SPACE.com -- A Bright Asteroid In the Evening Sky


	A Bright Asteroid In the Evening Sky By Jeff Kanipe Special to SPACE.com posted: 04:00 pm ET 14 July 2000

The asteroid Vesta, fourth-largest of the minor planets, but the only one capable of reaching naked-eye level, reaches opposition tonight

The asteroid Vesta, fourth-largest of the minor planets, but the only one capable of reaching naked-eye level, reaches opposition tonight. The asteroid rises as the sun sets and by midnight is overhead -- fully lit by the sun, and at its brightest. Hence, the rest of this month and into early August is an ideal time to look for this mountain-sized object.

Vesta is currently ensconced in the eastern reaches of the constellation Sagittarius. At magnitude 5.2, it is technically bright enough to be seen with the unaided eye. With the moon in the vicinity for the next few nights, however, this will prove difficult. On the other hand, the moon's light washes out numerous fainter stars, leaving the brighter ones still visible. This might make it easier to find in binoculars or a small telescope.

Vesta appears brighter than the three larger asteroids because its surface is unusually reflective. Astronomers refer to the reflecting strength of a planet or asteroid as its "albedo." Basically albedo is the ratio of the total amount of reflected light to the total amount of incident light.

An albedo value of 1.0 indicates a perfect reflecting surface while a value of 0.0 is a completely light-absorbing surface. Vesta's albedo is 0.38, which compares closely with that of Earth at 0.37. Venus wins the reflectivity contest hands-down. Its albedo is 0.65.

Use today's star map to isolate its location. Look just north of the kite-shaped asterism made up of four 4th-magnitude stars in southeastern Sagittarius. Vesta will appear fainter than these stars but slightly brighter than nearby HIP 97944. The whole region depicted in Map B fits into the field of view of 7x50 binoculars. Over the next few nights, the asteroid moves incrementally westward against the fixed stars.