NASA’s Dawn Space Craft is already a success story. It was launched on board a Delta II rocket in 2007, has spent 14 months investigating the asteroid Vesta, is using ion propulsion for acceleration and is now en route to Dwarf planet Ceres, the largest object in the asteroid belt. When Dawn arrives at Ceres on March 6, 2015, it will be the first space craft to have orbited two astronomical bodies and the first to orbit a dwarf planet. The space craft has been described using “acceleration with patience” and hopefully patience will pay off this March!
Ceres is one of the dwarf planets as described in the International Astronomical Union’s re- organisation of the planets in 2006. This same reclassification saw Pluto demoted from the ninth planet to a dwarf planet. To be a dwarf planet the astronomical object needs to be orbiting the Sun and have enough mass to have a spherical shape; it cannot be a moon nor will it have cleared its orbit. Ceres is located in the asteroid belt between Mars and Jupiter, so therefore has not cleared its orbit of any debris. Until 2006, Ceres was often considered the largest asteroid and was actually the very first asteroid ever to be discovered.
Ceres was first discovered by Italian priest and astronomer Father Giuseppe Piazzi (1746-1826) on New Year’s Day 1801. Piazzi gave it the name ‘Ceres Ferdinandea’, after the Roman goddess of agriculture and grain and the King Ferdinand IV of Naples. Ferdinandea was later dropped for its political status. He originally believed it to a fixed star, by following its movements; he called it a new star. Piazzi knew that Titius-Bode law predicted that the planets are in a logical spacing from one another, but that the space between Jupiter and Mars was too great and that another planet should be there. Uranus’ discovery in 1781 fitted into the pattern perfectly.
Piazzi thought he had discovered a new planet, however as Ceres disappeared into the glare of the Sun, its orbit could not be accurately predicted or its planetary status confirmed. So publicly he announced his findings to the press as a new comet yet revealed to a friend and colleague that he believed it to be something “better”. It was confirmed as a planet but over the next few years other objects were discovered in the same orbit and Ceres then became classed as one of the asteroids and thus sparked the discovery of the asteroid belt. Titius-Bode law was also rejected after the discovery of Neptune in 1846.
Ceres has a diameter of just 600 miles (950km), and is showing signs that water ice may exist beneath the surface of the planet. Last year it was announced that ESA’s Herschel Space Observatory has detected water vapour plumes shooting out of Ceres occasionally. It’s believed this occurs when its ice surface melts slightly. This is not typical of objects in the asteroid belt. Instead this melting water vapour is more characteristic of a comet. When the Dawn spacecraft arrives at Ceres it will map the planet to measure its mass and shape. Instruments on board Dawn will also reveal the elements and minerals present. This orbit around Ceres should also reveal the cause of the water vapour. This mission is designed to compare the two different objects,both fragments left over from the formation of the Solar System and to give insight into early conditions.
The Dawn spacecraft itself is technologically advanced. Instead of using regular methods of thrust, Dawn is using ion propulsion to power its way. An electrical charge changes the xenon gas atoms in the engine, turning them into ions. The ions are expelled causing the craft to move in the opposite direction. The ion thrusters need to run for a long time. This means speed and momentum need to be gathered over time rather than whizzing along. Mission Director Marc Rayman reckons to reach 0-60mph it would take Dawn four days! This ion propulsion system is more efficient than chemically powered craft. However, the thrust is quite gentle, so Dawn has also used Mars’ gravity to swing to the correct inclination when headed for Vesta in 2009. Using this gravity assist method saved an extra 100kg of Xenon fuel being carried on board.
When Dawn arrives at Ceres in just over a month, we hopefully will learn more about our dwarf planet, whether it has more water than here on Earth and hopefully more about the early Solar System. In anticipation of Dawn’s arrival, NASA is encouraging you to imagine what this planet may really look like. So far we have a grainy image captured by the Hubble Telescope as well as artist’s impressions. Do you have an idea of what you think Ceres will look like? Enter here http://dawn.jpl.nasa.gov/dawncommunity/imagine_Ceres_about.asp . This year is already looking pretty exciting as we patiently wait until the 6th of March!
(Article by Martina Redpath, Senior Education Support Officer)