100 meteoroid explosions on the moon since late 2005

100 Explosions on the Moon
05.21.2008

May 21, 2008: Not so long ago, anyone claiming to see flashes of light on the Moon would be viewed with deep suspicion by professional astronomers. Such reports were filed under "L" … for lunatic.

Not anymore. Over the past two and a half years, NASA astronomers have observed the Moon flashing at them not just once but one hundred times.

"They're explosions caused by meteoroids hitting the Moon," explains Bill Cooke, head of NASA's Meteoroid Environment Office at the Marshall Space Flight Center (MSFC). "A typical blast is about as powerful as a few hundred pounds of TNT and can be photographed easily using a backyard telescope."

As an example, he offers this video of an impact near crater Gauss on January 4, 2008: (go to link to watch video; caption of which is: Above: A lunar impact on Jan. 4, 2008. This is number 86 on the list of 100 impacts recorded by the MEO team since their survey began in 2005. Larger movies: 0.8 MB gif, 5.9 MB avi.)

The impactor was a tiny fragment of extinct comet 2003 EH1. Every year in early January, the Earth-Moon system passes through a stream of debris from that comet, producing the well-known Quadrantid meteor shower. Here on Earth, Quadrantids disintegrate as flashes of light in the atmosphere; on the airless Moon they hit the ground and explode.

"We started our monitoring program in late 2005 after NASA announced plans to return astronauts to the Moon," says team leader Rob Suggs of the MSFC. If people were going to be walking around up there, "it seemed like a good idea to measure how often the Moon was getting hit."

"Almost immediately, we detected a flash."

That first detection—"I'll never forget it," he says--came on Nov. 7, 2005, when a piece of Comet Encke about the size of a baseball hit Mare Imbrium. The resulting explosion produced a 7th magnitude flash, too dim for the naked eye but an easy target for the team's 10-inch telescope.

A common question, says Cooke, is "how can something explode on the Moon? There's no oxygen up there."

These explosions don't require oxygen or combustion. Meteoroids hit the moon with tremendous kinetic energy, traveling 30,000 mph or faster. "At that speed, even a pebble can blast a crater several feet wide. The impact heats up rocks and soil on the lunar surface hot enough to glow like molten lava--hence the flash."

During meteor showers such as the Quadrantids or Perseids, when the Moon passes through dense streams of cometary debris, the rate of lunar flashes can go as high as one per hour. Impacts subside when the Moon exits the stream, but curiously the rate never goes to zero.

"Even when no meteor shower is active, we still see flashes," says Cooke.

These "off-shower" impacts come from a vast swarm of natural space junk littering the inner solar system. Bits of stray comet dust and chips off old asteroids pepper the Moon in small but ultimately significant numbers. Earth gets hit, too, which is why on any given night you can stand under a dark sky and see a few meteors per hour glide overhead—no meteor shower required. Over the course of a year, these random or "sporadic" impacts outnumber impacts from organized meteor showers by a ratio of approximately 2:1.

"That's an important finding," says Suggs. "It means there's no time of year when the Moon is impact-free."

Fortunately, says Cooke, astronauts are in little danger. "The odds of a direct hit are negligible. If, however, we start building big lunar outposts with lots of surface area, we'll have to carefully consider these statistics and bear in mind the odds of a structure getting hit."

Secondary impacts are the greater concern. When meteoroids strike the Moon, debris goes flying in all directions. A single meteoroid produces a spray consisting of thousands of "secondary" particles all traveling at bullet-like velocities. This could be a problem because, while the odds of a direct hit are low, the odds of a secondary hit may be significantly greater. "Secondary particles smaller than a millimeter could pierce a spacesuit," notes Cooke.

At present, no one knows how far and wide secondary particles travel. To get a handle on the problem, Cooke, Suggs and colleagues are shooting artificial meteoroids at simulated moon dust and measuring the spray. This work is being done at the Vertical Gun Range at NASA's Ames Research Center in Mountain View, CA: full story.

Meanwhile, back at the observatory, the team has upgraded their original 10-inch (25 cm) telescope to a pair of telescopes, one 14-inch (36 cm) and one 20-inch (51 cm), located at the Marshall Space Flight Center in Alabama. They've also established a new observing site in Georgia with a 14-inch telescope. Multiple telescopes allow double- and triple-checking of faint flashes and improve the statistical underpinnings of the survey.

"The Moon is still flashing," says Suggs. Indeed, during the writing of this story, three more impacts were detected.

New title: 103 Explosions on the Moon.
(my comment: I guess there were 3 more since the article went up this morning, but they didn't change the title, just made this note at the end of the article)

Stay tuned to Science@NASA for a follow-up story describing how amateur astronomers can participate in this research.

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A map of the 100 explosions observed since late 2005. A complete list with lunar coordinates is available here (hotlink to http://www.nasa.gov/centers/marshall/news/lunar/).

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Lunar GRAIL
05.22.2008

May 22, 2008: Meet MIT professor of physics Maria Zuber. She's dynamic, intelligent, intense, and she's on a quest for the Grail.

No, not that Grail.

Zuber is the principal investigator of the Gravity Recovery and Interior Laboratory — "GRAIL" for short. It's a new NASA mission slated for launch in 2011 that will probe the moon's quirky gravity field. Data from GRAIL will help scientists understand forces at play beneath the lunar surface and learn how the moon, Earth and other terrestrial planets evolved.

