SKY REPORTER: Winter Rendezvous

These two views of Ceres were acquired by NASA's Dawn spacecraft on Feb. 12, 2015, from a distance of about 52,000 miles (83,000 kilometers) as the dwarf planet rotated. The images, which were taken about 10 hours apart, have been magnified from their original size.
Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

The ever shrinking roster of interesting objects in the Solar System not visited by machines from Earth will soon lose another member with the arrival of the Dawn spacecraft at the planet, minor planet, asteroid, (now) dwarf planet Ceres, orbiting the Sun between Mars and Jupiter. The mission, en route since 2007 driven by a revolutionary ion propulsion system, is now approaching Ceres a 600-mile-wide, roughly spherical world.
 
Dawn is managed by the Jet Propulsion Laboratory on behalf of NASA's Science Mission Directorate. UCLA has overall responsibility for Dawn’s science projects with instruments provided by teams from Germany, Italy, and the United States.
 
The mission is being propelled to Ceres by its novel and highly efficient ion thrust system. Electricity from solar panels ionizes Xenon gas which is rapidly attracted to and through perforations in an electrified plate, thereby producing an action-reaction thrust force on the plate and surrounding spacecraft. Before the ions are electrically yanked back toward the plate, counteracting the craft’s forward thrust, they are neutralized by an injected flow of electrons.
 
Designed to orbit the dwarf planet as it had previously orbited the asteroid Vesta, the mission’s eyes include a camera sensitive to visible light, a spectrometer for observing aspects of visible and infrared light, and another sensitive to gamma rays and neutrons. In addition, information gleaned from navigational data is expected to provide insights into the mass and internal structure of the dwarf planet.
 
Unlike Vesta, Ceres appears more akin to icy moons orbiting Jovian worlds than it is to terrestrial planets such as Earth. Comparisons between Vesta and Ceres are expected to reveal fundamental insights into how a vast nascent protosolar nebula of gas and dust contracted four and a half billion years ago and evolved into today’s Solar System.
 
Other important questions relating to the mission include the quest for specifics about a significant mass of water believed to exist beneath Ceres’ surface. Before we knew much about other worlds, Earth was sometimes described as a unique “water planet.” Space age studies show that water is not uncommon in the universe, with substantial amounts in comets, certain asteroids, a number of Jovian planet moons, and interstellar space.
 
Ceres was famously discovered by Giuseppe Piazzi on New Year’s Day 1801, the first day of the 19th century—remember that controversy about which year the new millennium began? Piazzi founded Palermo Observatory and used its three inch diameter telescope for his historic detection. It was attached to a five-foot-wide mounting circle with coordinate scales that were read with microscopes, and was regarded as the most accurate astronomical measuring system of its time. Piazzi was part of an organized hunt for an unknown planet believed to exist between the orbits of Mars and Jupiter. The discovery earned Ceres a temporary planetary designation, before its rather diminutive size became known.
 
During February the dwarf planet Ceres rises about two hours before the Sun, appearing as a faint ninth magnitude object in the direction of the constellation Sagittarius, just east of its Milk Dipper asterism.

The Moon

Planets

Mercury may be glimpsed during morning twilight during February. It is at greatest western elongation from the Sun on Tuesday the 24th when apparent separation is 27 degrees of arc.

Venus is now established as an easily seen feature of the southwestern sky for over an hour after twilight fades. During February this most brilliant planet sets progressively later each night. On the evening of the 20th from about six p.m. until nearly eight, the crescent Moon may be seen just two degrees from Venus, in the southwest, with Venus in turn less than a degree of arc from Mars. The two day old lunar crescent is thin and the trio offers a remarkable sight. The next evening the Moon will have moved toward the east and Venus appears even nearer Mars with their separation then being just 0.4 degree of arc, slightly less than the Moon’s diameter.

Jupiter was at opposition Friday the sixth, when we were at our closest for the year and the giant planet shines bright throughout February nights. Its apparent magnitude at opposition was –2.6 compared to –3.9 for Venus. Both planets are seen at nearly equal altitudes above the horizon at about 6 p.m. all month, Venus to the southwest and Jupiter in the southeast.