Get ready for a unique
cosmic collision! Early this coming Friday morning (Oct. 9), NASA's Lunar
Crater Observation and Sensing Satellite (LCROSS) will end its mission with a
bang — literally.
Currently carrying with it
the upper stage of the rocket that launched it on its way to the moon on June
18, the game plan is to send that spent rocket motor on a course to smash
into the lunar surface.
But just not anywhere on
the lunar surface, but to a thoroughly scrutinized crater called Cabeus that
lies near the moon's south pole and is enveloped in perpetual darkness. The
hoped-for resultant effects will be to find hidden water ice frozen inside the
crater.
And for seasoned skywatchers
here on Earth, it should also produce a visible cloud of ejected material.
However, only knowledgeable amateur astronomers with the right equipment will
be a able to detect the event. Others can watch the event live on NASA TV.
Smackdown!
The general belief among astronomers is that over the last
few billion years, the moon has been bombarded by countless numbers of comets.
The water from most of these comets completely sublimated away but if any
settled at the bottom of a crater near the moon's poles, these permanently
shadowed regions can keep the water from disappearing, remaining
as ice for a very long time.
Cabeus is a relatively flat crater about 60 miles in
diameter on the moon's south pole that scientists believe may be one of those
special cases that might hold water ice in its perpetually shadowed top soil. NASA
initially selected
a different target for LCROSS, the nearby crater of Cabeus A, but switched
to the larger Cabeus because data suggested it had a higher likelihood of
containing hidden water ice.
The impact is scheduled to occur this Friday, Oct. 9 at 11:30
UT. That's 7:30 a.m. EDT; 4:30 a.m. PDT. To convert
Universal Time to your local time, go here.
Impact will happen less than 10 hours after the spent LCROSS
Centaur rocket motor is released and its Shepherding Spacecraft maneuvers into
position to trail the Centaur en route to the lunar surface. The 5,000-pound
Centaur, is expected to slam into Cabeus at a sharp angle at a speed of 5,600
mph (9,010 kph).
If all goes according to schedule, the Shepherding vehicle,
carrying nine science payloads, will follow the Centaur's plunge into the moon,
beaming back data live to the Earth. Like a bullet hitting sand, the Centaur's
explosive collision is expected to create a crater roughly 60 or 70 feet wide
and perhaps as much as 16 feet deep, in the process dredging up approximately 385
tons of lunar dust and soil — enough to fill nearly 18 school buses. In
addition to recording the collision, the Shepherding Satellite will fly through
the regolith plume thrown up by the collision, just before it too slams into
the lunar surface some four minutes later, kicking up its own smaller plume of
debris.
But before it's sacrificed in the cause of science, the
1,500-pound Shepherding Spacecraft will utilize its specialized sensors and
will look for water's telltale chemical signature within the larger debris
plume created by the Centaur, possibly in the form of ice, hydrocarbons or
hydrated materials.
How to watch
NASA wants the amateur astronomer community to join in a
"citizen scientist" program. Jennifer Heldmann heads the LCROSS
observing campaign. "We would like to have as many eyes and instruments
watching the impact as possible because this is the way we'll get the most data
and the most information as possible."
Those who live to the west of Mississippi River will have
the best opportunity because the sky will still be dark. Those living east of
the Mississippi will still have the moon in the sky, but either dawn twilight
or — in the case of those living along the Atlantic Seaboard — sunrise will
have occurred, making for a much brighter sky background. A dark backdrop will
be an important prerequisite, since it's estimated that when the debris plume
forms on it will be no brighter than a sixth-magnitude star (the threshold of
naked-eye visibility), and quite likely even fainter.
If you want to attempt to see the impact yourself, here are some
important points to be aware of:
First, you're going to need a moderately large telescope,
somewhere on the order of at least 10 to 12 inches of aperture. Smaller
telescopes will probably not be able to do the job and you will definitely not
see anything using binoculars. You'll likely need to use magnifications in the
250 to 500-power range to have a legitimate chance of getting a glimpse of the
dim impact plume.
Keep in mind that high power dilutes the brightness of an
image, as well as aggravates any unsteadiness of detail. As a general rule of
thumb that the maximum amount of magnification for any telescope is 50-power
per inch of aperture.
"But wait a minute," you may protest, "my telescope comes
with a special Barlow lens, that the manufacturer promises will double or even
triple the magnification of my eyepiece."
True enough. In fact, that aforementioned 500-power is
likely achieved by pushing the scope's highest power eyepiece with that very
same Barlow lens, which ultimately will result in a dim, impossibly fuzzy
image. If you're a beginner, you need to understand that that using a Barlow
lens is similar to enlarging a photograph. The negative — like a telescope's
image — contains only so much detail, which can be blown up only so far before
all you can see is fuzz! So, if you have a 3-inch department store telescope —
even if it is blessed with perfect optics — the claim of 500-power is more than
three times the limit of the most practical magnification that it can
provide.
The impact will take place at the lunar south pole, or on
the lower limb of the moon along the dark portion immediately adjacent to the
terminator (the line that separates the illuminated
day side and the dark night side of the Moon). Try to keep the very bright
sunlit portion of the moon out of the field of view as much as possible.
The plume is expected to be in the shape of a "V"
but it will be exceedingly small in size relative to the moon itself. According
to NASA's Brian H. Day, the dusty material is only expected to rise about 6
miles (10 km) above the lunar surface. From Earth, that would be equal to about
one-quarter of 1 percent of the moon's apparent size or about 5.2 arc seconds.
To try and get an idea of just how large this is, point your
telescope toward Jupiter which conveniently shines in the southern part of our
current evening sky. Jupiter's disk currently measures 45-arc seconds in
diameter; so the dust plume from LCROSS would appear only about 1/9 as large as
that!
And the plume — if and when it's visible — will not last
very long. The best guesstimates are that it will last no more than 2 minutes.
You can get more detailed viewing tips from NASA here.
Observatories expected to participate in the study include
the newly refurbished Hubble Space Telescope, Hawaii's Keck and Gemini
telescopes, the Magdelena Ridge and Apache Ridge observatories in New Mexico, the MMT Observatory in Arizona and the Lunar Reconnaissance Orbiter (LRO) now
circling the moon.
Lastly, you can watch the event live here on NASA TV, beginning
at 6:30 a.m. EDT/3:30 a.m. PDT.
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other
publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.
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