Two bright asteroids are easily observable this month and next. Hovering at the naked-eye limit in brightness, all you need is a small pair of binoculars to track these slowly moving star-like objects from night to night.
Long before the first asteroid was discovered, astronomers took notice of a large gap between the orbits of Mars and Jupiter. Finally, on January 1, 1801, an Italian monk discovered Ceres, a 575-mile diameter rock circling the sun in the zone between the two planets.
Over time more asteroids or "minor planets" were discovered - Pallas in 1802, followed by Juno in 1804. Astronomers started suspecting that these objects were the remnants of a destroyed or ill-formed planet. The fourth asteroid, Vesta, was discovered in 1807.
The "asteroid belt" had been discovered. To date more than 200,000 individual asteroids have been discovered around the sun.
Not all asteroids occupy the primary belt. "Trojan" asteroids are trapped in Jupiter's orbit in two equally spaced clumps ahead of and trailing the planet. Balanced between the sun's and Jupiter's gravitational attraction, the Trojans maintain a constant distance from the planet.
Other asteroids live closer to the sun. "Amor" asteroids occupy the space between Mars and Earth. "Aten" asteroids circle the sun within the Earth's orbit. "Apollo" asteroids include the so-called "Earth crossers." Apollos are in elliptical orbits carrying them closer and farther from the sun than the Earth gets.
What about asteroids in the same orbit as the Earth?
Late last month, in July, NASA announced the discovery of Earth's own Trojan asteroid. This asteroid, currently designated "210 TK7," orbits the sun ahead of the Earth at a fixed distance of about 50 million miles.
Much farther out in the solar system another asteroid belt was discovered in recent years beyond the orbit of Neptune. This collection of icy bodies, nicknamed the "Kuiper Belt," includes the demoted dwarf planet Pluto as well as at least one object, named Eris, that astronomers believe is even larger than Pluto.
Now, back to the August sky. Asteroids Ceres and Vesta are well placed this month and the latter hovers at naked-eye visibility. Vesta is around magnitude six and Ceres is somewhat fainter, at magnitude eight. The naked eye limit is typically considered to be magnitude six or seven, depending on each individual's visual acuity.
Ceres is in the Constellation Cetus, moving into Aquarius in September. Asteroid Vesta spends August in the constellation Capricornus.
Downloadable star charts for each is available online from Sky & Telescope magazine. Information and a link to detailed charts may be found at www.skyandtelescope.com/observing/objects/asteroids. If you need any assistance locating the asteroids in the sky, print out the charts and bring them with you to the next Westminster Astronomical Society public star party at Bear Branch Nature Center September 2. Don't worry. Both asteroids will still be visible in the night sky in September.
For information about this and other activities, see the Westminster Astronomical Society public calendar at www.westminsterastro.org.
Coincidentally, NASA's Dawn spacecraft is currently orbiting asteroid Vesta. Current plans also include a future visit by Dawn at Ceres as well.

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