Starship Asterisk*

wallybru@comcast.net wrote:I am not an astronomer, but has there been a study done based on craters on the moon?
A survey of the craters with their age and size might add to the information on the likelyhood of an impact to earth.

karlhein wrote:There is continuing discussion about Earth impacts from asteroids and coments and the regular observation of extinction events. Has anyone plotted frequency distributions of the size of impact craters on the moon and on Mars (where erosion is a minimal issue), to make estimates of the size (and or velocity) of impacting objects? This might give an improved estimate of the frequency with which impact events occur on earth. Additionally, it might be of interest to know if the pattern of cratering is different on the side of the moon facing the earth vs the side facing away.

karlhein wrote:Somehow apodman copied my post, maybe he intended to reply. If anyone has information about my original post, I'd be interested in hearing from you.

karlhein wrote:Sorry ...

apodman wrote:

karlhein wrote:Sorry ...

No problem. I just thought cosmic forces might be at work planting the same idea in more than one mind at once.

dougie wrote:when was the moon last hit by a large meteorite

http://www.physlink.com/news/022503StuartEvent.cfm wrote: NASA Solves Half-Century Old Moon Mystery
Posted on: Tuesday February 25, 2003.
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On the left, Dr. Leon Stuart's image of the moon
On the right, Dr. Stuart; image courtesy: Jerry Stuart
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This fresh-looking lunar crater (arrowed) appears significantly brighter, and thus inherently bluer,
in a mosaic of blue-light and near-infrared images from the Clementine spacecraft.
Courtesy Bonnie J. Buratti (Jet Propulsion Laboratory).
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<<In the early morning hours of Nov. 15, 1953, an amateur astronomer in Oklahoma photographed what he believed to be a massive, white-hot fireball of vaporized rock rising from the center of the Moon's face. If his theory was right, Dr. Leon Stuart would be the first and only human in history to witness and document the impact of an asteroid-sized body impacting the Moon's scarred exterior.

Almost a half-century, numerous space probes and six manned lunar landings later, what had become known in astronomy circles as 'Stuart's Event' was still an unproven, controversial theory. Skeptics dismissed Stuart's data as inconclusive and claimed the flash was a result of a meteorite entering Earth's atmosphere. That is, until Dr. Bonnie J. Buratti, a scientist at NASA's Jet Propulsion Laboratory, Pasadena, Calif., and Lane Johnson of Pomona College, Claremont, Calif., took a fresh look at the 50-year-old lunar mystery.

'Stuart's remarkable photograph of the collision gave us an excellent starting point in our search,' said Buratti. 'We were able to estimate the energy produced by the collision. But we calculated that any crater resulting from the collision would have been too small to be seen by even the best Earth-based telescopes, so we looked elsewhere for proof.'

Buratti and Johnson's reconnaissance of the 35-kilometer (21.75-mile) wide region where the impact likely occurred led them to observations made by spacecraft orbiting the Moon. First, they dusted off photographs taken from the Lunar Orbiter spacecraft back in 1967, but none of the craters appeared a likely candidate. Then they consulted the more detailed imagery taken from the Clementine spacecraft in 1994.

'Using Stuart's photograph of the lunar flash, we estimated the object that hit the Moon was approximately 20 meters (65.6 feet) across, and the resulting crater would be in the range of one to two kilometers (.62 to 1.24 miles) across. We were looking for fresh craters with a non-eroded appearance,' Buratti said.

Part of what makes a Moon crater look 'fresh' is the appearance of a bluish tinge to the surface. This bluish tinge indicates lunar soil that is relatively untouched by a process called 'space weathering,' which reddens the soil. Another indicator of a fresh crater is that it reflects distinctly more light than the surrounding area.

Buratti and Johnson's search of images from the Clementine mission revealed a 1.5-kilometer (0.93 mile) wide crater. It had a bright blue, fresh-appearing layer of material surrounding the impact site, and it was located in the middle of Stuart's photograph of the 1953 flash. The crater's size is consistent with the energy produced by the observed flash; it has the right color and reflectance, and it is the right shape.

Having the vital statistics of Stuart's crater, Buratti and Johnson calculated the energy released at impact was about .5 megatons (35 times more powerful than the Hiroshima atomic bomb). They estimate such events occur on the lunar surface once every half-century.

'To me this is the celestial equivalent of observing a once-in-a-century hurricane,' said Buratti. 'We're taught the Moon is geologically dead, but this proves that it is not. Here we can actually see weather on the Moon,' she said.

While Dr. Stuart passed on in 1969, his son Jerry Stuart offered some thoughts about Buratti and Lane's findings. 'Astronomy is all about investigation and discovery. It was my father's passion, and I know he would be quite pleased,' he said.

Buratti and Johnson's study appears in the latest issue of the space journal, Icarus.>>

http://en.wikipedia.org/wiki/Moonquake wrote:
<<A moonquake is the lunar equivalent of an earthquake, i.e., a quake on the Moon. Moonquakes are much less common and weaker than earthquakes. Information about moonquakes comes from seismometers placed on the Moon by Apollo astronauts from 1969 through 1972. The instruments placed by the Apollo 12, 14, 15 and 16 missions functioned perfectly until they were switched off in 1977.

