let’s compare it to a big ocean liner—for example the Queen Elizabeth II:  displacement 44,000 tonnes.  So, by the difficult mathematical operation known as long division, we see that this asteroid (never mind its satellite) would deliver a mass equivalent to 700,000 ocean liners onto our unsuspecting heads.  WHAT?  The sensationalist media have underplayed the story by a factor of 80,000?

Why the disconnect?  Because of the vague use of the word “size”.  Some people use it to mean length, or area, or volume, or mass.  The length of an asteroid tells little about the size of a threat it presents—unless, of course, we know how to do arithmetic. 

But the real measure of its potential for wreaking havoc is its total kinetic energy.  Let’s take an impact speed of 16 kilometers per second as an example (many NEAs are moving a lot faster than that).  That puts it in the million-megaton  (1 teraton) league.  That would be comparable to 100 World War IIIs. 

Nine ocean liners?  Get real!

Tuesday, March 19, 2013

The Year of the Comets


Not since 1910 have we been treated to so fine a year for seeing comets.  Don’t miss the chance to see them yourself.  Space.com has shown a lovely photograph of two comets low in the western evening sky that should inspire anyone to make the effort.  Sadly, evening cloudiness over Puget Sound has denied me the opportunity—it’s not quite as nice for astronomy on the Washington coast as it was in Tucson!

Where do comets come from?  The simple answer, which the media pass on to us, is that they come from the Oort Cloud, a vast swarm of dirty snowballs that orbit in random directions around the Sun far outside the orbits of Neptune and Pluto.  This explanation has the advantage that it is sort or right—and the disadvantage that it is pretty inadequate.

Comets are usually divided into two families.  First we have the long-period comets, which typically take a million years to complete an orbit and spend most of their time 10,000 AU from the Sun.  These are the Oort cloud comets.  Their orbits are quite close to random: about half are traveling the “wrong way” around the Sun. which allows head-on collisions with planets at enormous closing speeds.  Only those that approach to well within Jupiter’s orbit ever get warm enough for wholesale evaporation of their ices, which blows off vast streams of gases and dust that give comets their “hairy” appearance, and hence the name “comet”, which comes from the Greek word for hair.  The overwhelming majority of the Oort Cloud comets have never (“what, never?  Well, hardly ever”) approached close enough to the Sun to light up, and hence to be discovered.  At best, such a comet has been observed only once.

Occasionally a long-period comet will pass close enough to Jupiter or Saturn to have its orbit strongly affected by the planet’s gravity.  These comets are diverted into relatively tame low-inclination orbits that cross the orbits of several of the terrestrial planets, often with orbital periods of 3 to 7 years and with aphelia close to the orbit of the planet that kicked it.  These are the short-period comets, which may be observed through dozens to hundreds of trips around the Sun.  They pass by repeatedly on regular schedules with well-known orbital periods, and for that reason are often called “periodic comets”.

There are several other fates possible for an Oort Cloud comet that ventures into our planetary system besides becoming periodic comets.  Some, after a traumatic close encounter with a giant planet, will be hurled outward at a speed well above the escape velocity of the Sun and become lonely wanderers in interstellar space.  The chances of such a body ever entering another planetary system and getting close enough to its star to light up as a bright comet are extremely remote.  Space is big, and stars are small.  No comet interloper from another planetary system has ever been observed.

But there are other fates in store for the long-period comets.  Some may fly by one of the giant planets and be diverted into orbits that have low inclination and cross the orbits of several of the giant planets.  These bodies cannot avoid collisions or violent gravitational interactions with these planets, and therefore have a short expected lifetime.  These bodies are called the Centaurs.  They and a vast dynamically related group called Trans-Neptunian Objects (TNOs), which, as their name suggests, orbit near and beyond Neptune, can be both former and future comets.  Pluto is one of the TNOs which happen to belong to a subfamily of bodies that have reached an orbital accommodation with Neptune, with a 3:2 orbital period resonance that prohibits them from ever approaching Neptune closely or colliding with it.  Bodies kicked into that neighborhood that were not lucky enough to enter a safe resonant orbit would soon collide with Neptune, be expelled from the Solar System, or become a Centaur.

