3200 Phaethon-- An Asteroid or not?

 The Near-Earth Asteroid 3200 Phaethon has -- in addition to being the most frequently misspelled asteroid -- a number of odd traits that call attention to it. 

First of all, it belongs to a rare and distinctive spectral type; it's a B type asteroid.  That suggests a tribal association with the big Belt asteroid 2 Pallas, which in turn implies cold and wet.

Second, Phaethon's orbit is more like that of a comet than that of a typical asteroid: on each trip around the Sun, it dives in to a perihelion passage a mere 0.140 AU from the Sun, far inside Mercury's orbit, and then coasts out to an aphelion distance of 4.025 AU, beyond the main asteroid belt, crossing the orbits of Mercury, Venus, Earth and Mars twice each on every orbit around the Sun.  Each circuit, with eight crossings of planetary orbits, takes 523.5 days

The intensity of sunlight on Phaethon's surface ranges from 50 times the intensity of sunlight at Earth's orbital distance when Phaethon is at perihelion to 6% of normal sunlight at aphelion.  The surface of Phaethon is quite dark, with an albedo (reflectivity) of only 0.1066: more than 89% of the incident sunlight is absorbed.  At perihelion, the daytime temperature can reach a peak of over 1000 Kelvin.  Another oddity of Phaethon is that its color is unusually bluish; we can be quite sure that it is not blue ice!

Phaethon's seemingly hazardous existence is greatly prolonged by the fact that its orbit is significantly tilted out of the plane of the ecliptic, with an inclination of 22.25 degrees relative to the ecliptic.  Generally, while passing across the orbits of the terrestrial planets, Phaethon is safely out of the plane in which they orbit.  The closest it can get to Earth in its present orbit, a hair less than 0.02 AU: 0.02 x 150 million kilometers, gives us 3 million kilometers of clearance when it is at its closest; about 8 times as far away as the Moon.

The orbital inclination of Phaethon spares us from the hazard of a collision with Earth (and the other terrestrial planets) over long time periods- but not permanently.  Phaethon's diameter is about 5.8 km; for comparison, Meteor Crater in Arizona, about 1.2 km in diameter, is the product of a (relatively) low-speed impact of a metallic asteroid with a diameter about 100 times smaller than Phaethon (and thus a volume about 1 million times smaller; allowing for its high density, a mass of about one 3-millionth of the mass of Phaethon).  The impact velocity of Phaethon, because of its extremely eccentric orbit, could easily be twice as high as that of the Meteor Crater impactor, delivering about 4 times as much energy per unit mass. Thus an impact of Phaethon with Earth would deliver roughly one million times as much energy as the Meteor Crater impactor.  Picture an impact crater 120 kilometers in diameter that produced a thick debris blanket over 300 kilometers in diameter.  The dust raised by the impact would throw Earth into an Ice Age for many thousands of years.

In the near future, Phaethon will not and cannot strike Earth.  In the long run, however, there is about a 50% probability that it will hit Earth; 40% chance of hitting Venus, and a few percent each for Mars and Mercury.

Getting a sample of the material of Phaethon would surely be interesting and informative, and nature does provide Earth with samples by natural means: dust expelled from its surface forms the Geminid meteor shower, which strikes Earth in the middle of December.  Unfortunately, the entry velocity is quite high, and acquiring samples of the dust is extremely challenging.  Spacecraft missions to retrieve surface samples from Phaethon are rendered impractical (but not completely impossible) by the body’s high orbital velocity.

So what is Phaethon? Where did it come from, and how did it get in its present orbit?

One clear possibility is that it was a short-period comet that has been stored in its present orbit long enough for solar heating to dispel its near-surface volatiles.  I have seen ludicrous claims that the intense heating during perihelion passages had heated the entire asteroid to the point of baking all the volatiles out of it.  The brief heating events during perihelion passage barely scratch the surface, and can have no effect on the composition of the deep interior.  Phaethon spends virtually its entire existence soaking in the frigid environment of the outer fringes of the Asteroid Belt.

Baby, it's cold out there.

