An asteroid may hit Mars next January 30. And while the reported 1-in-75 odds might not sound convincing, in astronomical parlance, that's enough to wonder because the initial odds were listed at 1-in-350.
So what are we talking about, here?
The asteroid, known as 2007 WD5, was discovered in late November and is similar in size to an object that hit remote central Siberia in 1908, unleashing energy equivalent to a 15-megaton nuclear bomb and wiping out 60 million trees.
Scientists tracking the asteroid, currently halfway between Earth and Mars, initially put the odds of impact at 1 in 350 but increased the chances this week. Scientists expect the odds to diminish again early next month after getting new observations of the asteroid's orbit, Chesley said.
...Speeding at 8 miles a second, a collision would carve a hole the size
of the famed Meteor Crater in Arizona.
While this is very interesting and all, the truth of the matter is the Earth gets smacked, hard, by fast-flying rocks at least 1-10m in diameter between 35 and 40 times a year, based on telemetry from early warning satellites whose primary job is to detect rocket exhaust plumes and nuclear flashes on the lovely, peaceful, equitably-distributed and ecologically-balanced planet rolling underneath them.
The big rocks like our new friend, Asteroid 2007 WD5, here can ruin your day, but for someone under an airburst as destructive as an SS-14 or MX missile warhead or "merely" as bad as the Hiroshima bomb, it's not that important a distinction. A rock that weighs on the order of 50-500 tons going at 10-40 kilometers per second is going to hurt.
Still, we should give the upcoming possible fireworks show a look.
Introducing 2007 WD5
Here's the original orbital ephemeris from November 21, so in one month's time the odds have shrunk by a factor of almost 5. As for why there is concern the odds might drop further, here is why:
For the next two weeks we can see the darn thing.
It seems the MOON is in the way.
Better estimates will become available as we wait to see its fate on January 30, 2008. Data is unavailable until 2007 WD5 comes out from behind the Moon, which is expected in a couple of weeks.
...
Large telescopes on Earth could easily see the plume of dust that results if the asteroid impacts the Martian surface.
The asteroid is traveling at about 8 miles (13 kilometers) per second. At that time, it would leave a hole similar in size to Meteor Crater in Arizona, which is about 4,000 feet (1,200 meters) in diameter and about 570 feet (170 meters) deep.
M-O-O-N. That spells Moon!
Last Time Something Similar Happened was NOT Tunguska
The most famous modern-era big impact on Earth happened on June 30, 1908 in Siberia
The Tunguska event, sometimes called the Tunguska explosion, was a massive explosion that occurred near the Podkamennaya (Under Rock) Tunguska River in what is now Krasnoyarsk Krai of Russia, at 7:40 a.m. on June 30, 1908.
The explosion was most likely caused by the air burst of a large meteoroid or comet fragment at an altitude of 5 to 10 kilometers (3–6 mi) above Earth's surface. Different studies yielded varying estimates for the meteor's size, including 50 meters, 60 meters, 90 to 190 meters, and up to 1200 meters in diameter. Although the meteor or comet is considered to have burst prior to hitting the surface, this event is still referred to as an impact event. The energy of the blast was estimated to be between 10 and 20 megatons of TNT — 1,000 times more powerful than the bomb dropped on Hiroshima, Japan. The explosion felled an estimated 80 million trees over 2,150 square kilometers (830 sq mi). It is estimated to have measured 5.0 on the Richter scale.
The Tunguska event is the largest impact event in recent history. An explosion of this magnitude had the potential to devastate large metropolitan areas had it occurred over a large city. This possibility has helped to spark discussion of asteroid deflection strategies.
However, more recent big rocks have come our way
Not as big, perhaps but let's take a look as some of them.
A Near Earth Object (or NEO) airbursted over the Eastern Mediterranean on June 6, 2002. At the same time, far to the east, India and Pakistan were on a high state of alert, the last time the two nuclear powers (and conspicuous non-friends) were at the brink.
The Eastern Mediterranean Event was a high-energy aerial explosion over the Mediterranean Sea, around 34ºN 21ºE (between Libya and Crete, Greece) on June 6th, 2002[1]. This explosion, similar to a small atomic bomb, has been related to an asteroid undetected while approaching the Earth. The object disintegrated and no part was recovered. Since it didn't reach the surface and it exploded over the sea, no crater was formed.