According to NASA, there are at least four different kinds of moonquakes:

* Deep Moonquakes (approximately 700 km below the surface, due to tidal stresses from Earth and Sun)
* Meteorite impact vibrations
* Thermal moonquakes (the frigid lunar crust expands when sunlight returns after the two week lunar night)
* Shallow moonquakes (20 or 30 kilometers below the surface, may be caused by large rims on young craters slumping)

The first three kinds of moonquakes mentioned above tend to be mild; however, shallow moonquakes can register up to 5.5 on the Richter scale. Between 1972 and 1977, twenty-eight shallow moonquakes were observed. On Earth, quakes of magnitude 4.5 and above can cause damage to buildings and other rigid structures. Vibration from shallow moonquakes usually continues for more than ten minutes compared to around one or two minutes on Earth. The moonquakes are much longer than earthquakes because there is no water on the Moon. Water weakens rocks through chemical weathering causing different minerals to expand. This makes the ground act like a sponge and so the vibrations are soon absorbed making the earthquake end sooner. The Moon does not absorb the vibrations as quickly as the Earth as it is more rigid and so keeps vibrating for longer, extending the moonquakes far beyond the biggest earthquakes.>>

http://www.skyandtelescope.com/news/3307351.html?page=1&c=y wrote: Lunar Flash Doesn't Pan Out
July 23, 2003 by J. Kelly Beatty

<<For the past few weeks, impact aficionados have been abuzz over the apparent confirmation that a house-size object struck the Moon on November 15, 1953. The bright flare captured that evening by Leon H. Stuart's backyard telescope matches the position of a small, fresh-looking crater recorded by a spacecraft four decades later. Bonnie J. Buratti (Jet Propulsion Laboratory) and Lane Johnson, a student at Pomona College, unearthed this 1½-kilometer-wide "smoking gun" in data from the Clementine orbiter, whose high-definition cameras mapped the entire Moon in 1994.

Although the annals of amateur astronomy chronicle hundreds of such transient lunar phenomena, almost all considered suspect by professionals, Stuart's event stands apart because it was both seen and photographed. That fact, together with the Clementine evidence, allowed Buratti and Johnson to make a convincing case in January's issue of the scientific journal Icarus. A press release even trumpeted "NASA Solves Half-Century Old Moon Mystery."

But a little more historical digging would have shown that there was no mystery to begin with. John E. Westfall (Association of Lunar and Planetary Observers) has discovered that the bright blip seen by Clementine also appears in a series of telescopic plates taken decades before Stuart snapped his controversial exposure. In particular, Westfall notes, the feature is "pretty obvious" in photographs made with Mount Wilson's 100-inch Hooker telescope in 1919. It also turned up on plates taken in 1937 with the 36-inch refractor at Lick Observatory and in others acquired with Catalina Observatory's 61-inch reflector in 1966.

"It's kind of disappointing," Buratti said when told of Westfall's revelation. "But it's more important to find that out." In researching their paper, she and Johnson had examined a few telescopic images for a small crater at the impact's presumed coordinates but found nothing. A search of Lunar Orbiter frames, taken during the 1960s, also turned up empty. They concluded that the candidate crater must be too small (roughly 0.8 arcsecond across) to be resolved by ground-based efforts.

Even before Westfall came forward, doubts had been growing about the Stuart-Clementine connection. For one thing, Stuart reported that the bright flare lasted at least 8 seconds, an implausibly long fireball for so small a crater. "We now know that an event of that scale should last no longer than a second, but [Stuart] didn't," comments impact specialist Alan W. Harris (Space Science Institute).

Other concerns were raised about the freshness of Buratti and Johnson's candidate crater. Solar-wind bombardment causes lunar material to darken and redden over time, but researchers believe such "space weathering" takes place slowly over millions of years. Thus, if 20-meter-wide objects slam into the Moon frequently (often enough to make Stuart's sighting statistically plausible), then the lunar landscape should be peppered with 100,000 bright, fresh-looking splashes — and it isn't. "You can't have it both ways," notes Harris.

Finally, the positional match wasn't as good as Buratti and Johnson first thought. Careful measurement of Stuart's image by Sky & Telescope editors Dennis di Cicco and Gary Seronik, as well as by Westfall, shows that the flare is centered a full 1°, or 30 km, from the Clementine candidate.

So, if it wasn't a flashy impact, what did Leon Stuart see and photograph a half century ago? Some have suggested that it was a "point meteor," headed directly at the camera, but that is ruled out by the flare's duration. Moreover, the spot on the photographic plate is perfectly round, arguing against a stray reflection or emulsion defect. In 1967 Stuart's original underwent a battery of tests at the University of Arizona's Lunar and Planetary Laboratory. But today the whereabouts of the original plate are unknown, and without it the true mystery of "Stuart's event" may never be solved.>>