Another Asteroid Flyby of Earth

The interloper this time is 2002AJ₁₂₉, a body with a highly eccentric, comet-like orbit.  Its perihelion is a near-suicidal 0.116397 AU, far inside Mercury’s orbit, and its orbital eccentricity is 0.915097, typical of short-period “periodic” comets.  That means that the orbital period is short enough so that it actually returns to the inner Solar System, where we humans presently reside, on human time scales.  Its orbital period is only 1.61 years, so we have had several opportunities already to see it since its discovery (in 2002, as its name tells us).

On each trip around the Sun it coasts out to aphelion at 2.625502 AU, comfortably far away from any planet.  The orbit is inclined 15. 47941 degrees relative to the plane of the ecliptic.

This asteroid, because of its size and Earth-crossing orbit, qualifies it as a PHA, a Potentially Hazardous Asteroid.  Its albedo has not been measured, so converting the observed brightness into a size is uncertain.  This uncertainty leads cautious astronomers to estimate a diameter of 600 meters to 1.2 kilometers. The aphelion distance suggests a possible origin in the heart of the asteroid belt, in a region in which dark C-type carbonaceous asteroids outnumber the brighter S-type stony asteroids.  An educated guess, that C-type is moderately more probable than S-type, would place the albedo (reflectivity for visible light) down around 0.04, which favors a size well above 1 kilometer.

By the way, it’s not going to hit us in the foreseeable future.  In the long term, however, all bets are off: since AJ₁₂₉ crosses the orbits of Mercury, Venus, Earth and Mars on every trip around the Sun there are lots of opportunities for tweaking its orbit.  Over millions of years the odds are about even that it will end its career by hitting Earth. 

The threat is real, but not imminent enough for me to change my investment strategy. 

Neither Fish nor Fowl; Neither Comet nor Asteroid?

The Hubble Space Telescope has presented us with telling images of a peculiar body that seems to share the attributes of an asteroid and a comet.  It has been christened with the double name 300163 2006 VW139/288P, alias P/2006 VW139, acknowledging its asteroid-like and comet-like traits and declining to make a decision between the two categories. 

The first part of the name attests that the body was discovered as an asteroid in October 2006, but the recognition of its strange appearance and its ambiguous identity had to await the Hubble observations a year ago.  The second part of its name identifies it as a body with cometary appearance (a tail) and a “periodic” orbit, meaning that it has a known orbital period.  And what should we call it?  The exceptional clunkiness of the name suggests nicknames such as “2006 VW”, which unfortunately conjures up a misleading (non-duplicitous) image.

A “video” of the body, constructed out of a number of snapshots taken as it rotates, can be found at http://hubblesite.org/videos/news/release/2017-32.  Form your own opinions (and suggest your own nicknames).

So what do we see when we look at that video?  Two bright, roughly equal-sized apparently solid bodies, just a few body diameters apart, trailing a bright “tail” of generally cometary appearance.  The tail appears to be a dust stream rather than a gaseous tail, tempting us to speculate about the chain of events that produced this strange display. 

The “just-so” story has to take into account the duplicity, proximity, and activity of the body.  There are several possible scenarios to consider:

1a and b. Collisional Fission.  An asteroid in the Belt was recently clobbered by a glancing impact sufficiently violent to get it rotating fast enough to fission into two bodies in very close orbit about each other, perhaps even a contact binary.  This happened so recently that the dust sprayed out by the impact has not had time to dissipate.  Sounds improbable, but there are examples galore to whet our imagination.  The Trojan asteroid 624 Hektor was studied in 1980 by a former student of mine, Stu Weidenschilling, and interpreted as a contact binary; a Wikipedia article on “Contact Binary: Asteroid” lists 13 strong candidates for the same distinction.  At least two comets (67P/Churyumov-Gerasimenko and 8P/Tuttle) also appear to have the same structure.  Such structures can arise either by (1a) the glancing collision of two large independently-orbiting bodies or by (1b) rotational fission of a single asteroid that was spun up to the point of splitting apart by a relatively small but sufficiently violent impact event.  Since the encounter velocity of two asteroids in independent orbits around the Sun is typically several kilometers per second, the overwhelming majority of asteroid/asteroid collisions must result in explosive disruption rather than capture into closed orbits.  Low-velocity encounters must be rare; but contact (or near-contact) binaries ARE rare.  The dust-generating activity can then be attributed to the collision event, which must have been fairly recent; otherwise, the dust would have been swept up by the bodies or blown away by solar radiation pressure.  Oh, we also know from independent evidence that 2006 VWs may survive low-velocity collisions. 