In addition, the outer satellites of the giant planets, those in retrograde orbits, are only very weakly bound to their planets.  It is clear that these bodies may be captured or lost into heliocentric orbits quite easily.  Such a lost satellite may become a Centaur; a newly captured satellite probably was a Centaur.

Periodic comets may make hundreds of perihelion passages before the supply of volatile ices near their surfaces is exhausted.  The body ceases to emit gases and dust, cometary activity fizzles out, and we are left with an icy comet core that is covered with a layer of fine, extremely black dust that not only blocks solar heating of the interior, but also has a very low thermal conductivity.  Once a dust layer a few meters thick has developed, all cometary activity ceases and the body has the appearance of an extremely dark (D-class) asteroid.  Many near-Earth asteroids (NEAs) not only follow orbits similar to those of the periodic comets, but some have even been observed to make the transition from comet to asteroid.  If a small impact event opens a hole in the dust blanket, solar heating can again reach the buried ice and a “jet” of gas and dust can erupt.  Many short-period comets are active thanks solely to one or a few such local jets.  And of course such a collision on a D asteroid may cause it to resume cometary activity.  Many NEAs that may be dust-mantled icy cores of “extinct” comets can be recognized by their orbits and their D-type reflection spectra.  All of these could again become comets.

The semantic distinctions between planetary retrograde satellites, Centaurs, TNOs, long-period comets, periodic comets, and dark NEAs give us useful ways of describing what and where a body is today, but they do not do justice to the complex histories these bodies may have had before fitting neatly into one of these convenient pigeonholes.

A Centaur may from time to time be perturbed into an Earth-crossing orbit by one of the giant planets whose orbits they cross.  Such a body, lighting up as it approached the Sun, would then be termed a giant comet.  The Centaur 10199 Chariklo is about 260 km in diameter, compared to 6 km for a typical large comet nucleus such as the body whose impact ended the Cretaceous Era and extinguished the last of the dinosaurs.  An impact of Chariklo with Earth would deliver about 100,000 times as much energy as that global extinction event, equivalent to about 4000 tons of TNT for each person on Earth.  That would be about 2000 times as severe as an all-out nuclear World War III.  Mankind would be extinguished and life on Earth would be set back to the pre-Cambrian Era.

Unlike the dinosaurs, we have technologies that allow us to find, track, predict, and even intercept potential impactors.  It would be criminally negligent to ignore the impact threat.

Friday, February 22, 2013

An Early Manned Mission to Mars in 2018?


On 27 February Dennis Tito, who paid his way to the ISS as a tourist back in 2001, will be announcing the plans of a new private space company, the Inspiration Mars Foundation.  The rumor mill has it that their purpose is to launch a manned expedition to Mars as early as January 2018. 

According to several sources, the mission would be a 501-day free-flying flyby (neither orbiting nor landing on Mars).  It would be lifted into space by a Falcon Heavy launch vehicle and with crew accommodation for two people in the form of a modified Dragon capsule, of recent ISS fame.  This scheme would incorporate ideas already put forward by SpaceX’s Elon Musk, who is a vocal advocate of both private space development and the exploration and eventual colonization of Mars.

The mission would be financed privately and would advance on a much more ambitious schedule than any governmental or intergovernmental project could reasonably expect to achieve.

For those who instinctively disbelieve the concept that private enterprise can provide access to space cheaper and on a larger scale than governmental entities can, a refresher course on SpaceShipTwo, the Bigelow inflatable space station module, the Dragon capsule, and the dozens of companies that have set their sights on providing low-cost private access to space would be in order.

This seems to be a typically American thrust, but in fact Canadian, European, and other companies are also engaged in these pursuits.  In fiction, the first manned mission to the Moon was envisioned by Jules Verne (De la Terre a la Lune; 1865) as being a private venture funded by rich American industrialists, building on Civil War military technology, and launched (fired!) from Florida by a giant gun.  In fact, strangely enough, the first technically plausible suggestion of how to get humans into space was in a novel, “Beyond the Planet Earth: In the Year 2000”, written by the pre-Soviet Russian visionary Konstantin Tsiolkovskii in 1916.  In it, the impetus for the development of manned spaceflight came from an international team of scientists and a group of private investors whom we would now call venture capitalists.

