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Leonids on the Moon


Leonids on the Moon
Marshall Space Flight Center
http://science.nasa.gov/newhome/headlines/ast03nov99_1.htm

Leonid meteorite impacts on the Moon might be visible from Earth and provide
a means for long-distance lunar prospecting.

Nov. 3, 1999: When the Leonid meteor shower strikes on the morning of
November 18, 1999, our planet won't be the only place in the cross hairs.
The Moon will also pass very close to the debris stream of comet
Tempel-Tuttle. Here on Earth, space-borne meteoroids will plummet into the
atmosphere and burn up, creating streaks of light called meteors. The vast
majority of meteoroids will burn and disintegrate well before they hit the
ground. The situation on the Moon, where there is no appreciable atmosphere,
is different. Every bit of comet debris that rains down on our satellite
will hit its surface. Some meteor enthusiasts hope that will create a
different sort of display. Rather than streaks of light in lunar skies,
there could be flashes of light on the Moon's surface each time a sizable
meteoroid hits the ground.

Last year, during the 1998 Leonid meteor shower, the phase of the moon was
new. It was so close to the sun in the sky that observing faint lunar
meteorite flashes was impossible. This year is different. During the 1999
Leonid shower the phase of the Moon will be just 2 days past first quarter.
That means the moon will visible in the night sky during the early evening
on November 17, and approximately 35% of the lunar disk as seen from Earth
will not be illuminated by sunlight. There will be plenty of dark lunar
terrain where flashes might be visible.

Is it possible to observe such flashes?

Maybe, say researchers. It depends a great deal on the mass spectrum of
particles in the Tempel-Tuttle debris stream and how efficiently kinetic
energy is converted into optical light as a result of the impacts. Both
factors are poorly known. Although flashes are unlikely to be seen with the
naked eye, they may be detectable through amateur telescopes.

"The impact of a one gram particle would generate of the order of 1023 to
1024 photons in the peak sensitivity range of the human eye," says Dr. Bo
Gustafson of the University of Florida Laboratory for Astrophysics. "Given
the distance to the Moon, we could expect a few times 106 photons per square
meter at the Earth. This should be barely detectable using a small
telescope."

In June 1999, Ciel & Espace reported that a Spanish team of astronomers led
by J.L. Ortiz had reached similar conclusions:

     Watching meteorites fall on the moon ... is within reach of
     (modest) amateur telescopes. Because the Moon doesn't have a
     substantial atmosphere, meteorite impacts there are much more
     violent than here on Earth liberating much more energy: 20 million
     joules for a 1-kg block. As seen from the Earth, this would
     produce a flash of magnitude 9 to 15. From Ciel & Espace, No. 349
     - Juin 1999, p. 17: Si, c'est possible! (Translation courtesy
     Bernd Pauli HD).

"The Leonid debris stream is in a retrograde orbit, and it's inclined just
22 degrees from the plane of Earth's orbit around the sun," says Professor
George Lebo of the University of Florida Department of Astronomy. "That's
why the Leonids enter the atmosphere with such a high velocity [72 km/s].
The Earth and the Leonids hit head-on, like a head-on collision between two
speeding automobiles."

"If you put yourself in the reference frame of the Earth it's pretty easy to
figure out where these meteoroids will hit the Moon, "continued Lebo. "On
November 18, at 0h UT the lunar sub-Leonid point [the spot where Leonid
meteoroids rain directly down on the Moon's surface] will be 9.4 degrees
north of the lunar equator and 9.5 degrees sun ward of the day-night
terminator. In other words, the greatest flux of Leonids are going to hit
nearly dead center on the lunar disk as seen from Earth, just over the
terminator on the sunlit side."

It won't be possible to see flashes on the Moon's sunlit surface, so
amateurs will have to look where the terrain is dark. The best approach will
be to train a telescope -- higher powers are best for discerning faint
flashes -- at a spot near the lunar equator on the night side of the
terminator, keeping the sunlit side of the moon completely out of the field
of view. Flashes observed with the naked eye would certainly be exciting,
but might have little scientific value. Instead, experienced observers
suggest using a low-light astronomical CCD video camera to make a permanent
record.

The Leonids radiant, in the constellation Leo, rises above the horizon at
mid-northern latitudes around midnight on November 17/18. That's about the
same time that the Moon sets. It's an ideal situation for observers who can
monitor the Moon for the first half of the night and then enjoy the Leonid
meteor shower from midnight until dawn.

