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Night Sky

December 2011 Night Sky

Charlie Christmann


There are many objects flying around our solar system too small to be called planets. These small bodies come in all shapes and sizes—we know them as planetoids, minor planets, asteroids, and comets. These objects have remained essentially unchanged since they coalesced at the birth of our solar system.

Most of the asteroids lay in a wide ring between the orbits of Mars and Jupiter. This is the main belt, where scientists believe more than 750,000 asteroids larger than half a mile in diameter reside, and millions more that are smaller than that. We know of at least two hundred asteroids larger than sixty miles in diameter.

Main belt asteroids can reach enormous sizes. Ceres, discovered January 1, 1801 by Giuseppe Piazzi, is more than 580 miles across, large enough to be considered a dwarf planet (though it is the smallest dwarf planet in our solar system). Compare that to the smallest asteroid discovered so far: 1991BA, is only about 20 feet across. Billions of years of collisions have shaped theses bodies into irregular shapes, gouging large crates in their surfaces, and causing them to spin erratically. Like their larger planetary cousins, some asteroids even have moons of their own. More than 150 asteroids are known to have at least one moon. There are some instances of binary asteroids, where two roughly equal sized objects orbit each other. Even a triplet asteroid has been found. Vesta, discovered by Heinrich Wilhelm Olbers on March 29, 1807, is the second largest object in the main belt. Vesta shows just how bad the damage can be from these collisions. Its largest crater is 285 miles in diameter.

Most asteroids fall into three classes based on composition. The C-type (or carbonaceous) are greyish in color and are the most common, making up more than 75 percent of known asteroids. They probably consist of clay and stony silicate rocks and inhabit the main belt’s outer regions. The S-type (or silicaceous) asteroids are greenish to reddish in color. These account for about seventeen percent of known asteroids and dominate the inner asteroid belt. These are made of silicate materials and nickel-iron. The M-type (or metallic) asteroids are reddish in color and make up most of the rest of the asteroids. These dwell in the middle region of the main belt and seem to be made up of nickel and iron.

Escapees, wandering outside of the main belt, have been known to have close encounters with the planets. Mars’ moons Phobos and Deimos, as well as the outer moons of Jupiter, Saturn, Uranus, and Neptune are thought to be captured asteroids that wander too close. Other wandering asteroids cross the orbits of Mars and Earth.

As of the end of October, 1256 asteroids have been classified as “potentially hazardous,” coming within five million miles to Earth. Luckily, none are known to be on a collision course with us.

On November 8, we had a close shave with asteroid 2005 YU55. This quarter-mile wide rock passed 201,900 miles from Earth, inside the Moon’s orbit. Astronomers enjoyed the chance to study this one up close and personal, since they had plenty of warning. This asteroid was discovered December 28, 2005 by Robert McMillan of the Spacewatch Program near Tucson Arizona.

This is the closest known approach by an asteroid of this size since 2010 XC15 missed Earth by 125,000 miles in 1976. The next known asteroid of this size to come our way will be 2001 WN5 in 2028. It should miss us by 154,000 miles.

As for our friend, 2005 YU55, it will pass 210,000 miles from Venus on January 19, 2029 and again menace Earth in 2041.


The Fall premier meteor shower, the Geminids, peaks on the 14th. This year, the moon rises at mid-evening and is up until sunrise, spoiling much of this shower’s glory. The best viewing of these meteors should be after midnight on December 14 and 15.

The Planets and the Moon

  • Check for Mercury starting mid-month rising in the east before sunrise.
  • Venus will be low in the southwest early in the month, rising higher each evening after sunset. One hour after sunset, see Venus and the Moon low in the southwest on the 26th.
  • Mars rises in the southeast from 11:00 p.m. early in December to 10:00 p.m. late in the month. See Mars and the Moon one hour before sunrise on the 17th in the southwest.
  • Jupiter is high in the east, shining brightly in the southeast after sunset. The Moon will be six degrees to the upper left of the waxing gibbous moon on the 6th.
  • Saturn rises in the east from 2:30 a.m. early in the month to 1:00 a.m. later in the month. A Saturn-Moon conjunction occurs on the 20th.
  • The Moon is full on the 10th at 7:36 a.m. and new at 11:06 a.m. on the 24th.
  • Winter begins at 10:30 a.m on the 22nd.
  • Early morning on the 25th, look for a fast moving red “star” bouncing from house to house.




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