2.  It is just a comet whose gas emission is undetectably small, but sufficient to maintain a dust cloud, according to H. H. Hsieh and 41 other coauthors, Discovery of main-belt comet P/2006 VW139 by Pan-STARRS1, Astrophysical Journal Letters, 748 (1). (L15) 1-7 (2012).  They interpret observations by the Pan-STARRS1 survey telescope as showing a striking similarity to the main-belt cometary object 133P/Elst-Pizarro, which has a rather similar orbit.  Both may be outliers of the Themis family of main-belt asteroids.  They argue that the stable maintenance of the dust cloud is indicative of cometary outgassing, most likely of water vapor, but they are also unable to detect any gaseous molecular emissions spectroscopically.

If I had to bet on the strength of present evidence, I’d have to prefer the latter explanation.  The implication of a very high water content in some Belt asteroids is worth remembering—especially if you are a water-based spacefarer.

See also: 

https://www.space.com/38214-spitting-asteroids-hubble-telescope-discovery.html?utm_source=sdc-newsletter&utm_medium=email&utm_campaign=20170922-sdc

More Weird News from Russia

A CNN news report this morning (19 February 2016) tells of Russian plans to modify existing ICBMs to carry warheads to intercept and blow up incoming asteroids.  This can be found at:

http://www.cnn.com/2016/02/19/politics/russia-icbm-asteroid-killer/index.html

The story is disturbing for a host of reasons. 

First, ICBMs can carry nuclear (thermonuclear) warheads over intercontinental range, for which purpose they can achieve a terminal velocity of 8 kilometers per second. They are designed so that achieving altitudes much higher than about 1000 km is not possible.  The simplest way of using an ICBM would be to intercept the incoming asteroid at an altitude of 1000 km, a move of such extreme stupidity that even the Russian Ministry of Defense would hesitate to do it.  A multi-megaton explosion in space so close to Earth would not only kill a large fraction of all the satellites operating in Earth orbit, but its EMP would knock out surface electrical power grids over a continent-sized area.  As a further bonus, fragments of the incoming asteroid would shower a wide area on the ground, likely inflicting several times as much damage as the intact asteroid would have done.  The unanimous conclusion of international Planetary Defense studies (with the participation and concurrence of leading Russian scientific experts) is that blowing up a threatening asteroid is a high-risk, damage-multiplying endeavor that should be avoided at all cost.

Second, redesigning a strategic missile for asteroid interception at a safe distance from Earth would require replacing the payload with an additional upper stage and a much smaller warhead.  The largest operational Russian ICBM, the SS-18-6, carries a 20 megaton thermonuclear warhead weighing about 9 tonnes; replacing that warhead with a new upper stage and a smaller (5 megaton?) warhead with a mass of about 2 tonnes would permit interception out to lunar distances.  But that raises another question:

Third is the question of how we deal with different kinds of targets.  There is no doubt that interception and destruction of a 10-meter diameter asteroid at the distance of the Moon would be safe: the problem is that asteroids of this size are extremely difficult to find.  Virtually all of the asteroids of this small size (>99.99% of them) remain undiscovered.  They can be found only if they approach Earth very closely.  In other words, an incoming 10-m asteroid on a collision course with Earth would almost certainly be unknown to us.  Discovery of a new asteroid of this size, even if it occurs by incredible good fortune while the asteroid is still at the distance of the Moon, would occur about one day before impact.  The asteroid would have to be discovered and tracked, and the mission would have to be planned and launched, within hours of discovery.  The asteroid would typically be traveling at 20 km/s and the interceptor rocket at 2 km/s, so interception would occur 1/10^th of the distance to the Moon, an altitude of about 40,000 km, which happens to be altitude of the Geosynchronous Orbit belt of communication satellites.  A 5 megaton explosion at that altitude would destroy most of the world’s communications assets.  An asteroid that, if it by incredibly bad luck should have hit a city, might have caused thousands of casualties, is destroyed at the cost of world-wide communication capabilities.  If we chose to leave it alone (or, more likely, never saw it coming) it is overwhelmingly more probable that it would have fallen in a remote and unpopulated area, probably over the ocean, and inflicted little or no damage.  The cure would probably be more lethal than the disease.

