Star Points for August 2011 by Curtis Roelle
Solar System's Largest Asteroids Visible in August

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 thousand 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 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 Earth 
gets.
 
What about asteroids in the same orbit as Earth? Late last month, in July, NASA announced the 
discovery of Earth's own Trojan asteroid.  This asteroid, currently designated "2010 TK7," orbits 
the Sun ahead of 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 6 and Ceres is somewhat fainter, at 
magnitude 8.  The naked-eye limit is typically considered to be magnitude 6 or 7, 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 
http://www.skyandtelescope.com/observing/objects/asteroids/122249184.html . If you need any 
assistance locating the asteroids in the sky, then print out the charts and bring them with you to the 
next Westminster Astronomical Society (WASI) public star party at Bear Branch Nature Center on 
Friday, September 2.  WASI members should be happy to assist you. Don't worry. Both asteroids 
will still be visible in the night sky in September. 

For information about this and other activities, see the WASI calendar at 
http://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.