Posted on 08/20/2006 3:13:34 PM PDT by SunkenCiv
History Of The Asteroid/Comet Impact Hazard[T]he first asteroid found to be in an orbit that crosses the Earth's orbit around the Sun ("Earth-crossing asteroid") was not discovered until 1932. Soon thereafter, a Harvard astronomer, Fletcher Watson (1941), wrote cogent sentences about the dangerous prospects of such an asteroid (a few miles wide) crashing into the Earth. With hindsight, the technical literature reveals that some other important scientists also forecast both the enormity as well as the rarity of a potential impact. Such eminent pre-Space-Age researchers included Ralph Baldwin (1949) and Ernst Opik (1957). From at least the 1950's through the 1970's there were also (we now recognize) prescient remarks (again by Opik, by USGS planetary geologist Harold Masursky [1967], and by one of the founders of modern planetary science, Nobel laureate Harold Urey [1973]) about the possibility that mass extinctions of species in the geological record might be explained by impacts of large asteroids or comets... Nothing was fundamentally different from the original insights of Fletcher, Baldwin, and Opik nor from the more thoroughly developed later analysis by Chapman & Morrison (1994), which was based on the preliminary thinking at Snowmass: there is a small, but non-zero, risk of a civilization-threatening impact happening -- a catastrophe that would be much smaller than the K/T boundary impact 65 million years ago but far larger than the 1908 Tunguska impact (which could have killed millions had it unluckily struck in the center of a city, but in fact killed only a couple of people at most).
by Clark R. ChapmanClosing in on Near Earth ObjectsSETI Institute astronomer Peter Jenniskens is hot on the trail of an elusive comet whose last visit was in 1976, and whose lingering debris may help scientists warn us about the imminent return of a mysterious class of Near Earth Objects (NEOs). We believe that prediction models tested on the Leonid showers can also be used to predict when these dust trails are steered in the Earth’s path by the gravitational influence of planets, and we are about to travel to South Africa to observe a new meteor shower thus predicted. When Comet C/1976 D1 Bradfield passed uncomfortably close to Earth’s orbit on its sweep through the inner solar system, it was a faint +8 magnitude binocular object in the Southern hemisphere. It’s passing was poorly communicated by observers who lacked today’s connectivity. The best determination astronomers can make of the comet orbit places a return visit about 1,000 years into the future. Before we all heave a sigh of relief, thousands of such comets remain undetected. A similarly sized comet in such a fast moving orbit in another solar system may long ago have wiped out a civilization before it could be detected in our SETI searches.
by Peter Jenniskens
Principal Investigator, SETI Institute
27 February 2003
Summer of Asteroids
Discover Magazine, 30 July 1998
Asteroids and comets are a hit with the public, judging from the success of the summer movies Deep Impact and Armageddon. The rogue bodies are a hit with scientists as well, who have released a flurry of related research announcements (some timed, no doubt, to catch the wave of media interest. David Tholen and Robert Whiteley of the University of Hawaii recently announced the discovery of a new kind of asteroid whose orbit lies entirely within the orbit of the Earth. Astronomers previously knew only of asteroids circling the sun entirely outside the Earth's orbit or the more threatening ones whose paths cross that of the Earth. Asteroids within the Earth's orbit are difficult to observe because they appear close to the sun in the sky. Many of these objects could be out there, as-yet unseen. If they approach or slightly cross the Earth at their most distant point from the sun, these sun-hugging asteroids could possibly strike the Earth.
On July 14, NASA established a new "Near-Earth Object Program Office" in order to get a better sense of the total population of Earth-threatening asteroids. The office will be headed by Donald Yeomans of the Jet Propulsion Laboratory, who recently helped debunk reports that an asteroid discovered in March might strike the Earth. NASA's goal is to locate, by 2010, at least 90 percent of the sizable (more than 1 kilometer wide) asteroids or comets that approach the Earth. Several current and upcoming missions will provide close-up information about the solar system's rogue members. The NEAR spacecraft is on its way to the asteroid Eros. Deep Space 1, an experimental high-technology mission to an as-yet unspecified asteroid, launches in October of this year. And Stardust, set for launch in 1999, will fly along Comet Wild-2, collect samples, and return them to the Earth for analysis.
--Corey S. Powell
Posted 7/30/98
"When we learned that our team had been assigned the topic of "Planetary Defense," we admittedly did what most people do when they first consider the subject: we laughed. This phenomenon is commonly referred to as the giggle factor, and we have seen it many times during the ensuing months of our research and briefings.
Gene Shoemaker said that, once an object gets to a mile in diameter, the energy released by its impact on the Earth would exceed the energy released if one took all the nuclear weapons in the world, put 'em in a pile, and set them off simultaneously. Of course, that's give or take a dozen miles per second of terminal velocity. ;')
You're just asking for it with that info. I don't think I ever posted this link before or not, but...
How to destroy the Earth
http://qntm.org/destroy
As far as I can see, the number of asteroids catalogued over 100 km radius is probably complete, although there could be one or two lurking. The number between 100 and 10 kilometers is being increased at a known and declining rate per year, which is a hint how many remain. The number catalogued between 10 and one kilometer is likewise increasing each year and the rate would be declining so as to hint how many would be left. Putting new instruments on the project would change the rate and the rate of change of the rate, but the limit would still be calculable. The estimate of 50% is probably in the ballpark of plus or minus 10%, or something like one sigma.
There's been a lot of study of the Teton fireball. I think (purely from memory) estimates of the size range from 30-100 meters. It did what the worst-case scenarios of the ballistic re-entrys for manned missions talked about -- it skipped out of the atmosphere.
Interesting. I heard that it was about the size of a bus.
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