Travel to Mars (“Barsoom”) was a standard theme of the writings of Edgar Rice Burroughs.  Percy Gregg’s novel “Across the Zodiac” (1880) recounts a visit to Mars.  Another early tale of interplanetary travel, like Tsiolkovskii’s novel also set in the year 2000,  was “A Journey in Other Worlds”, authored in 1894 by John Jacob Astor IV.  These and many other books, such as E. E. “Doc” Smith’s novels, generally attribute space travel ventures to innovators and private individual, not governments.

Perhaps Dennis Tito’s announcement will bring that spirit of non-governmental initiative not just into space, but all the way to Mars.

Thursday, February 21, 2013

Mining Asteroids, 2013


We now have two competing companies with their sights set on mining asteroids for commercial reasons.  Both companies are pursuing the dream I developed in my book, “Mining the Sky”, and both companies include long-time friends and students on their rolls.  To me the fact that there is competition in this endeavor is at least as important as the fact that it is being done at all.  It is through competition that new ideas are stimulated and old ideas are put to the test.
Which of these companies is the wave of the future?  I confess to having no crystal ball.  Being near the head of the line is no guarantee of long-term dominance—when’s the last time you used a Commodore PET or a TRS 80, not mention an Apple I?  Played any games on your TI-99 recently?  How’s the market for Xerox Altos?
Huge sales do not even guarantee long-term success: the best-selling personal computer ever was the Commodore 64, which, because of a price war with the TI-99, drove all players to the brink of bankruptcy (or over it).
The IBM PC and the Apple II were not “present at the creation”: they were just better…and quite different in design philosophy.  PCs and Apples still lead the personal computer world, although IBM has long since sold its own PC business to Lenovo in China, and armies of PC clones abound.
So are Planetary Resources Inc. and Deep Space industries the TI-99s and Commodore 64s of the space mining endeavor?  Or are they Apples and PCs?  Tune in again in ten years and maybe we’ll know.
A sure measure of the health of this new industry will be when even more competitors appear.
I have seen asteroid mining referred to as a “billion dollar industry”.  This is not correct: if the idea works, it is a multi-trillion dollar industry, making available to mankind more resources than the human race has used to date.  And if it is not successful, it will be known as a multi-million dollar flop.
I’m betting on long-term success.  Yesterday I joined the staff of Deep Space Industries as their Chief Scientist.  If, as the researchers are telling us, working Sudoku and crossword puzzles helps keep the brain functioning, then opening up the Solar System to the human race is likely to be an even more stimulating endeavor.  We no longer need fear “running out of resources” on a “finite planet”. 
The sky is no longer the limit.

Tuesday, February 19, 2013

Global Warming Update: What to Make of the Data


First, it is undeniably true that humans are injecting carbon dioxide and soot into the atmosphere at a record pace.  The data are uncontested.  Second, it is undeniably true that CO2 is a “greenhouse gas” which inhibits radiation of Earth’s surface heat into space and thus has a global warming effect.  Third, it is well established that water vapor has a far stronger warming effect than CO2.  As a further complication, clouds made by the condensation of that water vapor can lead to either cooling or heating, depending on the density, altitude, and particle sizes in the clouds.  Thus we are forced to estimate what effect warming by CO2 will have on the water vapor (and cloud) content of the atmosphere, a very difficult task.  Fourth, we must also come to grips with the warming effects of soot and carbon black, which also are products of human origin via everything from biomass cooking fires to coal-fired power plants to diesel engines.  Fifth, we need to understand all the correlates of natural processes such as solar variability and volcanic dust emission.  We can see clearly from the data in the HADCRUT4 graph in the previous post (Global Warming Update) that warming (and cooling) of Earth is far more complex than any one factor can explain: attributing all the warming in any time interval to CO2 makes CO2 appear more important than it really is, and biases all predictions in the direction of exaggerated warming.

