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There are currently two unmanned U.S. space missions currently at or preparing to arrive at dwarf planets in our solar system. Their names are Dawn and New Horizons and both have been traveling for years on long journeys getting to their target destinations.

A few years back, Pluto was demoted from major planet to dwarf planet. But Pluto is not the first "planet" to face demotion. On New Years Day in 1801 — the first day of the 19th century — a small object was discovered orbiting the sun in the gap between Mars and Jupiter. It was the culmination of a search for what was presumed to be a missing planet existing in that void. Named Ceres, the small world was heralded as a newly discovered planet.

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As more and more objects were discovered in the same general band around the sun, it was realized that they were all members of a new class of objects now known as asteroids. And so, all of these new planets were reclassified as "minor planets" and the number of known major planets remained at eight due to the discovery of Neptune in 1846.

Ceres managed to gain some ground back in 2006 with the reclassification of planets by the International Astronomical Union (IAU). Ceres was promoted from minor planet to the same "dwarf planet" status along with poor demoted Pluto.

The Dawn spacecraft was launched in 2007 from Cape Canaveral, Fla. Sixteen months later it sailed only 340 miles above Mars at its closest point which gave Dawn a gravity-assisted boost. Some two years later in 2011, Dawn entered orbit around Vesta, a giant 326-mile diameter asteroid. It analyzed the asteroid for nearly 14 months until its departure in 2012.

You may be asking yourself, if Vesta is such a huge asteroid, then why is it not considered to be a dwarf planet as well. The answer is rules, rules, rules! Vesta was considered too shapely by the IAU to be a true dwarf planet. Dwarf planets are supposed to be round, you see – "ellipsoidal" to use the precise nomenclature. Vesta has a huge crater making her physically unfit and therefore is designated as one of many thousands of other minor planets even though Vesta is the second-largest asteroid after Ceres.

In March, Dawn entered orbit around the 590-mile diameter dwarf planet Ceres. Even during its approach Dawn was returning tantalizing views. In one crater, two small bright objects were observed. Some scientists think they are specular reflections perhaps from ice deposits. After all, Ceres is thought to be one quarter ice. Dawn should remain active as it orbits Ceres until the end of the year when its primary mission comes to an end.

The Dawn spacecraft has four major scientific instruments. Its Framing Camera takes black and white images. Color images can be produced by shooting several black and white photos through color filters and combining them. The Visible and Infrared Spectrometer measures the intensity and bandwidth of reflected light while mapping planetary surfaces in order to determine properties such as temperature. The Gamma Ray and Neutron Detector maps the distribution and abundance of elements on the surface. The Gravity Science experiment measures subtle shifts in signals bounced off the surface in order to determine a body's mass and other properties.

Next month we'll talk about the New Horizons mission which is expected to fly past Pluto in July.

Curtis Roelle is a member of the Astronomical Society. His website is http://www.starpoints.org, and he can be reached at starpoints@gmail.com.

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