October 2009 Sky from the Keeble Observatory
Planets are constantly bombarded with debris from space, most of it leftover pieces
of the formation of our solar system, almost 5 billion years ago. Also, most of
the debris is very small, and burns up in the atmospheres of those planets which
have substantial atmospheres (which, fortunately, includes our home planet!). Last
summer we saw what happens when a large object strikes a large planet – it goes
“boom” but leaves no lasting scar. Over the ages, our planet has taken hits ranging
from inconsequential to damaging. Evidence ranges from “shooting stars” during a
meteor shower to the devastation at Tunguska, all the way up to the extinction of
On October 9th, there will be a human-caused impact on the Moon. The
Centaur upper stage rocket which took the Lunar Reconnaissance Orbiter (LRO) aloft
will crash into a crater near the Moon’s southern pole. It is expected to blast
a crater about 20 meters across and 3 to 5 meters deep, excavating some 250 metric
tons and throwing it into the sky. About four minutes later, a probe called the
Lunar Crater Observation and Sensing Satellite (LCROSS), which piggy-backed a ride
with LRO will fly through the debris cloud to analyze it for evidence of water ice.
Speaking of ice, recent small meteoroid impacts on Mars have excavated tiny craters
which revealed bright ice. The impacts were detected after the fact by the Mars
Reconnaissance Orbiter, which noted them in a mid-latitude region which had been
previously observed only a year earlier. The ice is about 10 inches beneath the
surface. Ironically, the 1976 Viking 2 probe might have found similar ice, if only
it had dug a trench 4 inches deeper than the 6 inch excavation actually performed.
In 2004, a previously unknown asteroid was discovered, and given the cryptic designation
2004 MN4. First observed in June, subsequent observations allowed determination
of its orbit, and by December it was announced that there was a 1 in 37 chance that
this object would hit Earth in 2029 on the 13th of April. Long odds,
to be sure, but it was the highest probability yet computed. And, this is no small
rock – rather, it’s over twenty billion kilograms (nearly 250 thousand tons) – and
the impact explosion would be 510 megatons, equivalent to 250,000 times the nuclear
bombs that destroyed Hiroshima and Nagasaki. The threat level, measured on the “Torino
scale” was evaluated at 4 out of 10, the highest ever assigned.
A sigh of relief accompanied further measurements, which showed that it would not
hit Earth, but would only pass by about 25000 miles from the surface (roughly where
the communication satellites orbit). But the Torino threat level was reduced only
to 1, because there was still a small possibility that the 2029 encounter would
bring it back for an impact in 2036. As of now, the threat has been reduced to Torino
level 0, but that doesn’t mean an impact is impossible, only that the odds are very
long. Currently the likelihood of any impact from Apophis in the 21st
century is set at less than 1 in 43,000.
Don’t sigh too soon, however. An object designated as VK184 has a Torino threat
assessment at level 1, with a 1 in 3000 chance of hitting us in 2048, 2053, 2055,
or 2057. At least it’s smaller than Apophis … it would only give a 150 megaton blast!
Lunar phases for October: Full Moon on the 4th at 2:10 am; Last
Quarter on the 11th, at 4:56 am; New Moon on the 18th, at
1:33 am; First Quarter on the 25th, at 8:42 pm.
Predawn planet watchers will have more fun than evening observers – most of the
visible planets are in the east before sunrise. Early in the month look for Saturn,
Mercury, Venus, and Mars, in that order starting at the eastern horizon. Saturn
is about 10 degrees above the horizon, rising some 45 minutes before sunrise. Mecury
is a bit higher, rising about 15 minutes earlier still. Venus is the brightest of
the three, rising more than a half hour before that. Mars is very high, 70 degrees
above the horizon. Later in the month, look for Mercury to move closer to the Sun
and get lost in the glare. Saturn and Venus will climb higher, and Mars will largely
hold its altitude, but moving above the southern horizon.
If you want to catch a planet in the early evening, you’ll have to settle for Jupiter,
which will emerge from evening twilight about 20 degrees above the southeast horizon.
It will set about 3:00 am in early October. By the end of the month look for it
only a bit higher, 30 degrees bearing the south-southeast. It will set around midnight
at month’s end.
Looking toward zenith, two hours after sunset at midmonth we will find the constellation
Cygnus. Deneb is now at the northeast end of the Swan, with fainter Albireo at the
southwest end. Altair, in the constellation Aquila (the Eagle), is now to the south-southwest,
while Vega (which was overhead at this time last month) is now to the west of zenith.
The Milky Way divides the sky from northeast to southwest. As we have previously
noted, Cygnus marks the direction toward which the Sun orbits about the center of
the Galaxy. Sagittarius, low on the southwest horizon, lies in the direction toward
the dynamical center of the Milky Way. That bright “star” you see to the south,
in the constellation Capricorn, is actually Jupiter!