What about kilometer-sized asteroids, which constitute a serious threat to areas the size of a country?  Asteroids of this size and brightness are much easier to discover and track: of all the Earth-crossing asteroids larger than about 1 km in diameter, we have discovered and tracked more than 95%.  Best estimates are that there about 980 such asteroids: of the estimated few dozen that have not yet been discovered, we are finding several new ones each year.  We know with surety that none of the ones discovered to date threatens impact with Earth in the next few centuries.  But suppose we were to discover a new one this year in an orbit that threatens Earth.  It is highly probable that we would have hundreds to thousands of years to prepare for that threat.  But the impact could be avoided by minuscule changes in the orbit of the asteroid.  As an example, suppose we find a km-sized body that would impact Earth in 300 years.  If we could change the orbit enough to miss Earth, we would buy ourselves thousands of years of additional time to deal with it.  Changing the asteroid’s orbit enough to displace its position by 10,000 km and guarantee that it would miss Earth 300 years from now requires changing the velocity of the asteroid by a minuscule 0.1 cm per second.  This can easily be effected by setting off a large nuclear explosion several km from the asteroid: the vaporized surface rock would exert a mild but entirely adequate vapor “puff” that would very slightly deflect the asteroid and change its speed without running the risk of turning the asteroid into a deadly shower of a thousand 100-meter sized chunks of shrapnel.

In short, this proposed “defense” scheme is sufficiently crazy that we would be well advised to look for other explanations of why Russia would want to suggest it.

Oh, by the way, the United States no longer has any operational ICBMs with multi-megaton “city buster” warheads.  These relics of the cold war survive only in Russia and China—and are effective threats only against population centers, not military targets.  This means the US doesn’t even have the option of doing something equally stupid with asteroids.

Extraordinary Near-Earth Asteroids I: 2014 PP69

April 2015

 We presently know of about 13,000 Near-Earth Asteroids, including nearly 1000 that are larger than 1 kilometer in diameter.  Typical NEAs range from Earth’s general vicinity out to the heart of the Asteroid Belt on each orbit around the Sun.  Their orbits typically have inclinations (relative to the plane of the Solar System) of 10 to 30 degrees, eccentricities of 0.2 to 0.6, and orbital periods of about 2 to 4 years.  The mean distance of any NEA from the Sun is usually near 2 AU.  But the NEAs are a wildly diverse collection of bodies that originated at widely separated locations in the Solar System.  The outliers of this population include some truly remarkable nonconformists.  One such asteroid is 2014 PP69. 

You will recall that the first five characters in an asteroid’s name tell us when it was discovered, in this case in 2014 in the second half of July.  This provisional name will be used until there is a long enough history of accurate tracking (usually at least one full synodic period, the time needed to “lap” Earth in its orbit around the Sun), to certify a precise, accurately predictable, orbit.  The synodic period is about 2 years for most NEAs.  At that time the asteroid will be given a catalog number such as 155629, at which point it will be referred to as 155629 2014 PP69.  Once an asteroid has been cataloged the discoverer may propose a name for it, such as Eros or Ceres; let’s call this one Egbert.  Then it will be called 155629 Egbert; just plain Egbert to its friends.  But the object of this post is just plain 2014 PP69: in the nine months since its discovery there has been no opportunity for it to pass by Earth again, and therefore no chance to assign it a very precise orbit and enter it into the catalog of numbered asteroids.  Once the refined orbit is determined, the discoverer of the asteroid gets to give it a name.