"We're going to study the moon's interior from crust to core," says Zuber. "It's very exciting."

Here's how it works: GRAIL will fly twin spacecraft, one behind the other, around the moon for several months. All the while, a microwave ranging system will precisely measure the distance between the two satellites. By watching that distance expand and contract as the two satellites fly over the lunar surface, researchers can map the moon's underlying gravity field1.

Scientists have long known that the moon's gravity field is strangely uneven and tugs on satellites in complex ways. Without course corrections, orbiters end their missions nose down in the moondust! In fact, all five of NASA's Lunar Orbiters (1966-1972), four Soviet Luna probes (1959-1965), two Apollo sub-satellites (1970-1971) and Japan's Hiten spacecraft (1993) suffered this fate.

The source of the gravitational quirkiness is a number of huge mascons (short for "mass concentrations") buried under the surfaces of lunar maria or "seas." Formed by colossal asteroid impacts billions of years ago, mascons make the moon the most gravitationally lumpy major body in the solar system. The anomaly is so great—half a percent—that it actually would be measurable to astronauts on the lunar surface. A plumb bob held at the edge of a mascon would hang about a third of a degree off vertical, pointing toward the central mass. Moreover, an astronaut in full spacesuit and life-support gear whose lunar weight was exactly 50 pounds at the edge of the mascon would weigh 50 pounds and 4 ounces when standing in the mascon's center.

To minimize the effects of mascons, satellite orbits have to be carefully chosen. GRAIL's gravity maps will help mission planners make those critical decisions. Moreover, the maps GRAIL scientists will construct are essential to NASA's intended human landing on the moon in the next decade. The gravity of the moon's far side and polar regions, where future landings are targeted, is least understood.

The GRAIL team aims to map the moon's gravity field so completely that "after GRAIL, we'll be able to navigate anything you want anywhere on the moon you want," says Zuber. "This mission will give us the most accurate global gravity field to date for any planet, including Earth."

GRAIL will also help students learn about gravity, the moon, and space. Each satellite will carry up to five cameras dedicated to public outreach and education. Undergraduate students supervised by trained adults will remotely operate the cameras from a facility at the University of California, San Diego, that currently operates similar cameras on the International Space Station.

Middle school students from all over the country will also get to join in the excitement of lunar exploration. "We'll have an interactive website where the middle school students can make recommendations for targets to photograph and then view the pictures of their suggested targets," she says. "This just has incredible potential to engage students."

Clearly, this is no ordinary Grail quest. Stay tuned to Science@NASA for updates as the adventure unfolds.

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A gravity map of the moon made by the Lunar Prospector spacecraft in 1998-99. Mascons are shown in orange-red. The five largest all correspond to the largest lava-filled craters or lunar "seas" visible in binoculars on the near side of the Moon: Mare Imbrium, Mare Serenitatus, Mare Crisium, Mare Humorum and Mare Nectaris. Image reference: Alex S. Konopliv et al, Icarus 150, 1–18 (2001). [more - hyperlink to http://science.nasa.gov/headlines/y2006/06nov_loworbit.htm]

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Bizarre Lunar Orbits
11.06.2006

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.

What happened? The Moon itself plunged the subsatellite to its death. That's the conclusion of Alex S. Konopliv, planetary scientist at NASA's Jet Propulsion Laboratory in Pasadena. He and several colleagues have been analyzing the orbits of various Moon-orbiting satellites since PFS-2, notably the 1998–99 mission of Lunar Prospector.

"If the Moon were a uniform sphere, you could have an orbit that was perfect ellipse or circle," Konopliv explained. "The Moon has no atmosphere to cause drag or heating on a spacecraft, so you can go really low: Lunar Prospector spent six months orbiting only 20 miles (30 km) above the surface."

So why did PFS-2, which was inserted into an elliptical orbit that originally carried it from 52 miles (97 km) to 66 miles (120 km), end up as a kamikaze blast of broken aluminum struts and solar panels?

"The Moon is extraordinarily lumpy, gravitationally speaking," Konopliv continues. "I don't mean mountains or physical topography. I mean in mass. What appear to be flat seas of lunar lava have huge positive gravitational anomalies—that is, their mass and thus their gravitational fields are significantly stronger than the rest of the lunar crust." Known as mass concentrations or "mascons," there are five big ones on the front side of the Moon facing Earth, all in lunar maria (Latin for "seas") and visible in binoculars from Earth.

The mascons' gravitational anomaly is so great—half a percent—that it actually would be measurable to astronauts on the lunar surface. "If you were standing at the edge of one of the maria, a plumb bob would hang about a third of a degree off vertical, pointing toward the mascon," Konopliv says. Moreover, an astronaut in full spacesuit and life-support gear whose lunar weight was exactly 50 pounds at the edge of the mascon would weigh 50 pounds and 4 ounces when standing in the mascon's center.

"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.

Alternatively, if there are mission reasons for choosing a non-frozen orbital inclination, plan to do frequent course corrections. Lunar Prospector had to do a maneuver every two months to keep itself in its initial circular orbit of 60 miles (100 km)—and more often than once a month when it was orbiting at only 20 miles (30 km) altitude. When its fuel tank was nearly empty, the scientists knew its end was near, so they deliberately crashed it on July 30, 1999, near the Moon's south pole to observe its plume of lunar dust. After a year and a half, the Moon had claimed the spacecraft for its own.

Bottom line, says Konopliv: "Carry plenty of fuel."

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