This diagram shows the phases of the August 28 lunar eclipse as the Moon passes through the Earth's shadow. The phases are marked at the times they occur at pacific daylight time in the U.S. – 2:52am PDT is 7:52pm Sydney time. Image: F. Espenak/NASA's GSFC

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This week, Earth's shadow will settle across the Moon for a 90-minute total eclipse. In the midst of the lunar darkness, Cooke hopes to record some flashes of light – explosions caused by meteoroids crashing into the Moon and blasting themselves to smithereens.

Catch the lunar eclipse starting at 6:51pm Sydney time tonight, Tuesday 28 August. The full eclipse will be visible between 7:52pm and 9:23pm Sydney time (2:52am and 4.23am U.S. Pacific time), according to the Sydney Observatory, and may turn the Moon blood red.

Another recent lunar eclipse, in March this year, was visible across much of the World, but only the western coast of Australia (see, Lunar eclipse to shade Moon red, Cosmos Online). Today's eclipse will be visible across the entire nation, as well as the Pacific and USA.

Impact spotting

"The eclipse is a great time to look [for impacts]," says Cooke, who heads up the U.S. space agency's Meteoroid Environment Office (MEO) at the Marshall Space Flight Centre in Huntsville, Alabama. The entire face of the Moon will be in shadow for several hours, offering more than 18 million square kilometres of dark terrain as target for incoming meteoroids.

Lunar explosions are nothing new. Cooke's team has been monitoring the Moon since late 2005 and they've recorded 62 impacts so far. "Meteoroids that hit Earth disintegrate in the atmosphere, producing a harmless streak of light. But the Moon has no atmosphere, so 'lunar meteors' plunge into the ground," he says.

Typical strikes release as much energy as 100 kg of TNT, gouging craters several metres wide and producing bursts of light bright enough to be spotted 385,000 km away, through ordinary backyard telescopes on Earth.

"About half of the impacts we see come from regular meteor showers like the Perseids and Leonids," says MEO team-member Danielle Moser. "The other half are 'sporadic' meteors associated with no particular asteroid or comet."

Helions are go

The MEO observatory at the Marshall Space Flight Centre, consists of two 36-centimetre telescopes equipped with sensitive low-light video cameras. Moser and colleague Victoria Coffey will be on duty Tuesday morning local time, to capture the action.

During the eclipse, they hope to catch an elusive variety of meteor called helions. "Helion meteoroids come from the direction of the sun," Cooke says, "and that makes them very difficult to observe [on Earth]." They streak across the sky most often around local noon when the Sun's glare is too intense for meteor watching.

Wait a minute. Meteors from the Sun? "The Sun itself is not the source," Cooke explains. "We believe helion meteoroids come from ancient Sun-grazing comets that laid down trails of dusty debris in the vicinity of the sun."

Nobody can be certain, however, because helion meteoroids are so devilishly difficult to study. Astronomers see them only in small numbers briefly before dawn or after sunset. Attempts to study helions via radar during the day have been largely foiled by terrestrial radio interference and natural radio bursts from the Sun—both of which can drown out meteoroid "pings."

Enter the eclipse

This where the eclipse comes in. During the eclipse, the Man in the Moon (the face we see from Earth) will be turned squarely toward the sun—"perfect geometry for intercepting helion meteoroids," says Moser. "And with Earth's shadow providing some darkness, we should be able to see any explosions quite clearly."

"Watching helion meteoroids hit the Moon and studying the flashes will tell us more about their size, velocity and penetration," she adds. That, in turn, will further the MEO's goal of estimating meteoroid hazards to spacecraft and future Moon-walking astronauts.

No one has ever seen a lunar impact during an eclipse, "but there's a first time for everything," Cooke says.