Bob Doyle, Columnist
In last week’s column, I described how sizable numbers of asteroids pass through the inner solar system.
On the average, each day, one of these objects the size of a basketball hits the Earth. Our atmosphere will burn off the outer layers of such objects and only the central core will reach the Earth.
Much larger objects, the size of a large house would likely impact us about once a century. The last such object exploded over Siberia in 1908, laying waste to an area the size of a large city.
This column will describe the techniques used to detect such objects and what can be done to deflect one heading our way. My reference is “Near Earth Objects: Finding Them Before They Find Us” by Donald Yeoman, of the Jet Propulsion Laboratory.
In the late 1700s, astronomers noted the big gap between the orbits of Mars and Jupiter. A group of observers got together and divided the sky into 24 segments. Then each observer would scan the sky with his telescope.
The organization was informally called “The Celestial Police”. On the first day of 19rh century, January 1, 1801 a Sicilian priest, Giuseppe Piazzi discovered a new planet, which he called Ceres, for the patron goddess of Sicily.
The first five asteroids (means ‘star like’) were called minor planets. As the discoveries of these objects increased with the advent of astrophotography, the minor planet category was dropped and these mini planets were called asteroids.
The first near Earth asteroid to be discovered was Eros, found in 1898. For the next 60 years, the number of near-Earth asteroids slowly grew.
Then in 1973, a team of astronomers at Mount Palomar (California) began a systematic survey of the sky for asteroids. The next big advance came in 1984 with digital cameras, which were much more sensitive than film cameras.
The first big program was called Spacewatch, using a camera with 2,000 pixels by 2,000 pixels or 4 Megapixels. Today’s digital cameras for capturing asteroids have hundreds of millions of pixels.
Once the threat of the near Earth asteroids was understood, Congress in the 1990’s charged NASA to conduct sky searches for any objects one kilometer (0.6 miles) or larger that could impact Earth.
The next generation asteroid searcher programs are designed to detect asteroids as small as 140 m across (size of a large discount store). Infrared detectors as well as space telescopes (eliminates daytime glare) will be used.
The latest data suggest that there are 20,500 near Earth objects in the 100 meter range.
The big question is “what if a sizable asteroid is projected to impact the Earth a year or more in the future?” (If less than a year, there is insufficient time to prepare an interceptor mission.)
One important factor is that most near Earth asteroids rotate every few hours. Radar images of nearby asteroids allow us to determine the shape and geometry of these objects.
It is also estimated that 15 percent of near Earth asteroids are burned out comets, whose ice content is low because of repeated passages around the sun.
The simplest plan to deflect an asteroid is to send a massive space craft to collide with it.
A rocky asteroid of 100 meters width, if hit by a 5 ton spacecraft at 10 km/sec would change the asteroid’s velocity by 10 cm./sec or 4 inches/sec. Over a 10-year period, the asteroid’s position would be moved by 2 Earth’s radii.
Another approach is the gravity tractor, where a spacecraft is parked near the asteroid, slightly altering its path.
There are studies where a nuclear bomb is detonated close to the asteroid.
At M.I.T. in 1967, a senior course did a study to use a 100 megaton bomb launched on a Saturn V rocket (which carried astronauts to the lunar surface) to deflect an asteroid (named Icarus). There would be a trailing interceptor satellite to send back images of the asteroid following the explosion.
SKY SIGHTS AHEAD: Today at dusk, there will be a line up of the crescent moon and the brilliant planet Venus, seen best at 8:20 p.m. very low in the western twilight.
Tomorrow night, the moon will have moved above Venus and will be at the same level as the planet Saturn (to the right of the moon). On Sept. 12 the evening moon will appear half full.
This is the best occasion to view the lunar craters and mountains; using binoculars, look along the straight edge of the moon where the sun is rising.
Bob Doyle invites any readers comments and questions. E-mail him at firstname.lastname@example.org . He is available as a speaker on his column topics.