Leonid Lunar Prospecting

Although optical flashes were not observed on the moon during last year's
meteor shower, a team of scientists from the Boston University Center for
Space Physics discovered indirect evidence for Leonid impacts.

The Moon has an extremely tenuous atmosphere that contains, among other
things, sodium atoms. Just above the Moon's surface the density of sodium is
50 atoms per cubic centimeter. For comparison, the sodium density in Earth's
lower atmosphere is 1019/cc! Although the Moon's atmosphere is incredibly
thin, researchers at Boston University's space physics lab have built
sensitive cameras that can trace its sodium component out to several lunar
radii.

In mid-November 1998 the Boston University group were using their sodium
camera to monitor Earth's atmosphere for changes due to Leonid meteors. To
their surprise they detected a bright sodium spot on November 17 that grew
in brightness, peaked on November 19, and then faded away. The spot was
almost 180 degrees away from the new Moon in the night sky. Nevertheless,
the source of the sodium was apparently Earth's satellite. When Leonid
meteoroids crashed into the Moon's dusty soil they kicked up an extra
helping of sodium atoms, increasing the density of the Moon's thin
atmosphere. A long lunar sodium tail formed (much like the tail of a comet)
which swept by our planet two days later.

The Boston University experiment showed for the first time that intense
meteor showers might be one way of "lunar prospecting" from a distance -- by
looking at materials blasted off the surface as meteoroids strike. A team of
scientists from the University of Texas and NASA tried something similar
earlier this year when they crashed NASA's Lunar Prospector spacecraft into
the Moon. The probe was sent hurtling into a south polar crater on July 31
in hopes that the impact would vaporize shadowed water-ice and send a cloud
of water vapor and OH flying over the lunar limb. Telescopes, including the
Hubble Space Telescope, looked near the impact site after the crash, but
failed to detect evidence for water. That doesn't mean there's no water on
the moon, say scientists. Lunar Prospector may simply have hit a dry spot,
or perhaps the water vapor didn't rise high enough to see.

Dr. David Goldstein, a professor at the University of Texas who proposed the
Lunar Prospector impact experiment, is wondering if the Leonids might
succeed where the Lunar Prospector crash failed. Data from Lunar
Prospector's neutron spectrometer indicate that water-ice on the moon is
concentrated around the Moon's poles where shadowed areas would allow
pockets of water to remain frozen (see the figure below). The 1999 Leonids
won't reach the Moon's south pole, but many meteoroids should strike the
north pole.

"The Leonids will be coming in from above the ecliptic plane," says
Goldstein. "Given the Earth-moon geometry on November 18th that means that
the lunar north pole will be exposed, but not the south pole. That's
unfortunate because there's thought to be more water around the south pole
where we crashed Lunar Prospector. There's no chance of a Leonid meteoroid
hitting the crater where Prospector crashed. Near the north pole the
meteoroids will be coming in at several degrees above the horizon -- very
similar to the Lunar Prospector trajectory."

"Compared to Lunar Prospector, Leonid meteoroids are light weight and tiny,
but they move a lot faster," Goldstein continued. "The mass of Lunar
Prospector was 160 kg and it was moving 1.7 km/s when it hit the moon on
July 31. Leonid particles are going about 72 km/s. That means that a Leonid
the mass of a golf ball (about 0.1 kg) would deliver the same kinetic energy
as the Lunar Prospector crash."

"If a Leonid meteoroid did hit a spot near the north pole with frozen water,
it's not clear what we would see. The Lunar Prospector collision was like a
car crash -- it was moving at relatively slow speed. When it hit, we hoped
it would kick up water vapor that would be dissociated into OH by
ultraviolet sunlight. In theory we would then see the OH by looking above
the sunlit lunar limb with appropriate spectrometers. A Leonid crash would
be much more violent. Instead of water vapor gently wafting above the lunar
limb, we might see ionized, hot plasma. It's possible that we would also get
some warm water vapor that didn't sustain such a damaging shock wave, but
it's really hard to say. We haven't done the high speed simulations yet."

Goldstein says that he and his colleagues may not have time to organize a
search for signs of water kicked up by Leonids this year, following so
closely on the heels of the Lunar Prospector experiment. However, with some
experts predicting significant Leonid activity into the next millennium,
there will be time to arrange an observing campaign for next year and
beyond.

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