So here’s what’s unusual about 2014 PP69: its perihelion distance of 1.25 AU, which qualifies it as an Amor asteroid, contrasts sharply with its aphelion distance of 41.79 AU, well outside the orbits of Neptune and Pluto and well into the Kuiper Belt.  Its orbital period is an incredible 99.84 years, longer than that of Halley’s Comet.  But that’s not all: the inclination of its orbit is 93.63 degrees, meaning that it orbits almost at right angles to the plane of the Solar System—in fact, the orbit is slightly retrograde, moving around the Sun in a direction opposite to that followed by the planets.  The eccentricity of its orbit is 0.942, higher than that of the typical short-period comet.  At perihelion, closer to Mars’ orbit than to Earth’s, it is traveling at a whopping 40 kilometers per second.

What do we know about the asteroid itself?  Almost nothing.  The discovery images show that it has a visual (H) magnitude of 20.17, which, by the crude “rule of thumb” used for newly discovered NEAs (an assumed average albedo of 0.14; 14% reflectivity in visible light) corresponds to a diameter of about 330 meters.  However, the orbit is cometary, suggesting that a more realistic albedo would about 0.035.  If it’s that bright and that black, then its cross-section area must be four times as large, and its diameter twice as large, as this crude guess would suggest.  That implies eight times the volume and about eight times the mass, raising the question of its impact hazard.  The good news is that, despite its large size and kinetic energy, the point at which it crosses the plane of Earth’s orbit is far outside our neighborhood. 

The body is almost certainly of cometary composition, similar to the Centaurs and the Kuiper Belt bodies and to short-period comets.  A reasonable guess would be that it is about 60% by mass ices and about 40% rock, which in turn contains perhaps 5-10% of organic matter, mostly complex polymers.

Sending a spacecraft to visit 2014 PP69 would be extremely difficult because of its very high relative velocity.  And then there is the problem that the next optimal launch opportunity is a century off.

How soon will 2014 PP69 qualify for a catalog number?  On its next pass through the inner Solar System we will have an opportunity to track it again with such a long span of observations (a century!) that a very accurate orbit can be calculated.  That will be in the year 2114.  The bad news is that the discoverer will no longer be alive to exercise the option of naming his baby!

Dawn at Ceres

March 2015

The Dawn spacecraft, having completed its lengthy survey of the asteroid 4 Vesta, and having survived the interplanetary cruise from Vesta to Ceres, is now safely in orbit around the largest asteroid in the Belt, 1 Ceres.  One of the first results from Dawn’s survey of Ceres is the discovery of small, intensely bright spots on its surface.

Vesta and Ceres, though nearly at the same distance from the Sun, are not twins; in fact, they are very different creatures.  Vesta is unique among the large (>100 km diameter) asteroids in the Belt: it is a thoroughly reworked body, having undergone extensive melting and differentiation into layers with different composition and density, with a surface dominated by rocks closely similar to terrestrial basalts.  Ceres, in contrast, is a modestly altered body that is genetically related to the very dark, volatile-rich C-type asteroids that dominate the outer half of the Belt.

Back in 1977 Larry Lebofsky studied the infrared reflection spectrum of Ceres and found an absorption feature near a wavelength of 3 micrometers (µm).  This is a region in which water, in all its many chemical forms, is a strong absorber.  Articles in the press tend to assume that any mention of “water” means liquid water, which equates to a well-watered Eden for life.  But the 3-micron feature is simply due to excitation of the stretching mode of the O-H chemical bond: water vapor, liquid water, solid water-ice polymorphs, clay minerals, micas, and hydrated salts such as gypsum all have broad absorption bands in this same spectral region.  Liquid water, if present today, could not occur stably close to the surface (too cold; hard vacuum), but might persist at modest depths if some solute is present to lower the freezing point and depress the vapor pressure.  A plausible candidate for that role is ammonium chloride, NH₄Cl, which I regard as a far more plausible solute than the often-quoted ammonia.  Many years ago (in Low-Temperature Condensation from the Solar Nebula, Icarus 16, 241 (1972) I pointed out that chemical synthesis of ammonia is strongly favored by high pressures, suppressing ammonia synthesis in the Solar Nebula.  My colleagues Ron Prinn and Bruce Fegley showed that the higher pressure in dense protoplanetary nebulae favored ammonia formation there: ammonia should be an important constituent of ices in planetary satellite systems, but not in asteroids, because they are formed in the low-pressure regime of the Solar Nebula. 

