Jul 26, 1999

Lunar Prospector in Eclipse





The partial lunar eclipse of July 28, 1999 poses a last-minute obstacle to Lunar Prospector, which is scheduled to crash into the Moon three days later.


November 1993 lunar eclipse.  Image credit: Andy Steere
July 26, 1999: This Wednesday, July 28, sky watchers in the Americas, east Asia and the Pacific have a chance to view the last lunar eclipse before the year 2000 when the full moon passes through Earth's umbral shadow for over 2 hours. This eclipse will be a partial one. At maximum approximately 40% of the Moon will be covered by the darkest part of Earth's shadow giving the moon an eerie, orange-colored glow.

Right: This photo of the November 1993 total lunar eclipse shows how a completely eclipsed Moon may not appear completely dark. Sunlight scattered into the Earth's shadow after passing around the planet's edge and through its dusty atmosphere can make the Moon take on dramatic shades of red during totality. Image credit and copyright: Andy Steere [more information]

Lunar eclipses are fairly common -- they happen about once every 6 months, on average -- but there's something uncommon about this one. It could mark the premature end of NASA's Lunar Prospector mission.


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Lunar Prospector is scheduled to crash into a crater near the Moon's south pole on July 31 in a dramatic experiment designed to detect water on the moon. Before that can happen it must first endure the July 28 eclipse that will pose the biggest challenge to its survival since it was launched in January 1998. According to the UT Austin Lunar Prospector Impact site, Prospector's battery was designed to handle maximum shadow periods of 47 minutes before being recharged by solar arrays mounted on the side the spacecraft. During the upcoming eclipse, Prospector will pass in and out of regions of partial and total Earth/moon shadows for nearly 3 continuous hours as it orbits the moon. Components of the spacecraft will experience temperatures beyond their normal operating range as the battery runs down to dangerously low levels and is unable to operate electric heaters. There is a possibility that key subsystems required to operate the spacecraft may be damaged and control of the spacecraft could be lost.

Nevertheless, mission scientists are cautiously optimistic.

"Lunar Prospector has gone through a couple of eclipses similar to this one and experienced no difficulties," says Lisa Chu-Thielbar, the Lunar Prospector Mission Office Outreach Coordinator. "However, this eclipse is near the end of Prospector's intended lifetime so the operations team is justifiably cautious about turning systems off and on and surviving eclipses. Will Lunar Prospector experience any unusual difficulties on July 28? We hope not!"

If the eclipse on July 28 poses a hazard to Lunar Prospector, why schedule the crash three days after the eclipse rather than before? According to Dr. David Goldstein of the University of Texas at Austin, there are good scientific reasons to wait, including the simple fact that the spacecraft will fly right over the impact site on the July 31.

Also, Goldstein and his colleagues want to observe the crash after the full moon on July 28 to take advantage of better solar illumination around the lunar limb.

"We expect to only be able to see gas that is over a warm sunlit surface and above the limb of the Moon," he explains. "During a full moon, the terrain just behind the visible limb is dark. That's not optimal because if the ground is dark and cold it will tend to trap and re-freeze gases liberated by Prospector's impact. Waiting until after the full moon will improve the prospects for a longer-lasting exosphere - an extremely thin local atmosphere - which the astronomy team led by Dr. Edwin Barker of UT Austin hopes to observe."

For more information about plans to observe the impact, including tips for amateur astronomers, see our July 21, 1999 headline Bracing for Impact.



Above: The July 28, 1999 partial lunar eclipse will be visible from the Americas, east Asia, Australia and the Pacific. Image adapted from material at Fred Espanek's eclipse web site.

If Lunar Prospector survives the eclipse on July 28, 1999, it will be a temporary respite before it crashes into the Moon's south pole in search of water 3 days later. Scientists are optimistic that the spacecraft can clear this last hurdle before the end of the mission, but even if it doesn't, the Lunar Prospector mission must be considered an impressive success. Since its launch in January 1998, Prospector has scored one scientific triumph after another including a precision gravity map of the lunar surface, global maps of elemental composition, the detection of mini-magnetospheres related to large impact sites, and evidence pointing toward a small iron-rich lunar core. Undoubtedly the most important finding has been the possible detection of substantial water-ice deposits at both the North and South lunar poles. Project scientists estimate that at least 200 million metric tons of water in the form of ice crystals are mixed with the top 18 inches of soil near the poles. If these figures are correct, there might be enough water on the Moon to support substantial human colonies. The planned crash of Lunar Prospector into a shadowed crater on July 31 is intended to test this latter proposition.

By the light of the shadowy Moon...

The Moon will enter the outskirts of Earth's shadow early in the morning of July 28 at 8:56 UT (1:56 a.m. PDT). The darkening of the Moon's southern hemisphere will accelerate at 10:22 UT (3:22 a.m. PDT), and the eclipse maximum will occur at 11:34 UT (4:34 a.m. PDT). In North America the best viewing sites will be along the west coast. Observers on the eastern seaboard will be able to see just the beginning of the eclipse at dawn. [See Fred Espanek's eclipse visibility map for more information.]



Above: A diagram from Fred Espanek's eclipse web site showing the progression of the July 28, 1999 partial lunar eclipse. Before entering the main umbra, or cone of shadow, the Moon must pass through the penumbra, or zone of partial shadow, which exists because the sun is a disk and not a point source of light. Wednesday's eclipse is partial -- only 40% of the Moon will be darkened by Earth's umbral shadow. Even during a total lunar eclipse, when the Moon passes entirely through the umbral shadow, the Moon is not completely dark. That's because sunlight is refracted as it passes through Earth's atmosphere and bent toward the lunar surface. The depth of shadow and the orange hue of the lunar surface during an eclipse are influenced by the amount of dust or ash in the earth's upper atmosphere. Eclipses following volcanic eruptions on Earth can appear especially dark and coppery-colored.

A lunar eclipse is always a beautiful sight, but the fate of Lunar Prospector may add some drama to the event for sky watchers viewing the orange-colored, shadowy Moon next Wednesday morning. For more information about the lunar eclipse, and eclipses in general, see Fred Espanek's Eclipse Home Page at the Goddard Space Flight Center.

Please visit the Lunar Prospector project web site from NASA/Ames and the Lunar Prospector impact page from UT Austin for more information about the Lunar Prospector mission and its impending crash into the Moon.

Lunar Prospector was the first of NASA's competitively selected "faster, better, cheaper" Discovery-class missions. The $63 million mission is managed by NASA Ames Research Center, Moffett Field, CA.


Web Links


Bracing for Impact -- Astronomers prepare to observe the crash of Lunar Prospector on July 31, 1999. Includes observing hints for amateurs, July 21, 1999, NASA Science News

Destined for a Watery Grave -- NASA scientists have decided to send Lunar Prospector crashing into the Moon's south pole in search of water, June 4, 1999, NASA Science News

Zeroing in on Lunar Ice -- Astronomers explore the Lunar Prospector crash site using radar, June 4, 1999, NASA Space Science News

Lunar Prospector set to make science "splash" -- NASA/Ames press release

NASA Press Release (3 September 1998) -- announcing enhanced estimate of quantity of water on the Moon

NASA Press Release (5 March 1998) -- announcing the detection of ice on the Moon

Lunar Prospector Home Page -- from NASA/Ames

Ice on the Moon -- informative article about lunar water -- where it is and how to find it.

Lunar Prospects -- Astronomy Picture of the Day, Sep. 18, 1998

Impact Moon -- Astronomy Picture of the Day, Mar. 26, 1999

The Nine Planets: the Moon -- from SEDS

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Author: Dr. Tony Phillips
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