Temperatures are influenced by the amount of radiation absorbed by gases—but not in a linear fashion.  The temperature increase caused by multiplying the abundance of a gas such as CO2 by, say, a factor of two is called the “climate sensitivity”: temperature is related to the logarithm of the absorbing gas abundance.  Doubling the CO2 abundance from the 19th-century level of less than 300 parts per million (ppm) to about 600 ppm would have the same warming effect as doubling the CO2 pressure again, to 1200 ppm.  So what is this “climate sensitivity”?  Climate modelers have used numbers ranging from about 1.5 to 6.5 oC per doubling of CO2.  Current wisdom favors a number near 1.5 or 1.6, right at the very bottom of the range used for generating dire climate predictions, for the short-term effects of solar heating.

Prof. Berntsen in a previous post suggested that the rapid warming of the 1978-1998 time period was due to a random combination of natural factors, carbon dioxide warming, and soot warming.  If we wrongly attribute all the observed warming to CO2, we are led inevitably to a gross overestimate of its warming power, predicting unreasonably high “climate sensitivity” and leading computer models to exaggerate the future warming trend.  If Prof. Berntsen’s estimate holds up, the climate sensitivity of CO2 after taking out the effect of soot is only about 60% of 1.5-1.6 degrees: call it 1 ÂșC.

How would we describe the temperature graph in the previous post without making reference to theories and explanations?  We could break the graph up into five “eras”: 1850 to 1927, with no significant net temperature change and a temperature anomaly of -0.3 oC; 1927 to 1940, with a warming of about 0.3 oC; 1940 to 1978 with gentle cooling of 0.15 oC; 1978 to 1998 with a strong warming of 0.65 oC, and 1998 to the present, with no significant change.  The “noise” in the data is striking: there are many independent effects of similar size at work, which sometimes work in synchrony.  Of course, if we included data extending back to the “Little Ice Age” of the 1600s, all of the data on this graph would be termed “very warm”.  And if we were to reach back to the “Roman Warm Period” 2000 years ago we would find temperatures closely similar to those of today.  Going back 9000 years to the early Holocene (the present interglacial period) we would have found an Earth that was warmer than today without any human influence or record-high CO2 content, powered solely by natural variability.

It is sobering to realize that most of the “noise” in the temperature graph is not random measurement errors, but real climate effects that are not adequately treated in (and were not predicted by) present models.  But remember that, no matter how complex our modeling of the atmosphere, some important factors such as volcanic emission of dust and sulfur gases and the effects of the variability of solar activity and solar wind strength will still defy prediction. 

Climate modeling is one of the most difficult computational problems known.  Like any science, the body of available data expands rapidly, and computer models must constantly be updated to include those data.   Many effects, such as the role of clouds or of soot, or the variability of the Sun, are recognized as important factors even while we still lack the detailed quantitative understanding of them that we would need to incorporate them into computer models.  Critical thought is the essence of science: we learn from experience and constantly improve our theories in the light of new data.  Similarly, theory points out what data we need to improve our understanding, and may even suggest how to go about acquiring them.  Skepticism is not the enemy of science; it is the very heart of the scientific endeavor.

We need an end to name-calling and personal attacks and threats.  We need to remove the discussion of global warming from the realm of politics and economically involved interest groups of both extremes.  We need to accept that anthropogenic global warming is not a “settled science”, but a vigorous and ambitious area of research in which new knowledge is of critical importance.  Remember that Newtonian physics was once a “settled science”: and then along came Einstein.  For about a century, celestial mechanics was also viewed as “settled science”: then along came spaceflight and modern mathematics.  Climate science is neither “settled” nor “fraudulent”: we must stop repeating and amplifying the most strident rhetoric, very little of which emanates either from scientists or from those in the media with real understanding of the issues.  We need less activism and more understanding.

There is one final very simple point to make: the phenomena of nature are incredibly complex.  Simplistic slogans such as “big industry is destroying our planet” and “climate science is a left-wing plot” are not only ignorant; they endanger our future.  Let’s bury that simplistic rhetoric and strengthen the science of complexity.

Monday, February 18, 2013

Chelyabinsk Update


      1.      Yield:  Current evidence suggests an explosive yield of a little less  than 1 megaton of TNT, comparable to an ICBM warhead.  We should be very grateful that it did not detonate closer to the ground, or we would be looking at tens of thousands of civilian deaths.

