March 2013 Sky from the Keeble Observatory
While professional astronomers and a few doomsayers were concentrating on a 50 meter
rock that would pass inside the orbits of most communication satellites on the 15th
of February, a bus sized asteroid exploded high in the atmosphere over the Urals.
Perhaps Chicken Little was right! At least, he may have been justified in thinking
so. What’s going on here?
Even a cursory examination of craters on the Moon and other airless worlds in our
solar system gives ready evidence that some pretty big chunks of solid material
are still orbiting the Sun. Formally, any “small” – i.e. smaller than a planet -
solid object orbiting the Sun is called a meteoroid. Sometimes these hit an atmosphere.
At their extreme relative speeds (typically 30 kilometers per second in the Earth’s
vicinity – that’s about 20 miles per second) they heat up quickly from friction
and incandesce. We can see these as “shooting stars” – these are formally known
as meteors. If one of these strikes the surface of a planet it is called a meteorite.
The overwhelming majority of these meteors are small grains that vaporize completely
before getting anywhere near the ground.
The Russian meteor/meteorite came out of the direction to the Sun, so it was not
detected until it hit the atmosphere. Fortunately it was moving almost parallel
to the ground when it exploded at about 20 miles altitude with the equivalent yield
of a 500 kiloton hydrogen bomb. As it was, millions of windows were shattered by
the shock wave and a factory roof collapsed from the overpressure. In 1908 a rocky
asteroid perhaps 100 meters in diameter exploded over the Tunguska region in Siberia,
leveling over 800 square miles of forest. This blast has been estimated at 3 – 30
megatons, i.e. perhaps 1000 times the yield of the Hiroshima bomb. The recent event
is the largest recorded in over a century. The good news or bad news is that we
can expect something like that to happen roughly once per century. But they’re not
timed events. What we really mean to say is that there is roughly a 1% chance that
we will see such an impact in any given year.
Larger still meteoroids are rarer. Some 65 million years ago one such larger object
struck off what is now the Yucatan peninsula. Estimated to have been roughly 100
meters in diameter, this asteroid exploded with a yield of 100 teratons (that’s
10 million megatons – more than 2 million times the largest nuclear device ever
exploded). The Chicxulub crater remnant is 110 miles in diameter, making it the
largest known crater on Earth. It is this event which is thought to have ended the
time of the dinosaurs and made it possible for mammals to eventually evolve into
humans like those reading this column.
Next month we’ll look at efforts to identify and track these potentially dangerous
objects, and what we might do to protect ourselves.
Lunar phases for March: Last Quarter on the 4th, at 4:53 pm; New
Moon on the 11th, at 3:51 pm; First Quarter on the 19th, at
1:27 pm; Full Moon on the 27th, at 5:27 am.
Planet watchers may want to find another hobby this month. Only Jupiter and Saturn
will be readily visible. Jupiter will emerge from evening twilight to the south
early in the month, then drift slowly to the west. Saturn rises before midnight
(about 10:30 pm early in the month, 9:30 after the switch to daylight time on the
10th). Saturn is high to the south in the predawn skies. Venus passes
behind the Sun, and Mars is too close to pick out of the glare. Mercury will return
to the predawn skies by month’s end, but will be very low and close to the Sun.
Our overhead view at midmonth, roughly three hours after sunset, finds the view
toward zenith remarkably empty of bright stars. Castor and Pollux are perhaps the
closest, high to the west and above Orion. Jupiter is near Aldebaran in Taurus,
and is actually brighter than this bright star. Lower and to the southwest we see
Sirius for what should be our last chance until its return next winter. Ursa Major
is inverted, with the “Big Dipper” asterism high to the northeast. This is a good
time to check out the middle star in the “handle.” It’s actually a complex binary
with its two brightest constituents Mizar and Alcor close enough that you may have
difficulty resolving them without binoculars. A better telescope reveals that each
of these is also binary. The end star in the handle is called Alkaid, yet another
Arabic name. On a clear moonless night, binoculars may enable you to see the Whirlpool
Galaxy, also known as M51 for its place in Charles Messier’s catalog of objects
not to be confused with comets. It will be found about 5 degrees above and to the
right of Alkaid. You should be able to pick it out with binoculars, while a larger
telescope will give you a sense for why it’s called the whirlpool, but your best
view will be to find a deep image on line. The bright spiral arms tracing out regions
of star formation may be the byproduct of a near collision with its companion galaxy.
This spiral structure was first noted by the Earl of Ross with his 72 inch telescope
in 1845. We now know that a massive black hole lies at the core of M 51, much as
a million solar mass black hole occupies the center of our own Milky Way.
Another face on spiral is found below and to the left of Alkaid. This one is M 101,
also known as the Pinwheel Galaxy. Yes, astronomers do have a way with words!