Similar to the Tunguska event, the asteroid —about 30ft in diameter— exploded as a consequence of the energetic alteration of atmospheric entries[citation needed]. It was detected by satellites and seismographic stations, with a calculated yield of about 26 kt (doubling Hiroshima, approximately Nagasaki, a small modern nuclear bomb). Had it detonated on a populated area, the consequences would have been catastrophic.
Okay, compared to Tunguska that ain't much. "Only" as destructive as the only nuclear weapons ever used in anger. "Only" enough to destroy a couple of hundred thousand lives, tops. That is, if such a surprise suicidal visitor to the Earth's upper atmosphere does not kick-start a nuclear exchange:
This summer, as India and Pakistan faced-off over the disputed Kashmir region, US early warning satellites detected an explosion in the Earth's atmosphere with an energy of 12 kilotonnes of explosive.
The detonation, equivalent to the blast that destroyed Hiroshima, fortunately occurred over the Mediterranean Sea.
But according to US Brig Gen Simon Worden, if it had occurred at the same latitude a few hours earlier, the result could have been much worse.
Had the explosion occurred over India or Pakistan, the resulting panic could have sparked a nuclear war, Worden says.
General Worden's full comments are in a Speech given July 10, 2002 on what current near-space object surveillance can and cannot give us:
Perhaps the most urgent mid-term task has already been begun. This is the systematic observation and cataloguing of close to all potentially threatening NEOS. We are probably about halfway through cataloging "large" NEOS (greater than a kilometer in diameter). It's interesting to note that the most effective sensor has been the MIT Lincoln Lab LINEAR facility in New Mexico. This is a test bed for the next generation of military ground-based space surveillance sensors. But this ground-based system, however effective, can only really address the "large", highly unlikely threats. We find out every few weeks about "modest" asteroids a few hundred meters in diameter. These are often caught as they sail by the earth, often closer than the Moon, unnoticed until they have nearly passed. Most recently the object 2002MN had just this sort of near miss - this time only a few tens of thousands of kilometers from the earth! Moreover, ground-based systems such as LINEAR are unable to detect one of the potentially most damaging classes of objects, those such as comets that come at us from the direction of the sun. New space-surveillance systems capable of scanning the entire sky every few days are what's needed.
...
Regardless of how well we know NEO orbits and how well we can predict their impacts the fact remains that today we have insufficient information to contemplate mitigating an impact. We do not know the internal structure of these objects. Indeed, we have reason to believe that many, if not most are more in the nature of "rubble piles" than coherent objects. This structure suggests that any effort to "push" or divert a NEO might simply fragment it - and perhaps turn a single dangerous asteroid into hundreds of objects that could damage a much larger area.
The message being:
- As of 2002 we were only halfway through cataloguing the dino-killers
- Every few weeks we get surprised by a mid-sized civilization killer doing a drive-by
- We cannot even begin to see rocks and comets coming up at us from inside the Earth's orbit with ground-based detection (think: telescopes during the day)
- We are just speculating as to what asteroid structure and composition are like; just getting spectrographic readings is no substitute for comprehensive physical sampling. Short form: We don't know what we are shooting at or trying to shove into a safer orbit, not really, even if we had the means to do so.
- Attempting to relocate or pulverize a rock might turn a single bullet into a shotgun blast..
Okay, that was cheerful. Now for a couple of other often-overlooked impacts
In September 25, 2002 (a busy year), a smaller 0.5-5 kiloton yield event occured near Bodaybo, Russia.
Going a bit farther back in time, perhaps as recent as the 1890s, we find the Wabar Craters in Saudi Arabia, which according to this Scientific American pdf had a explosive force of 12 kilotons of TNT.
One of the most famous close call was the Daylight Fireball of August 10, 1972 which did not strike the Earth or blow up in its atmosphere but it was well-observed and documented.
The Great Daylight 1972 Fireball (or US19720810) is an Earth grazer meteoroid which passed within 57 km of the surface of the Earth at 20:29 UTC on August 10, 1972, or 1.01 Earth radii from the centre of the Earth. It entered the Earth's atmosphere in daylight over Utah, United States (1430 local time) and passed northwards leaving the atmosphere over Alberta, Canada. It was seen by many people, recorded on film and by space borne sensors.[1]
By comparison, the Otto C. Winzen developed balloon reached 51.8 kilometres in 1972 and Low Earth Orbit satellites orbit from about 200 kilometres from the surface of the Earth.
