The Earth dodged one bullet Friday with the close passage of asteroid 2012 DA14, which came within 17,000 miles of the Earths surface. There was never any danger the calculation of its orbit showed it would not hit us. One thing we astronomers and rocket scientists are good at is computing orbits and trajectories. That rock was about the size of a 10-story building and half a city block long. Such rocks do hit us about every thousand years. The mile-wide Meteor Crater in Arizona was the result of such a hit about 50,000 years ago.
The Russians were a little less lucky Friday, with a rock perhaps a tenth that size vaporizing above them. The shock waves blew out hundreds of windows, injuring more than 1,000 people. A similar but larger object exploded over Siberia in 1908, flattening trees for more than 1,000 square miles.
We should pause to note that 2012 DA14 was discovered only last year. How many unknown killer rocks are out there?
While the close passage of 2012 DA14 was not visible to U.S. viewers, a couple of less threatening rocks are going to be visible this year. In March we will hopefully catch a glimpse of comet PanSTARRS, which could become a fairly bright object in the dusk sky. One obstacle is that it will be very low at a half hour after sunset it will be only about 5 to 10 degrees above the western horizon. Ten degrees is about a fist-width at arms length. Look for it in starting about 5 degrees to the south of due west on March 10 and moving night-to-night higher and northward until it is about 10 degrees north of west on March 20. It will continue northward and higher, but fainter, through May. It will be a good object for binoculars.
PanSTARRS was discovered by a robotically operated telescope in Hawaii that searches for asteroids and comets that could strike Earth. Comets are named for their discoverers, and in this case it was this automatic telescope.
We think this comet will be bright but more difficult to predict, with more unknowns than in orbit calculations. Comets are dirty snowballs, or maybe snowy rocks; the mixture of rock and ice varies from comet to comet. The ice is heated by the sun, sublimates to gas and is excited by the sunlight. Dust that escapes reflects sunlight, too. So, forecasting the brightness of comets is difficult.
In November we will bookend our year with the appearance of another, potentially much brighter comet, also named after its discovery telescope, ISON. Stay tuned for details.
If we are still here.
Daniel B. Caton is a physics and astronomy professor and director of observatories at Appalachian State University. Email: email@example.com. More on this months column: www.upintheair.info.