2.      Optimum burst height:  The nuclear weapons literature, including the classic 1977 analysis entitled The Effects of Nuclear Weapons, shows that the effective range of destruction from an aerial explosion depends sensitively on the altitude of the explosion.  An explosion at sufficiently high altitude strikes a very large area with a weak shock wave, rattling windows but doing negligible damage.  In the daytime, or in cloudy weather, there may be no sightings of a fireball.  A little lower, and the same explosion would break windows.  Glass shards accelerated by the blast wave are the principal hazard.  This is the Chelyabinsk event.  Move the explosion a little closer to the ground, and radiant heating of the surface becomes important.  Fires can be ignited by the flash, especially clothing, window curtains, and automobile upholstery.  In rural areas, trees and brush ignite.  This is the Tunguska event of 1908, which flattened hundreds of square kilometers of forest and burned 2200 square kilometers.  A little closer to the ground, and blast overpressures become high enough to cause structural failure of reasonably well-constructed buildings.  The “premature” failure of the factory building in Chelyabinsk probably owes more to its Soviet-era construction quality than to the severity of the blast.  At about the same explosion altitude, the air blast that follows the flash (traveling at the speed of sound rather than the speed of light) hits hard enough to blow out many of the fires, but potentially fanning others into a firestorm.  In this sequence from high altitude to very low altitudes, each successive blast strikes with greater intensity (higher blast overpressure) over a smaller target area.  A body that reaches the surface either intact or as a compact swarm of high-velocity fragments can excavate a crater, depositing almost all of its kinetic energy in an area about 100 times the actual area of the crater by means of high-speed explosive ejection of debris from the crater.  This is Meteor Crater in Arizona.  Very large impacts eject vast quantities of dust and vapor and shock-produced nitrogen oxides in the form of a mushroom cloud, which lifts them to high altitudes and spreads them widely over the Earth.  The very biggest impacts seen in the geological record actually blast away the atmosphere above a plane tangent to Earth’s surface at the point of impact, hurling crater eject worldwide.  This is the Chicxulub event at the end of the Cretaceous Era, the famed dinosaur-killer.  For a given explosive yield there is an altitude, called the “optimum burst height”, at which the area of devastation is maximized.  For a 1-megaton explosion the optimum burst height is about 1700 meters (a mile) and widespread structural damage occurs for any blast below about 5000 m (3 mi).  For a 10-megaton explosion the optimum burst height is near 5000 m and the threshold for structural damage is near 12000 m (7 miles).  At yields of 1000 megatons (1 gigaton), a 10,000-year event, severe surface damage occurs at just about any plausible burst height. 

3.      Entry Angle and Velocity:  It is aerodynamic pressure that causes an entering body to crush and shear itself into fragments.  The aerodynamic pressure is proportional to the density of the atmosphere and to the square of the velocity.  The density of the atmosphere drops off roughly exponentially with altitude, and is therefore very low at 100 km altitude.  As a general rule, bodies that enter at lower speeds penetrate deeper than those that enter at higher (cometary) speeds.  They contain less kinetic energy per ton, but are more efficient at delivering that energy to the ground.  Bodies that enter the atmosphere at shallow grazing angles (nearly horizontal motion) spend a relative long time at high altitudes where the atmosphere is thin and crushing is least probable.  They tend to decelerate rather gently and therefore are traveling slower at any altitude; therefore they penetrate deeper before exploding than a vertically-entering body of the same size and speed.  Note that, for any given material, the higher the velocity, the higher the altitude of explosion: the faster the bullet, the less its penetration.  There is also a huge range of strengths for asteroidal and cometary material: cometary “fluff” fails at high altitudes; iron meteorites (M-class asteroids) often penetrate all the way to the ground before exploding, and hence deliver their full original kinetic energy to a crater (or small cluster of craters) with high efficiency.  This is the Sikhote-Alin meteorite fall in eastern Siberia in 1947.

