UPDATE:
22 November 1999 - The first recorded impact of a meteorite on the Moon may have been captured
on video during the 1999 Leonids meteor storm. Astronomers call for
confirming data. Click
here for details. |
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.
 Left: These all-sky images show
an unusual patch of brightness in the sky. It is the Moon’s
sodium tail after the 1998 Leonid meteor shower. The sodium emission
was about one-hundredth the brightness needed to be visible to
the unaided eye. Image Credit: Steven M. Smith/Boston University.
[more information]
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.
 Left: During its 18-month mission
orbiting the Moon, Lunar Prospector used a method called neutron
spectroscopy to look for lunar water. Neutrons are subatomic
particles that are continually ejected from the lunar soil by
cosmic rays. In this graphic, the coincident dips in medium-energy
neutrons at both lunar poles (see arrows) are a possible signature
of water. Scientists hoped that by crashing the spacecraft into
a shadowed crater they would be able to find direct evidence
of water-ice. More
information about lunar water. Credit NASA/Ames. |
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