Bakersfield Night Sky – July 16, 2011
By Nick Strobel
Saturn and Mercury grace our sky this evening. Both are in the western sky by the time the sky gets dark enough to see them. Mercury will be low near the western horizon by half an hour after sunset. As shown in the second star chart below, it will be the brightest star low in the west. Up a little higher will be Regulus at the end of the sickle part of Leo. We have about five more evenings of decent viewing of Mercury before it drops back down toward the Sun. By the 26th, Mercury and Regulus will be close enough together to comfortably fit within the same field of view of your binoculars. Saturn will be in the southwest about a third of the way up in the sky on the right side of Virgo. To its left will be Spica on the left side of Virgo. The stars of Scorpius, Sagittarius, Ophiuchus, and the central bulge of our galaxy, the Milky Way, will be rising in the southeast at that time. They will be in the south by about 10 PM. The Waning Gibbous Moon will be rising at about 9:30 PM with the stars of Capricornus. Above the Moon will be the bright stars of the Summer Triangle: Deneb on the left in Cygnus, Vega at the top middle in Lyra, and Altair on the right in Aquila. Above Vega and almost directly overhead will be the bowtie or butterfly shape of the central part of Hercules. The southeastern view is shown in the first star chart below.
The king of the planets, Jupiter, rises at about 1:30 AM between the dim stars of Pisces and Aries. Orange-red Mars is now mid-way between the horns of Taurus. On the morning of July 25th Mars will be on the line between the tips of Taurus' horns. The Waning Crescent Moon will be next to the Pleiades star cluster at the shoulder of Taurus, close enough to fit within the same view of your binoculars. Two early mornings later an even thinner crescent Moon will be right next to Mars. See the third star chart below.
In other astronomy news the Dawn spacecraft is expected to begin orbiting the third largest asteroid, Vesta. I say "expected to" because Dawn's orbit insertion won't be the usual slam of rockets firing to slow it down in a few minutes time. Dawn uses a newer type of propulsion once found only in science fiction stories called "ion propulsion" instead of the usual chemical rockets. Dawn shoots a beam of Xenon gas out the back end at a speed ten times greater than the usual rocket's propellant. The "ion" part of the name comes from the Xenon is charged while it is accelerated to such incredible speeds (the Xenon is neutralized as it leaves the spacecraft). Despite such fast speeds, the overall thrust is very small—about as much as a piece of standard computer printer paper pushing down on your out-stretched hand. Ion propulsion achieves speeds much faster than chemical rockets by accelerating a L-O-N-G time. Its changes in speeds (including orbit insertion) are very gradual. We won't know that Dawn is in orbit until a few days after the fact because only very careful measurements will show when Vesta's gravity has changed Dawn's speed enough to put it in orbit.
Dawn's main goal is to figure out the formation process of the planets. During their formation the planets grew from dust grains clumping together to form rocks that in turn stuck together to make boulders. Those boulders accumulated into protoplanets that then collided to make the planets. In some parts of the solar system this process proceeded faster than other parts. Jupiter was able to grow very fast and its enormous gravity stirred up the protoplanets between it and Mars sufficiently enough to prevent those protoplanets from becoming a planet. Vesta and the dwarf planet Ceres are the two most massive protoplanets in the asteroid belt (Pallas is slightly larger than Vesta but less massive). Ceres will be investigated by Dawn in a few years after Dawn finishes exploring Vesta for a year and then slowly accelerates away toward Ceres on the other side of the asteroid belt. Such a trajectory would be impossible with chemical rockets but is no problem for ion propulsion with a bit of patience. Ceres and Vesta appear to be quite different from each other: Ceres is more primitive and seems to have a lot of water (perhaps as much as a 100-kilometer frozen layer below its rocky surface) while Veta is more evolved and very dry. Why are they so different? How did their size and the amount of water affect their development (and by extension the planets)?
Asteroid missions also help us to determine the mineral resources available for possible future extraction and the internal structure of the asteroids. Besides all of the scientific information, knowing the internal structure would enable us to know how to best nudge an asteroid away from a collision with the Earth. Unlike what you see in the movies, we can only give small nudges to asteroids and comets heading our way and trying to blow them up would more likely lead to multiple (smaller) chunks heading our way instead of just the one larger piece. Also, the internal structure will determine how the asteroid or comet would absorb the energy of a nearby blast from a nuclear warhead—a rubble pile responds differently than a single solid, stiff piece the same size as the rubble pile. If the small nudge is given while the asteroid is still far away from the Earth, the asteroid will cross the Earth's orbit at a significantly different place than where the Earth would be.
Finally, speaking of asteroid collisions, are there any dangerous asteroids heading our way? No. No large ones that are going to hit the Earth in the next hundred years at least. Large asteroids a hundred meters to a kilometer in size would cause tremendous damage on a local scale. Larger than that, the effects of a collision would be global. The asteroid that buzzed by Earth on June 27th was only 5 to 20 meters in size. Every so often I will get a call from someone worried about some upcoming asteroid or comet collision they heard about on a website, in an email message, or on a TV show (those stories seem to come about as often as the end-of-the-world stories). Usually, the story has taken a grain of truth and blown it all out of proportion. To find the grain of truth, here's what I do: I check out the news headlines on NASA's Near Earth Object Program website at http://neo.jpl.nasa.gov and the IAU's Minor Planet Center at www.minorplanetcenter.net. I will also look at Sky and Telescope's news section at www.skyandtelescope.com for something that hasn't been reported yet in the weekly news email they send out. Take a look at the NEO and MPC websites when evaluating any scary story of an upcoming collision. There are a lot of "junk science" websites and TV shows out there, most designed to capture your attention by scaring you in order to sell the products advertised on the website or show. Such techniques also work in political campaigns too but let's get to that later when we're in the midst of the election year next year. <smile>
Want to see more of the stars at night and save energy? Shield your lights so that the light only goes down toward the ground. See www.darksky.org for how.
Director of the William M Thomas Planetarium at Bakersfield College
Author of the award-winning website www.astronomynotes.com