4.      Linear Explosion:  The energy dissipated by a strong, deeply penetrating bolide is often released nearly in the form of a point explosion, with almost all the original kinetic energy being given off in the same moment.  But many smaller bodies deposit their energy along a lengthy path through the atmosphere as they break up in many stages.  This is especially true of bodies with shallow entry angles.  Since the impactor may be traveling at 20 km per second, its speed is about Mach 30.  We think of the shock wave from a supersonic aircraft traveling at Mach 2 or 3 as a cone with an opening angle of, say, 30 degrees originated at the nose of the aircraft.  But at Mach 30 the opening angle is only about 2 degrees: the energy released is very nearly in the form of a linear explosion.  Some theorists talk of the “exploding wire” model, which is not a bad way to picture it.  Imagine a “wire” stretching across the sky that detonates nearly instantaneously.  The first sound to reach you is not from the point where the explosion began but from the segment of the wire nearest to you.  That sound reaches you as a strong, sharp blast, a “sonic boom”, after which the sound reaches you from ever more distant locations on the wire.  Thus after the first sharp boom you hear simultaneously the noises emitted both before and after the body passed closest to you.  These explosions and “rumbling” continue until, at last, you hear the first sounds given off during entry.  The first sounds, having traveled so much farther, reach you last.

5.      Crater:  There have been reports on the internet, some illustrated by photos of a burning crater, that purport to show the impact point of the Chelyabinsk bolide.  The photos are simply a hoax, showing file pictures of a natural gas fire that has been burning for decades in an oil field in Kazakhstan.   If there is an impact crater, it is a hole found in the ice of a lake.  That suggests a low fire hazard.

6.      Meteorites:  Meteorite recovery from the bolide would be enormously valuable, and this morning’s news claims over 50 stones recovered to date.  My guess is that there is a potential for recovery of hundreds or even thousands of stones, and that they will prove to be ordinary chondrites (the most abundant types of meteorites, of H, L, and LL classes).  Much weaker (carbonaceous) material would explode at high altitudes; strong (iron or stony-iron) meteorites could penetrate to the ground intact and make a huge crater.  Let’s keep our eyes on this: as the many images of the event are carefully studied we should soon know the precise path of the bolide and hence know where to look for any other meteorites it may have dropped. 

7.      Russian Defense Ministry Spokesman: A high-ranking Russian military officer has been quoted as saying that “this was no meteor; it was an American military test.”  If you can see any military advantage to breaking windows in Chelyabinsk, you’re more imaginative than I am.  Also, Russian scientific sources are quoting entry speeds of 18-20 kilometers per second, which is far above entry velocity for return from the Moon (about 11 km/s) and insanely larger than the top speed of any military weapons system ever devised.  The energy content of the explosion suggests a mass of 10,000 tons, 100 times the lifting ability of a Saturn 5 or the Space Shuttle (neither of which is in service), and about equal to the displacement of a guided missile cruiser such as the Ticonderoga.  This officer would profit from conversing with the Russian scientists who investigated the Tunguska event, and who actually do know something about these events.  Besides, if we take his explanation seriously, we would have to credit those aggressive Americans with having had even higher technology in 1908.

 

Friday, February 15, 2013

The Chelyabinsk Event, 15 February 2013


Early today a huge aerial explosion rocked the Siberian city of Chelyabinsk, collapsing or damaging buildings and shattering windows throughout the city.  Slivers of window glass accelerated by the blast wave from the explosion sent at least 500 people to hospitals for treatment, with many more injured less severely.  The media are trumpeting a “meteor” explosion and speculating about a link to this afternoon’s flyby of the Near-Earth Asteroid 2012 DA14.  I am being barraged with requests for information, even though the amount of solid quantitative date now available is minimal.  Nonetheless, there are several points that can confidently be made.

1.       This was not a meteor.  A meteor is an optical phenomenon, a flash of light seen in the sky when a piece of cosmic debris (usually dust- or sand grain-sized) enters Earth’s upper atmosphere, converts its huge kinetic energy into heat, and “burns up” (vaporizes), usually at an altitude of at least 100 km.  The Chelyabinsk object was a fragment of asteroidal or cometary origin, probably several meters in diameter, properly called a “meteoroid” or, more loosely, a “small asteroid”.  A brilliant fireball seen in the atmosphere is called a bolide.  Some bolides, caused by entry of large pieces of hard rock, drop meteorites on the ground: a meteorite is a rock of cosmic origin that reaches the ground in macroscopic pieces (not dust or vapor).  Some bolides are cometary fluff, of which nothing is strong enough to survive as a meteorite.  This body was fairly strong, and is therefore more likely to be an asteroid-derived meteoroid.  Indeed, some Russian sources are claiming that a meteorite from the blast fell in a lake in nearby Chebarkul, Russia, but this has not been verified.  Such judgments are tricky because the distance to the fireball is usually wildly underestimated (“it cleared my barn, so it must have been at least 50 feet up”).