Indeed, the freshly fallen Orgueil CI chondrite, which contains all the minerals mentioned above, as well as veins of soluble salts deposited from solution in water, was reported to give off a strong odor of “smelling salts”, ammonium chloride. 

And what about the bright white spots on Ceres?  Liquid water released from the interior of Ceres would boil well below the surface, producing a jet of rapidly cooling vapor.  Water vapor vented from a warm interior into the frigid vacuum of Ceres’ surface would expand irreversibly to produce a jet of snow, which would fall to the ground near the vent.  This would happen whether the source of the water vapor is a shallow layer containing liquid water or deep, hot rocks containing –OH minerals.  Linking water venting on Ceres to the local origin of life is sufficiently far-fetched to deserve skepticism.

Hayabusa 2: On its Way to an Asteroid

March 2015

 

Japan’s second asteroid sampling mission is under way.  Hayabusa 2 was launched from the Tanegashima Space Center in January on its 6-year trip to the Near Earth Asteroid (NEA) named 1999 JU3.  The mission is a follow-up to the ambitious but trouble-plagued Hayabusa 1 flight of 2003-2010, which aspired to grab a substantial sample of the NEA Itokawa, but suffered several failures, including malfunction of its sampling equipment.  Hayabusa 1 nonetheless returned successfully to Earth bearing traces of asteroid dust on its surface.

Hayabusa 2 carries, in addition to a sample-acquisition system, four small probes, one of which is patterned after the Philae probe that recently landed on Comet 67P/Churyumov-Gerasimenko.  These probes are capable of “hopping” about on the asteroid surface; indeed, the main spacecraft is intended to land and collect samples in three different places.

The present target asteroid, despite its uninteresting name, has particular attraction for people interested in the discovery and use of the native resources of space: it is a very dark rock, similar in its reflectivity and spectrum to the carbonaceous chondrite meteorites.  These meteorites contain up to 20% water by weight, plus about 6% of tarry organic polymers and interesting amounts of many other compounds of the volatile elements hydrogen, carbon, oxygen, sulfur, nitrogen, chlorine, and so on.  The dominant minerals are water-bearing clays, magnetite, and a variety of metal sulfides loosely cemented by the organic gunk that coats the mineral grains.  The CI chondrite meteorites also contain veins of various water-soluble and water-bearing minerals, mostly sulfates and carbonates, that run through the otherwise very black groundmass.  Moderate heating releases water vapor; strong heating drives off a rich variety of gases and causes the organic matter to react with the oxygen-rich mineral magnetite to “burn” the organic matter and release copious amounts of carbon oxides and water.  All told, strong heating of CI material drives off ~40% of the total mass of the meteorite as gases of H, C, O, N, S, and Cl compounds. 

1999 JU3 is about 920 meters (0.6 miles) in diameter, with a total mass of about 1 billion tonnes, which upon heating would release some 400 million tonnes of volatiles.  By way of comparison, a fully-fueled Saturn V rocket or Space Shuttle contains about 2000 tonnes of rocket propellant: 1999 JU3 contains enough hydrogen, carbon, and oxygen to fuel 200,000 such flights.

Deep Space Industries (deepspaceindustries.com) is presently studying processes for turning the volatiles extracted from carbonaceous asteroids into fuels and oxidizer for future space missions, and into air and water for life-support and agricultural uses by future spacefarers.  The byproducts from extraction of volatiles, including metals, are also of great economic interest. Plans to manufacture these products await the successful return of samples to Earth.

Return of Hayabusa 2 from the asteroid will commence in late 2019, with entry into Earth’s atmosphere and recovery of the return capsule scheduled for December 2020 in the interior of Australia.

Hayabusa 2 is a difficult and challenging mission.  The Japanese Space Agency JAXA is to be congratulated for learning valuable lessons from their earlier asteroid mission and designing an ambitious and well-conceived successor.  I wish them the very best of luck.

“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!

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.

 

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.”