--------- #1309 - Vesta and Ceres Asteroids Get a Visitor?
- Attachment : Vesta Asteroid and Dawn Spacecraft.
- On September 27, 2007 we sent a spacecraft on a journey of 1,700,000,000 miles to visit an asteroid named “ Vesta”. The “Dawn” spacecraft became a satellite in orbit around Vesta on July 16, 2011. Its mission is to study two asteroids over the next four years.
- After Vesta the spacecraft will start another 2 ½ year trip to visit the asteroid “Ceres” which it will also orbit for several months. Lots of data is coming back from this mission already. Science is trying to learn about these rocks that were the same rocks that formed the planets 4,000,000,000 years ago. Vesta’s age is put at 4,567,000,000 years old.
- The entry into Vesta orbit is a gentle one. Both are orbiting the Sun at 46,000 miles per hour with their relative speeds only differing by 110 miles per hour. The initial altitude of orbit was 9,900 miles above the asteroid.
- These asteroids reside inside the Asteroid Belt that orbits between Mars and Jupiter. They represent a part of the original accretion disk that never clumped big enough to form planets.
- Cameras aboard the spacecraft have seven color filters each designed to isolate a specific wavelength. The data photographed will tell scientists the mineral composition of the asteroid’s surface. Other detectors will measure Gamma Rays and neutron emission that emit from the surface when it is bombarded by Cosmic Rays. Cosmic Rays are not really “rays” but particles, mostly nuclei of hydrogen and helium atoms. By studying these impact emissions scientists can identify the elements of potassium, thorium, and uranium. These elements can be identified down to a depth of 3 feet below the surface.
- In total over 400 different element wavelengths will be detected to allow science to identify the emission and absorption spectral lines unique to each element.
- The influence of the asteroid’s gravity on the spacecraft will alter its orbit slightly, speeding it up and slowing it down. These changes in speed will be detected as Doppler Shifts in the frequency of the data transmissions. With this additional data science can accurately calculate the mass of each asteroid. We calculate the volume and then the density of the asteroid to better understand the asteroid’s composition and internal structure. Data so far suggests Vesta to have a metallic iron-nickel core with a rocky olivine mantle and a surface crust.
- Several descending orbits will be used to gather more data. Initially the orbit will be 1,700 miles altitude. Each orbit taking 3 days. The asteroid itself completes one revolution every 5 hours, 20 minutes. At this altitude the resolution of the cameras will be down to 820 feet.
- Later a closer orbit will get to 410 miles altitude taking only 12 hours to complete one orbit. This elevation is designed to make a topographic map of the surface.
- The next orbit at 110 miles altitude takes only 4 hours to complete an orbit. This lowest altitude will allow collection of the highest resolution data and the Gamma Ray and neutron emissions data. Upon completion of these measurements Vesta will be ready to gently fire its ion rockets to proceed on its journey to Ceres. We will have to wait until 2015 to get first hand reports on this part of the mission. It takes another 2 ½ years to reach Ceres
- Already we have learned that Vesta is 330 miles diameter. There is an enormous crater on Vesta where it has lost 1% of its mass due to an enormous collision that happened 1 billion years ago. The Earth has collected some of this debris in meteorites that we have recovered on Earth.
- Vesta is 2.36 AU from the Sun, the Earth is 1 AU. Its mass is 2.59 *10^20 kilograms, 0.35% the mass of the Moon. The density is 3.42 grams / cm^3, about the same as our Moon which is 3.34 grams / cm^3. Earth is 5.52 grams / cm^3. Vesta’s temperatures range from -20 C to -190 C. From Earth Vesta can be seen with binoculars. Its brightness Magnitude is 6.1. 6.0 Magnitude brightness is considered the average limit for the naked eye.
- The next asteroid is Ceres which is the largest, it is 33% of the mass of the entire Asteroid Belt that contains millions of smaller asteroids, including Vesta. Ceres is 590 miles diameter and takes 4.6 years to orbit the Sun. Ceres’ mass is 9.43*10^20 kilograms, or 1.3% the mass of the Moon. Its density is 2.077 grams / cm^3. Pure water is 1.0 grams / cm^3. Ceres takes 9 hours to rotate. It is orbiting the Sun at 40,000 miles per hour. Early indications are that Ceres may have a weak atmosphere and water frost on its surface. It is thought to have a rocky core and an icy mantle, judging from density calculations. Wait till 2015 to learn the better answers.
- An interesting part of this mission is the spacecraft’s propulsion system. There is no way this spacecraft could carry enough conventional rocket fuel to complete this mission. Instead its fuel tank contains 937 pounds of Xenon. Xenon is neutral atoms having 54 protons, 54 electrons, and 70 to 82 neutrons, depending on the Xenon isotope. The rocket engine uses ion propulsion. To get the ions an electron beam is fired at the Xenon dislodging the atom’s electrons and causing the nucleus to be ionized with a positive charge. The thruster is a strong electric field that fires these positive ions out the back of the rocket at 89,000 miles per hour. Action equals reaction and the spacecraft gently reaches the orbital speeds it needs to reach the asteroids.
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