2.      The path of 2012 DA14 is well understood.  It is in a generally Earth-like orbit, except that its orbit is inclined relative to the plane of Earth’s orbit around the Sun.  To first approximation, it is neither “catching up with Earth” or “being swept up from behind by Earth”: Its motion relative to Earth it basically at right angles to the direction of our orbital motion.  It will pass us from south to north.  Think of two cars on the freeway traveling in the same direction at the same speed, one of them in lane 2 and the other switching from lane 1 to lane 3.  Chelyabinsk is basically “behind the Earth” as seen by the approaching asteroid.  In other words, the Chelyabinsk object is not associated with 2012 DA14.

3.      There is also speculation about 2012 DA14 being accompanied by debris and even small satellites.  This is well founded, but these fragments, produced by collisions of small rocks with the asteroid, must follow paths that are closely similar to that of the parent asteroid.  If they exist, and if they hit Earth, they will do so near or to the south of the Equator.  Incidentally, the orbits of satellites of NEAs are usually close in, simply because distant satellites will be stripped away by the tidal forces of the Sun (and now, during a close flyby, by Earth also), and their orbital speeds are tiny (centimeters to meters per second).

4.      There was an early report of Russia scrambling jet fighters to intercept the object.  Here’s how that works: suppose the bolide is traveling at the absolute minimum entry speed of about 10 km/second and radar picks it up at a range of 1000 km.  This radar detection tells them the speed of the bolide.  From detection to arrival they have 100 seconds, tops.  Then they have the interesting task of intercepting something moving 10 (or 20) km/s with an airplane that has a top speed of, say, Mach 2.5.  That’s about 0.75 km/s.  See the problem?  The real military significance of impact airbursts is not that it is impossible to intercept them with jet aircraft: it is the danger of a completely unpredicted high-yield aerial explosion occurring over a major city in a heavily armed, politically unstable region: think, Tel Aviv, Tehran, etc.  Instant World War III.

5.      There’s a lot of talk and speculation about how rare such events are.  Any meaningful statistics would require that we know how big it really was (the bigger the rarer).  But a reasonable first guess is that this is a decadal object: ten per century hitting Earth, of which typically nine are in sparsely populated or unpopulated areas, such as the Tunguska Event of 30 June 1908 and the two Brazilian events around 1930.  We’ll know more about the size and blast energy soon.  So my take is that these events are not rare, but having one over a city is unusual.

               In the 1997 edition of my book Rain of Iron and Ice I included a lengthy table of reports from public media and scientific journals documenting injuries, deaths, property damage, and near-misses due to cosmic impact events, ranging from a meteorite knocking off a girl’s hat to a powerful airburst showering a city in China with tens of thousands of stones and killing over 10,000 people [Ch’ing-yang, Shansi, 1490 AD; source: Kevin Yao, Paul Weissman, and Don Yeomans, Meteoritics 29, 864-971 (1994)].   My Monte Carlo models of the long-term effect of impact events in my 2000 book Comet and Asteroid Impact Hazards on a Populated Earth provide quantitative estimates of the events occurring in hundreds of 100-year computer models.  In it, Model H89 generates a low-altitude airburst of 83 megatons yield at an altitude of 19 km.  A random location generator placed this blast over the city of Orleans, France, killing 40,000 people and igniting a firestorm.  After this model was published, Pete Worden, who was then Commandant of Falcon AFB in Colorado Springs, sent me an account that he had found in Bishop Gregory of Tours’ History of the Franks:  “580 AD  In Louraine, one morning before the dawning of the day, a great light was seen crossing the heavens, falling toward the east.  A sound like a tree crashing down was heard over all the countryside, but it could surely not have been any tree, since it was heard more than fifty miles away… The city of Bordeaux was badly shaken by an earthquake… The city of Orleans also burned with so great a fire that even the rich lost almost everything.”