Chimney Rock moonrise, December 2004 photo by Helen Richardson

by Kim Malville

Sky Almanac

Before the chill of winter sets in, October is a good month for meteor-watching. There are two meteor showers this month, streaming out of the constellations of Orion and Taurus.  The Taurids come from a very broad stream of comet debris and the shower runs throughout October, November and part of December. Under dark skies one might be able to spot six per hour of these slow and graceful meteors. Amongst the small particles of the meteor stream there are some larger chunks from the breakup of the comet and sometimes there are spectacular fireballs. If you see one, trace it backward to see if it intersects with Taurus. There is the possibility that the brightest meteor in recorded history that produced the 10-15 megaton Tunguska was a Taurid. So, stay alert!

The Orionids are just the opposite kind of meteor. They are fast and come from a narrow meteor stream, peaking between October 20-24. They radiate from the top of Orion’s club, arriving at a rate of 20 meteors per hour. They are the results of debris peeling off of the icy head Comet Halley.

Mars is the only bright planet visible in the evening. Jupiter rises about 10pm in the beginning of the month. For early risers, Venus is a spectacular object rising about three hours before the sun.

October 1-4: Venus and Regulus come close to each other at dawn. October 3 will be the best time for viewing the conjunction. Venus will 150 times brighter than Regulus and the pair will be a wonderful sight in binoculars.

October  17-18: The crescent moon forms a compact triangle with Mars and Antares at dusk.

October 20-24: Orionid meteors.

Jupiter, the giant Hoover, is hit again

A megaton explosion on Jupiter photographed by George Hall on September 10.

An impact on Jupiter early Monday, September 10, created a fireball on the planet so large and bright that amateur astronomers on Earth spotted the flash. Judging from the brightness of the flash, it released megatons into the atmosphere of Jupiter, produced either by a comet or an asteroid. This power is about 5-10 times less than the greatest explosion in human history, the Tunguska explosion in Siberia on June 30, 1908. Last month’s impact was actually the fourth report of objects striking Jupiter since July 2009. One of the most dazzling Jupiter impacts was the Shoemaker-Levy 9 comet crash in 1994. The comet broke into more than 20 pieces as it approached Jupiter, with each piece crashing into the giant planet.

While Earth comes close to getting hit by a 30-foot-wide object about every 10 years, Jupiter gets hit with these objects a few times each month. The giant plant is a huge gravitational Hoover, sucking up comets and asteroids that come within its reach. Thank you Jupiter: because of you, there are fewer dangerous object near the earth.  But we still need to be vigilant. On September 8 this year, a 30-foot-wide asteroid flew close to our planet without hitting us. A slightly smaller rock burned up in the atmosphere above Sudan in 2010.

Goodbye Vesta

Vesta, the asteroid wishing to be a Dwarf Planet courtesy NASA

On September 4, 2012, the Dawn robotic spacecraft left Vesta, the second most massive asteroid, and headed for the largest asteroid, Ceres, which it should reach in 2015. Dawn was launched on 27 September 2007, and almost four years later, on 16 July 2011, it began orbiting Vesta’s orbit, the first spacecraft ever to visit this tiny world lying between Mars and Jupiter. Vesta looks old, a leftover from the early years of our solar system. Vesta’s surface shows heavy cratering and long troughs created by ancient impacts. Its low gravity allows for surface features like huge cliffs and one of the highest mountains in the solar system, which at 12 miles is twice the height of Mount Everest. Such a mountain (visible on the bottom of the figure) is a spectacular ornament on a rock that is only 326 miles in diameter.

Vesta turns out to be an utterly fascinating object, the last remaining example of the kind of rocky proto-planet that formed the terrestrial planets (Mars, Earth, Venus and Mercury). Other proto-planets in the asteroid belt have been destroyed by collision. Vesta has suffered two major collisions, but has remained intact. It lost some 1% of its mass less than a billion years ago in a collision that left an enormous crater occupying much of its southern hemisphere. That collision scattered material that is normally deep underground and has reached the earth as some 200 meteorites. As a result of these meteorites we have an extraordinary knowledge of the interior of Vesta. In particular, it has a nickel iron core, which has a radius half that of the asteroid. The core has now cooled, but in the past the molten core was a magma ocean, some of which erupted, spreading magma across the surface. Its rotation rate is 5 hours, which is fast for an asteroid. That rapid rotation plus a molten metallic core means that it once had a strong magnetic field, which is confirmed in some of the meteorites.

Vesta’s shape is close to an oblate spheroid formed by its gravity. Such a shape is a fundamental characteristic of a planet. However the major irregularities such as collision basins and high mountains have so far prevented it from being identified as a dwarf planet like Pluto and the largest of the asteroids, Ceres.

Vesta is currently one of only six identified bodies in the solar system for which we have physical samples. The other others are from the Earth, of course, meteorites from Mars and our moon, and samples returned from our  moon, the comet Wild 2, and the asteroid Itokawa. Rocks from the earth and meteorites are the cheapest way to collect these samples.  We just have to wait for them to come to us, hoping that they will not be too large.