April 2013 Sky from the Keeble Observatory
Last month’s near miss (near hit?) by one small asteroid and the atmospheric detonation
of another prompted a discussion of the hazards from interplanetary stuff hitting
the Earth. It actually happens all the time, with tons of meteoroids being vaporized
in the atmosphere and relatively few of them exploding or hitting the ground. Indeed,
since 75% of our planet’s surface is covered by water and much of the land area
is uninhabited, most of these objects go completely unnoticed. And yet, there is
a non-zero chance of something significant hitting, as the Ural event showed only
too clearly. History and prehistory show us that these impacts do have an effect.
The so-called Chicxulub crater led to the extinction of the dinosaurs. The Tunguska
event in 1908 leveled hundreds of square miles of forest. The Ural event broke windows,
injured thousands, and certainly rattled a lot of nerves.
Perhaps triggered by a series of asteroid disaster movies in 1998, which were in
turn triggered by the collision of Comet Shoemaker-Levy 9 with Jupiter, Congress
has directed NASA and the Air Force to identify and track all potentially dangerous
“Near Earth Objects” to provide warning. Of course, the budgets to do this have
not been forthcoming, and it looks even worse with the sequester. It is now projected
that we might know of 90% of potential hazards by 2030.
What does it take to be “dangerous?” Let’s look at the energy involved, and compare
to the cost of electricity. A 1 kilogram (2.2 lb) rock dropped from a few hundred
kilometers will have to dissipate about 2 million joules of energy to come to rest.
The same kilogram dropped from 1 meter will make a sound on impact equivalent to
about 10 joules for comparison. A kilowatt-hour is 3.6 megajoules, so at current
rates from Dominion Virginia Power, you could buy 2 megajoules for less than a dime.
That’s not a lot of energy, though it sounds like a big number. A thousand tons
of high explosives (1 kiloton) yields about 4.2 trillion joules, or about $160,000
equivalent. Of course the objects we’re talking about arrive at the top of the atmosphere
traveling somewhere between Earth-escape speed (11 km/s) and Solar-escape (42 km/s)
so we have to account for that energy, too – add between 60 and 800 megajoules per
kilogram! A one meter rock, about 4 metric tons, arrives with 4000 times the energy
of a one kilogram rock. At 10 meters we have another factor of 1000 in mass, bringing
the explosive yield to 50 kilotons – a modest nuclear bomb. Every factor of 10 in
diameter increases the yield by a factor of 1000, so watch out for the 1 kilometer
Watching out we are, as Yoda might say. NASA’s Near Earth Asteroid program is charged
not only with detection, but orbit calculation and risk assessment. Currently they
are tracking over 1300 Potentially Hazardous Asteroids, though none are currently
projected to hit Earth. For a partial list you can go to
http://spaceweather.com or directly to the NEO project site at
What would we do if we found one that was actually likely to hit us? That’s next
Lunar phases for April: Last Quarter on the 3rd, at 12:37 am;
New Moon on the 10th, at 5:35 am; First Quarter on the 18th,
at 8:31 am; Full Moon on the 25th, at 3:57 pm.
Predawn planet watchers can still catch Saturn to the southwest early in the month,
but it will be slipping lower and lower as we move through April. The good news
is that it will also be visible in late evening skies by month’s end as the ringed
planet reaches opposition on the 28th, rising as the Sun sets. Jupiter
will be visible to the west after sunset for a few hours. It starts the month about
45 degrees above the horizon at twilight, setting three hours later. By month’s
end it will be lower and setting two hours after sunset.
Our midmonth overhead view of the sky, about three hours after sunset, finds us
staring at an apparently empty patch of the heavens! Not actually, of course, but
the lack of bright stars near zenith might lead us to consider the possibility.
It just happens that at this particular time you are looking out of the plane of
our home Galaxy. Wait a few hours and you’ll see that the Milky Way was hugging
Although no “famous” constellations are directly overhead, look for some a little
lower. Leo is to the southwest, about 60 degrees above the horizon. The bright star
Regulus is the “heart of the Lion” and Denebola marks the lion’s tail nearly 50
degrees to the left. Castor and Pollux in the constellation Gemini are about 45
degrees above the western horizon, while Jupiter is settling toward the horizon
to the west northwest.
Ursa Major is high to the north, with the bowl of the Big Dipper inverted above
the north star Polaris (which isn’t actually above the north pole, but it’s close).
Vega is rising low to the northeast. Arcturus in the constellation Bootes is about
48 degrees above the east southeast. Saturn is below and to the southeast, about
24 degrees above the horizon and rising.