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September Night Sky—looking south at 8:00 p.m. on September 15

September 2011 Night Sky

Charlie Christmann

Minor Planets

Orbiting between Mars and Jupiter, a collection of rocks called asteroids circle the sun. Several hundred thousand of them in the asteroid belt have been discovered and given a designation. More are being discovered every year. Astronomers believe that there are even more that are too small to be seen from Earth.

It was Giuseppe Piazzi who, on January 1, 1801, first discovered an object which he first thought was a new comet. But, after determining its orbit more precisely, it became clear that this object was more like a small planet. He named it Ceres, after the Sicilian goddess of grain. Ceres is the largest known asteroid in the belt.

Astronomers of the day had been looking for something to occupy the space between Mars and Jupiter. Johann Daniel Titius formulated a hypothesis in 1766, after the discovery of Uranus, that the location of planets in the solar system follows a simple arithmetic rule: a=(n+4)/10, where n=0, 3, 6, 12, 24, 48... Mars filled the slot for n=12 and Jupiter fills the n=48 position. Position n=24 was missing. Ceres fills the bill perfectly.

A year after Ceres was discovered, astronomer Heinrich Olbers unexpectedly found another body between Mars and Jupiter that was almost as bright as Ceres. He called it Pallas after Pallas Athena, an alternate name for the goddess Athena. It was Sir William Herschel who coined the term “asteroid” for any object too small to be a “planet,” but not a comet.

From there, the rush to find asteroids was on. Astronomers spotted Juno in 1804 and Vesta in 1807.

Recently, NASA sent the spacecraft Dawn to investigate the asteroid Vesta up close. The craft entered orbit in June of this year. It took four years for the ion-powered spacecraft to reach Vesta, the second largest known object in the asteroid belt with a diameter of 330 miles. Pictures returned to Earth show a surface that is thought to be made from basaltic rock that flowed out of a presumably hot interior, and then quickly cooled, when the asteroid formed some 4.5 billion years ago. Vesta may hold one of the oldest and most pristine surfaces in the solar system.

Earlier observations by the Hubble Space Telescope showed a large impact crater near Vesta’s South Pole some 285 miles across and eight miles deep. Compared to the size of the asteroid, this is a monstrous crater. A proportionately sized crater on Earth would be roughly the size of the Pacific Ocean.

Photographs from Dawn show huge ripple-like grooves stretching around the asteroid’s equator. The groves are believed to be a result of the crushing impact that happened at the South Pole.

Vesta is thought to be the source of a large number of meteorites that fall to Earth. In October 1960, two fence workers in Millbillillie, Western Australia, observed a fireball overhead, and pieces of the fallen meteorite were found ten years later. Unlike most meteorites, this one could be traced to Vesta by its unique chemical signature. Both the meteorite and Vesta contain pyroxene with a different combination of oxygen isotopes. This is a common mineral found in lava flows, meaning that the meteorite was created in an ancient lava flow on Vesta’s surface. The meteorite also has the same pyroxene signature as other small asteroids, recently discovered near Vesta, that are considered chips blasted off Vesta’s surface. This debris extends all the way to an escape hatch region in the asteroid belt called the Kirkwood gap. This area of the solar system is swept clean of asteroids because Jupiter’s gravitational pull sweeps material from the main belt and nudges it onto a new orbit that crosses Earth’s path around the Sun.

For more pictures and information on Vesta, see NASA’s website at www.dawn.jpl.nasa.gov. For a personal view of Vesta, use a small telescope to find it in the southern September night sky around 8:00 p.m.

The Planets and the Moon

  • Mercury will be in the eastern sky before sunrise the first part of the month.
  • On the 3rd, look one hour before sunrise near the horizon; Mercury will be at its greatest elongation (highest in the sky).
  • Look for Castor, Pollux, and Mars about 10 degrees above Mercury and Procyon, 30 degrees to the upper right. Bellatrix, Betelgeuse, and Sirius will be nearby.
  • Venus will be in the sun’s glare this month.
  • Mars is in the East before sunrise. A Mars-Moon conjunction occurs on the 23rd.
  • Jupiter is in the east-northeast after sunset.
  • Two hours after sunset on the 16th, look for the Moon seven degrees to the left of Jupiter.
  • Saturn will be low in the west at sunset.
  • The Moon is full on the 12th and new on the 27th.
  • Fall begins at 3:04 a.m. on the 23rd.
   

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