August 2011

Tony Martinez

Each month I will describe sights of interest in the night skies of South Carolina. These sights will be broken down into three sections; what you can see with the naked eye, with binoculars, and with a small telescope. The best time to view the night sky is at and around the times when the Moon is not visible, what is known as a New Moon; which will occur this month on August 28th. For August, your best viewing nights will be from August 1st through August 5th and August 20th through August 31st. The Star chart below is set for Florence, SC on August 15th at 10 pm.


This month I would like to challenge everyone to try to locate the asteroid Vesta. Vesta is now in an empty region of the sky, making it difficult to locate. However, the reason that I am featuring Vesta this month is that NASA’s Dawn space probe just went into orbit around this asteroid on July 17th. This is the first probe that has been placed in orbit around a main belt asteroid. Dawn will orbit Vesta for about one year, at which time it will leave Vesta and visit the largest asteroid, Ceres.

When we hear the term asteroids, we immediately think of a wide band of rocky objects between the orbits of Mars and Jupiter. Indeed the vast majority of the asteroids are located in this “Main Asteroid Belt.” However there are two other groups of asteroids that rotate around the Sun. The second group lies in the same orbit as Jupiter. These asteroids are called “Trojans” and “Greeks,” and are located 60 degrees behind and in front of Jupiter. The third group of asteroids is called “Near Earth Asteroids” (NEA’s).

The NEA’s are especially important to us because they are in orbits that may result in a collision with Earth. Even a small asteroid only a few miles across would cause devastating effect if it hit our planet. Several small NEA’s have been found that will make very close passes by our Earth in the next fifteen to thirty years.

Before I help you locate Vesta, I would like to discuss the origins of the main asteroid belt. Over the years there have been various theories about the formation of the asteroid belt. First, consider the layout of our solar system. The four planets closest to the Sun are rocky planets (Mercury, Venus, Earth and Mars). Then there is a large “gap,” filled with rocky asteroids, followed by the gaseous planets, (Jupiter, Saturn, Uranus and Neptune). Some theories state that a rocky planet had formed between Mars and Jupiter, and the gravitational forces of Jupiter tore the new planet apart. Another theory is that the asteroids are merely rocky solar system debris that never formed a planet. I personally like the second theory, but it is seldom explained further. So let me try to elaborate. The theory concerning the formation of the inner planets is that after our Sun formed, the rocky/metallic debris that circled near the Sun was extremely hot and was therefore in a molten state. Gravitational forces caused some of these molten pieces to collide, and form larger pieces. As these larger molten pieces grew in size, they then attracted more pieces until they became a planet like our Earth. The key here is when molten or semi-molten objects collide, they can be easily combined to make larger molten “globs.” Also, because the new planets were molten, and spinning, they easily formed into spheres instead of irregular shapes. This process of planet building can take millions of years, and during this time the molten debris could cool down and become solid. The farther away the molten rocks were from the Sun, the faster they could cool down. The farthest rocky/metallic molten rocks were in the region of the asteroid belt, and being so far from the sun, they could cool down much faster than the regions that would form the Earth or Venus, etc. So the debris in the region of the asteroid belt most likely cooled down to solid rocks before they could form into larger molten pieces. Once they cooled to solid debris, when they hit other debris they would more likely bounce off then be adsorbed as did the molten pieces nearer to the Sun. Only the largest asteroid Ceres (about 500 miles across) formed into a sphere, implying that it formed in a molten state. We hope the Dawn Probe will give us better understanding into the true origin of the asteroid belt.

Now the challenge to locate Vesta. This month Vesta is in the constellation Capricorn. Capricorn is not an easy constellation to see, but it is just to the left of the constellation Sagittarius. Sagittarius is in the south, and is an easy to spot summer constellation, known as the “Teapot Asterism.”

To locate Vesta, we will use a stepwise method called star hopping. Vesta can be seen with binoculars, so we will do the star hopping with seven power binoculars. To star hop, you start with an easy to find target that fits in you binocular’s field of view (FOV), and then move step wise to other targets that can fit into your binocular’s FOV.

We will begin Step 1with the “handle” part of the Sagittarius “teapot” asterism. This handle will fit into your FOV, see below. Now slowly scan to the left until you see the Step 2 group of stars in your FOV.

Next scan more to the left and find the Step 3 right triangle. This month, Vesta will move into and though this right triangle.

Good luck in your search.

Naked Eye Sights: The well-known Perseids meteor shower peaks on the 13th, however, the full Moon will wash out all but very large meteors.

Binocular Sights (7 to 10 power): Vesta.

Telescope Sights (60-100mm): Once you find Vesta using binoculars, try to locate it with a telescope.

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