For some years now I have been asking about and discussing the question of how far lunar dust might have traveled during the Apollo landings and ascents. Since some of the factors are difficult to assess, (ejection angle, speed range of impinged particles, etc.,) the only opinions I have received have been speculative. I wondered if any of the particles might have actually been launched into orbit.
As luck would have it, I found what is probably the best available answer while reading the NASA recommendations about preservation of landing sites on the moon.
http://www.nasa.gov/sites/default/files/617743main_NASA-USG_LUNAR_HISTORIC_SITES_RevA-508.pdfFor anyone who may have wondered about this or who has discussed it with me in the past, here is what I found, beginning on page 11.
NASA analysis using gas flow codes has indicated that rocket exhaust plumes from the landing
stages can induce high injection velocities of the top layer of the lunar surface; this analysis is
further supported by the mathematical analysis performed prior to the Apollo program before
such codes were developed. The plume modeling also predicts that the impingement from the
descent engine(s) on the loose lunar material creates a nearly flat sheet of blowing material, a
broad cone of particulate ejecta that rises at an angle between 1 and 3 degrees elevation
above the local terrain on all sides around the landing spacecraft. This predicted ejection
angle of 1 to 3 degrees is confirmed by photogrammetric techniques applied to the blowing
dust clouds seen in the descent videos of the Apollo landings.
Analysis further indicates that these particles can achieve ejection velocities between 300 and
2000 meters per second (m/s) with the smaller particles generally traveling faster. Because
there is negligible ambient lunar atmosphere outside the plume, the particles continue at that
velocity until striking the lunar surface far away. Some particles travel almost all the way
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around the moon before impact. The smallest, dust-sized particles achieve near-lunar escape
velocity, 2.37 km/s, and even exceed it by a significant margin, sending them into solar orbit,
according to some plume simulations. These conclusions are corroborated by the observations
of the Apollo crews. Several crew members reported that the blowing material was a flat
sheet close to the surface, so that rocks could be seen through the sheet and/or protruding
through the top of it. During Apollo 11, Buzz Aldrin reported that while this material was
blowing, the lunar horizon became “obscured by a tan haze,” which indicates that the ejected
particles were moving fast enough to travel over and beyond the horizon. For the dust-sized
particles, the highest velocity achieved is equal to the plume gas velocity, which depends on its
combustion temperature and thus chemical composition and, to a lesser degree, the vehicle’s
thrust. Thus, a smaller landing vehicle (with comparable propellants) can eject dust-sized
particles at comparable velocities, although in lower quantities (mass) per second.
Careful review of the landing videos, and comparison to plume modeling, shows that gravel
and rocks 1 cm to 10 cm in diameter were also ejected by the plume at speeds between 5 and
50 m/s. Ballistic calculation indicates that these rocks impacted the lunar surface up to 1.5 km
from the LM. It is the inertia of these larger particles (in contrast to the low inertia of dustsized
particles) that prevents them from achieving velocities comparable to the plume gas
before they run out of the plume into lunar vacuum. Thus, for a smaller lander with less thrust
(lower plume gas density), the rocks and gravel will achieve an even smaller fraction of the
plume gas velocity and will travel a shorter distance from the landing site. Vehicles larger
than the LM could eject rocks a greater distance.
Experiments have shown that lunar soil is highly abrasive and effective as a sandblasting
medium. The Apollo 12 LM landed 155 m from the Surveyor 3 spacecraft and retrieved
material samples from the spacecraft for later analysis. Even though Surveyor was in a crater
and below the horizontal plane by 4.3 m and thus “under” the main sheet of material blown
from the LM, the Surveyor spacecraft received significant sandblasting and pitting from the
Apollo landing. This suggests that collisions between the ejected particles within the main dust
sheet scattered them out of that sheet into a much broader but lower-density spray than
described above, and it was the scattered particles that impinged on the Surveyor.
Comparison with the optical density of the blowing soil indicates that if the Surveyor had been
directly impinged by the main sheet, it would have sustained several orders of magnitude
greater surface damage, including dust implantation, scouring, pitting, cracking, and
microscopic crushing of the surface materials. Thus, the Surveyor’s damage under-represents
the degree of damage that could have occurred from an LM-sized vehicle’s plume at that
distance. Also, the damage to the Surveyor would have been greater if any of the ejected
gravel pieces or rocks had struck it (the odds of such an occurrence have not yet been
quantified).
Other cases of plume impingement effects have been documented in addition to the Surveyor
damage. These include two cases (Apollo 15 and 16) where the launch of the Apollo LM
Ascent Stage (AS) blew blankets that had been left on the surface, and the blankets almost
impacted and damaged the deployed scientific instruments. O’Brien reported that when the
AS lifted off in both Apollo 11 and Apollo 12, the solar cells in the Dust Detector Experiments
(DDE) had an immediate change in their received sunlight – sometimes less and sometimes
more – attributable to dust being delivered to or knocked off the cells, respectively. In Apollo
11, the DDE was 17 m away from the LM, and in Apollo 12 it was 130 m away. At the latter
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distance, the plume gas would be too rarefied to have much effect, so the observed removal of
dust from the DDE was due to the impingement of high-velocity ejecta.
At every distance from a spacecraft landing on the Moon, there will be ejected particles that
impact at that distance. At large distances the impingement flux becomes small and
eventually negligible. However, requiring large distances to protect the Apollo sites could
make it impractical for missions to visit them. A landing distance that is specified as a means
to protect the sites while still enabling access must be a compromise that reduces the
impingement effects without entirely eliminating them. The lunar horizon is roughly 1.8 km
from any given point on the lunar surface. By targeting the landing point at 2.0 km from the
closest lunar artifact, the main sheet of high-velocity dust-sized particles – which constitutes
the largest fraction of the lunar soil – will fly over the top of the artifact site and thus minimize
direct impingement. Larger rock or gravel-sized ejecta, which will travel at lower velocities,
will impact the lunar surface well short of the horizon, thus also missing the artifact site. The
intermediate range of particle sizes (larger, sand-sized particles), which will travel over the
horizon but with sufficient downward curvature to strike the artifact site, are a minority
fraction of the lunar soil and will have a much lower flux density at impingement from that
distance than if the spacecraft were landing closer, thus reducing damage.
