Bizarre Lunar Orbits - 11.06.2006

What goes bump in the morning? Oh, not our car accident several years back but something else that could make for a bad hair day.

When you put a satellite around the Moon, you would think that it should just keep orbiting. Satellites going around Mars seem to stay up, but going around the Moon is not quite the same. The Moon it turns out is bumpy in terms of how the gravitational field looks to the spacecraft. A tug this way and that as you go around will disturb your orbit. Without adjustments, you will find yourself bumping into the mountain tops.

Science at NASA has an interesting article about Bizarre Lunar Orbits. When the Lunar Reconnaissance Orbiter goes to the Moon, the orbit selected will affect how much fuel will be needed to maintain the desired track. Hope they note some of the past experiences with lunar orbiters.
http://lunar.gsfc.nasa.gov/
http://lunar.gsfc.nasa.gov/missions/index.html
- LRK -

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NASA Science News for November 6, 2006

Mysterious concentrations of mass in the Moon's ancient lava seas disturb the orbits of Moon-circling spacecraft. NASA is taking these "mascons" into account as the agency prepares to return to the Moon.

FULL STORY at
http://science.nasa.gov/headlines/y2006/06nov_loworbit.htm
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Thanks for looking up with me.

Larry Kellogg

Web Site: http://lkellogg.vttoth.com/LarryRussellKellogg/
BlogSpot: http://kelloggserialreports.blogspot.com/
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Newsltr.: https://news.altair.com/mailman/listinfo/lunar-update
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http://science.nasa.gov/headlines/y2006/06nov_loworbit.htm

Nov. 6, 2006: Near the end of the mission of Apollo 16, on April 24, 1972, just before returning back home to Earth, the three astronauts released one last scientific experiment: a small "subsatellite" called PFS-2 to orbit the Moon about every 2 hours.

The intention? Joining an earlier subsatellite PFS-1, released by Apollo 15 astronauts eight months earlier, PFS-2 was to measure charged particles and magnetic fields all around the Moon as the Moon orbited Earth. The low orbits of both subsatellites were to be similar ellipses, ranging from 55 to 76 miles (89 to 122 km) above the lunar surface.

Instead, something bizarre happened.

The orbit of PFS-2 rapidly changed shape and distance from the Moon. In 2-1/2 weeks the satellite was swooping to within a hair-raising 6 miles (10 km) of the lunar surface at closest approach. As the orbit kept changing, PFS-2 backed off again, until it seemed to be a safe 30 miles away. But not for long: inexorably, the subsatellite's orbit carried it back toward the Moon. And on May 29, 1972—only 35 days and 425 orbits after its release—PFS-2 crashed.

Snip

"Lunar mascons make most low lunar orbits unstable," says Konopliv. As a satellite passes 50 or 60 miles overhead, the mascons pull it forward, back, left, right, or down, the exact direction and magnitude of the tugging depends on the satellite's trajectory. Absent any periodic boosts from onboard rockets to correct the orbit, most satellites released into low lunar orbits (under about 60 miles or 100 km) will eventually crash into the Moon. PFS-2 released by Apollo 16 was simply a dramatic worst-case example. But even its longer-lived predecessor PFS-1 (released by Apollo 15) literally bit the dust in January 1973 after less than a year and a half.

So what does this mean for eventual lunar exploration?

Be careful of the orbit chosen for a low-orbiting lunar satellite. "What counts is an orbit's inclination," that is, the tilt of its plane to the Moon's equatorial plane. "There are actually a number of 'frozen orbits' where a spacecraft can stay in a low lunar orbit indefinitely. They occur at four inclinations: 27º, 50º, 76º, and 86º"—the last one being nearly over the lunar poles. The orbit of the relatively long-lived Apollo 15 subsatellite PFS-1 had an inclination of 28º, which turned out to be close to the inclination of one of the frozen orbits—but poor PFS-2 was cursed with an inclination of only 11º.

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Author: Trudy E. Bell | Editor: Dr. Tony Phillips | Credit: Science@NASA
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More to the story...

Lunar mascons are a mystery. Although scientists generally agree they resulted from ancient impacts billions of years ago, it’s unclear how much of the excess mass is due to denser lava material filling the crater or how much is due to upwelling of denser iron-rich mantle material to the crust. Regardless of composition or origin, the mascons make the Moon the most gravitationally "lumpy" body known in the solar system. Although mascons also exist on Mars, none have been found on Venus or Earth; those two larger planets, however, have had an active tectonic (geological) past that has drawn their crusts down into their interiors several times in the past few billion years, homogenizing the distribution of mass.

Details about the subsatellites of both Apollos 15 and 16, including their orbital parameters, appear on p. 5-5 of Apollo 16 Mission Report.
http://history.nasa.gov/alsj/a16/a16mr.html

[NOTE: This link will take you to an 18 Mb PDF version of the Apollo 16 Mission Report courtesy of Glen Swanson and Ron Wells. P.5-5 starts on page 44 of 393 pages.
Thanks Ron. - LRK -]

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More about the mascons in the context of the gravitational lumpiness of the moon is "Improved Gravity Field of the Moon from Lunar Prospector," by Konopliv and colleagues.
http://www.sciencemag.org/cgi/content/full/281/5382/1476

An account of the deliberate crashing of Lunar Prospector into the Moon may be found here.
http://www.ae.utexas.edu/research/cfpl/lunar/pressrelease/discussion.html

The Vision for Space Exploration
http://www.nasa.gov/mission_pages/exploration/main/index.html

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http://lkellogg.vttoth.com/LarryRussellKellogg/links.html

What does it take to get to the Moon?

The following files require Acrobat Reader to view. Get Acrobat Reader.
http://www.adobe.com/products/acrobat/readstep2_allversions.html

See LUNAR PROSPECTOR MISSION DESIGN AND TRAJECTORY SUPPORT.
AAS98-323.pdf (387kb PDF file)
http://lunar.arc.nasa.gov/project/pdf/AAS98-323.pdf

The complete Status Report archive in an Adobe Acrobat Reader PDF file.
LPStatus.pdf (255 kb PDF file)
http://lunar.arc.nasa.gov/datavis/LPStatus.pdf

LUNAR PROSPECTOR End of Mission & Overview Press Kit (550 kb PDF file) http://lunar.arc.nasa.gov/resources/LPBckgrn.pdf

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WHAT THE MIND CAN CONCEIVE, AND BELIEVE, IT WILL ACHIEVE - LRK

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