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Leonids on the Moon


UPDATE22 November 1999 - The first recorded impact of a meteorite on the Moon may have been captured
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a leonid meteor and the moon. copyright bishop web works

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.

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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 Moons 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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