Analysis of its appearance and trajectory showed it was a meteoroid about 2 to 10 metres[2][3] in diameter in the Apollo asteroid class in an orbit that would make a subsequent close approach to Earth in August 1997.[1] In 1994 Zdenek Ceplecha re-analysed the data and suggested the passage would have reduced the meteoroid's mass to about a third or half of its original mass.[2]
Eye Candy Department
from Australia
the 1972 meteorite
NOT a fireball but an old Soyuz rocket burning up in the atmosphere over Denver, Colorado - Still very cool.
How often do we get smacked?
Technically, we get smacked -- a lot -- every single day. From page 27 of this this fine piece we learn that "At Earth the current annual infall of meteoric and meteoritic material hitting the top of the atmosphere is estimated, from satellite, radio and photograpic meteor measurements, at
16,000 tons, with 12,400 tons of that concentrated in the particles in the 10^-6 to 10^-2 gram mass range (Hughes, 1978)." However, longer-run measures suggest a far larger average figure. ("Kyte infers a long-term infall rate of 77,000 + 25,000 tons per year, about five times the current infall estimated by Hughes.")
My guess is this accommodates the occasional big rocks that show up.
And what matters to most is how often to we get struck (or at least our atmosphere gets struck) by a rock significant in mass to set off nuclear readiness alerts or lay waste to neighborhoods, small towns, major metropolitan regions, continents, global ecologies...that sort of thing. Again, we go to the Wiki well:
Many impact events occur without being observed by anyone on the ground. Between 1975 and 1992, American missile early warning satellites picked up 136 major explosions in the upper atmosphere. In the 21-Nov-2002 edition of the journal Nature, Peter Brown of the University of Western Ontario reported on his study of US early warning satellite records for the proceeding 8 years. He identified 300 flashes caused by 1m to 10m sized meteors in that time period and estimated the rate of Tunguska sized events as once in 400 years. Shoemaker estimated that one of such magnitude occurs about once every 300 years, though more recent analyses have suggested he exaggerated by an order of magnitude. Even at that, this is not a long interval, and it is a somewhat nerve-wracking question to consider when the next "Big One" will be, and more to the point, where.
This guess for Tunguska, though, is based on an assumed mass that might be overstated based on recent conjectures based on supercomputer research at Sandia Labs:
"The [Tunguska] asteroid that caused the extensive damage was much smaller than we had thought," says Sandia principal investigator Mark Boslough of the impact that occurred June 30, 1908. "That such a small object can do this kind of destruction suggests that smaller asteroids are something to consider. Their smaller size indicates such collisions are not as improbable as we had believed."
Because smaller asteroids approach Earth statistically more frequently than larger ones, he says, "We should be making more efforts at detecting the smaller ones than we have till now."
The new simulation — which more closely matches the widely known facts of destruction than earlier models — shows that the center of mass of an asteroid exploding above the ground is transported downward at speeds faster than sound. It takes the form of a high-temperature jet of expanding gas called a fireball.
This causes stronger blast waves and thermal radiation pulses at the surface than would be predicted by an explosion limited to the height at which the blast was initiated.
The original assumption was that Tunguska was a once in 600-1000 years type of event. If the destructive force represented there was 3-5 megatons rather than 10-20 we are talking a range of 2-7 times more likelihood, and such events in the once every 150-500 year range plausible, or roughly once every 250 years, per this article on the same topic. When I do some simple spreadsheet guesswork, I come up with a range between 160 and 270 years, averaging about once every 215 years.
The real threat
From the survey of early warning satellite data, we are getting estimates of better 0.5 kiloton-yield airbursts happening close to 30-40 times a year based on the 2002 study, ranging (usually) up to one 25 kiloton-yield airburst every year. This is still (usually) insufficient to punch all the way through the Earth's atmosphere. And perhaps (usually) any surviving fragments do not make it all the way down to the surface.
And most of the time, no one is inconvenienced anyway. It is just those occcasions where a large unexpected explosion happens near well-armed ignorant people who are looking for an excuse to hit something that you have to worry.
The good news is the world is run by calm, well-educated persons who know not to go off half-cocked.
Yep. We're perfectly safe, even if the sky does fall about 40 times a year. :)