[meteorite-list] Scouring the moon

Sun Feb 17 01:22:58 EST 2008

While people talk about the first footprints on the moon being preserved for
long times, I've assumed that they were probably erased by the firing of the
engines on the ascent stage-- Glenn's one giant leap was erased as he left.
This article talks about just how much damage the blasts (from the descent)
could be.

http://www.msnbc.msn.com/id/23130776/

How lunar landers sandblasted the moon 
Seemingly gentle touchdowns were actually incredibly violent events
By David Powell
Space.com
updated 2:30 p.m. ET, Tues., Feb. 12, 2008
New research has revealed the seemingly gentle touchdowns of the six Apollo
Lunar Modules (LMs) on the moon between 1969 and 1972 were actually incredibly
violent events.

The Lunar Module's descent engine blew out high-velocity lunar particles that
strafed the landscape.

"The smallest particles were seen by the Apollo astronauts to fly right out over
the horizon and keep on going," said Philip Metzger of NASA's Kennedy Space
Center (KSC). "Depending on the actual velocity they may have gone halfway
around the moon or more. In most cases they would only travel until they hit a
natural terrain feature, such as a crater rim or a mountain range."

These minute specks of lunar dust are estimated to have been propelled at speeds
of between 0.6 and 1.5 miles per second (up to 5,400 mph). That's nearly fast
enough to escape the moon's gravity and enter orbit around the sun.

Better knowledge of the phenomenon will inform plans to put people and habitats
on the moon.

Force in a vacuum
Pinning down the mechanics of these Apollo induced dust storms was achieved by
Forrest Lumpkin at the Johnson Space Center together with Jacobs Engineering in
Houston. Lumpkin and his team achieved the difficult task of simulating how the
Lunar Module exhaust gases behaved in a vacuum.

A study of original Apollo landing footage undertaken by John Lane and Christ
Immer, working for the Arctic Slope Research Corporation at the Kennedy Space
Center, was also an essential factor in understanding just how a descending
Lunar Module (LM) interacts with the lunar surface.

"New methods were developed to measure the shadows of the LM during its descent
in the old Apollo landing videos," Metzger said. "From measuring these shadows,
they have determined the shape of the blowing dust clouds under the LM. This is
one of our main sources of experimental data on the ejection of soil by rocket
exhausts." 

The Apollo footage also shows that the blast from the LM engine gases were also
powerful enough to move rocks up to 6 inches in size.

Surveying the damage
While the rocket exhaust may roll large rocks a short distance, it is the
smaller particles that have the most widespread effect on the landing area.

This is clearly demonstrated by the damage inflicted on the Surveyor 3 unmanned
craft, which rested only 600 feet from the descending Apollo 12 LM in November
1969.

Study of the Surveyor camera and mechanical scoop returned to Earth by the
Apollo 12 astronauts revealed it had been sandblasted by lunar dust.

"There were what looked liked permanent shadows cast into the Surveyor," Metzger
said. "Cosmic radiation in the lunar environment had darkened the surface of the
Surveyor and then the spray of fine dust from the Apollo 12 LM removed that
darkening wherever the spray could reach." 

This same scouring process could easily damage the sensitive surfaces of future
lunar hardware and fine lunar dust could also be forced into the interiors of
surface equipment causing vital parts to jam.

"When the support collar was removed from the Surveyor camera, a small sample of
soil and dust particles were found inside, where they had been injected through
a small inspection hole that happened to be facing in the direction of the LM,"
Metzger said.

Such fine dust, around 10 microns in size, makes up the bulk of what was kicked
up by the LM, but larger particles around 60 microns wide (the size of a very
fine sand grain) also made their mark on Surveyor 3 in the form of hundreds of
micro-craters.

"The paint on the Surveyor camera shroud was fractured in a mud-cracking
pattern," Metzger told SPACE.com. "Each intersection of cracks was at the
location where a tiny particle had impacted, drilling a tiny cylindrical hole
down into the paint and causing the fractures to spread out from there like
spider-legs in a car windshield."

The particles that caused this damage are estimated to have been traveling at
around 1,300 feet per second. The figure which ties in well with the expected
velocity of particles blasted across the lunar surface by the LM engine just
before touchdown. (Bullets exit a rifle at between 600 and 5,000 feet per
second.)

Protecting our interests
The sleet of particles shot at Surveyor 3 by just one nearby landing serves as a
warning for humankind's return to the moon, and if spacecraft are to land near a
future lunar outpost some precautions are going to have to be made. (Similar
efforts are underway to figure out the effects of Mars landers.)

"We are currently investigating several different techniques. One idea is to use
the natural terrain to block the spray between the landing site and the lunar
outpost," Metzger explains. 

"Another idea is to create an artificial terrain feature, a berm, around part of
the landing site to block the spray in the crucial directions. It may also be
possible to modify the landing surface to prevent the spray altogether."