Deep Impact

The NASA LCROSS spacecraft is scheduled to smack into the south pole of the moon at 4:30am on the morning of October 9.

They’re looking for water ice that might be hidden in polar craters. The hope is that the impact will spray up evidence of water along with the other debris.

The impact itself probably won’t be visible from earth, though that shouldn’t stop you from trying. But it is expected to generate a plume of dust – and, perhaps, water vapor – visible for up to a minute after the impact. Estimates of the size of the plume vary wildly – I’ve seen predictions ranging from 40 feet to 5 miles, but of course the debris will disperse and get harder to see as it rises higher. Most people agree that it should reach roughly 5th magnitude, possibly slightly brighter.

5th magnitude is still a little tough to see against a gibbous (20 day old) moon, but certainly possible. I’ve seen suggestions that a large aperture (10” or more) may help.

The moon will have a favorable libration: the south pole will be tipped an extra 2-1/2° toward us, so we’ll be able to see a bit farther than usual into those southern craters. The target is a 48 km crater called Cabeus A, located west of Malapert and south-southwest of Newton. Cabeus A is nearly always in shadow, which is what makes it a great candidate for possible water ice.

I’ve heard tentative suggestions of LCROSS observing parties put on by SJAA at Houge, or by TAC at Montebello, but as of press time I haven’t heard anything definite. If you’re looking for a group to observe with, check the web to find the latest plans. Local astronomer Brian Day of PAS is coordinating amateur observations for NASA’s Ames Research Center; you can find more information at http://lcross.arc.nasa.gov/observation.htm. There’s also a Google group, http://groups.google.com/group/lcross_observation. And if you’ll be awake during the impact but don’t feel up to dragging a telescope outside – or if the weather doesn’t cooperate – you can watch the Exploratorium’s live webcast at http://www.explo.tv.

What do you look at during the rest of October?

Jupiter is high (well, as high as it gets this year, about 35 degrees) in the southern sky at nightfall and remains visible most of the evening. It should make an ideal showpiece for all those witches and goblins coming by on the 31st, if you want to offer them some “eye candy” along with the more traditional sugary kind.

Uranus and Neptune are also well placed for early evening observing, Uranus on the border between Pisces and Aquarius and Neptune sitting a couple of degrees above the left corner of Capricornus. Pluto runs a bit ahead; it’s already past its prime when night falls, so start early if you want to catch it in its complicated Milky Way field in northwestern Sagittarius.

Mars rises a little after midnight and is at its best in the predawn hours. Saturn, Mercury and Venus rise just before dawn. Early October is Mercury’s best show this year, provided you don’t insist on seeing a crescent – it’s gibbous, as is Venus.

Saturn’s rings have opened a little since we last saw them, to slightly over 2 degrees, and it makes two close passes with other planets this month. It passes a third of a degree away from Mercury on the morning of the 8th, then half a degree from Venus on the 13th.

Finally, if you’re out in the dawn hours watching the morning planets or the LCROSS impact, take a look for another plume of fine particles, much fainter and more spread out than the ones LCROSS will stir up: the morning zodiacal light. This thin band of dim light stretching up along the ecliptic from the horizon – caused by sunlight glittering off dust left over from the formation of the solar system – is at its best in the second half of October.