[meteorite-list] Vesta in Virgo: A Naked-Eye Asteroid

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:23:48 2004
Message-ID: <200303160542.VAA27828_at_zagami.jpl.nasa.gov>

http://skyandtelescope.com/observing/objects/asteroids/article_895_1.asp

Vesta in Virgo: A Naked-Eye Asteroid
By Greg Bryant
Sky & Telescope
March 16, 2003

During the first half of 2003, observers with binoculars and small
telescopes will be able to watch the asteroid 4 Vesta loop gracefully
through the constellation Virgo, making its most favorable return in
several years. On March 26th Vesta stands at opposition to the Sun and
can be observed throughout the night. For several weeks before and after
that date, with the help of the chart (left), it may even be glimpsed with
the unaided eye.

Vesta's favorable apparition continues through June and July, the
period when it becomes best placed (highest in the sky) for viewing
during early-evening hours. The constellation Virgo is famous for its
galaxies, and Vesta's journey takes it past many such deep-sky denizens
near the heart of the Virgo Galaxy Cluster. By the start of March Vesta
is already magnitude 6.4; it peaks at 5.9 on March 27th, then fades to 6.4
by end of April. It continues a gradual decline to 7.0 on June 1st, 7.4 on
July 1st, and 7.6 at the beginning of August.

For some observers the mention of Vesta brings back memories of moonlight
when it last reached opposition in our night skies, in July 2000. That
year the asteroid became even brighter, peaking at magnitude 5.4, but on
a night that coincided very nearly with the full Moon - and a total lunar
eclipse! This year the Moon is well out of the way for observers
wishing to catch sight of Vesta.

In the Footsteps of Vesta's Discovery

Among the tens of thousands of numbered asteroids in our solar
system, Vesta is the only one that ever becomes so easy to spot. It
actually brightens past magnitude 6.0 every few years. Such was the
night of March 29, 1807, when German physician Heinrich
Wilhelm Olbers (1758-1840) discovered the "unknown star" that
would soon be hailed as the fourth known asteroid.

Olbers is best known today for the paradox he investigated: why the
sky appears so dark at night (S&T: December 2001, page 44). But he
was no stranger to discoveries. He had earlier found 2 Pallas, the
second known asteroid, in 1802. Three comets bear his name, and
one of them returns every 70 years. He also devised an extremely
efficient method for calculating a comet's orbit that is still in
use today (for an initial solution, when the orbit can be assumed
to be parabolic).

Virgo was a charmed part of the sky for Olbers. In 1796 he found
his second comet south of Virgo's brightest star, Spica. In 1802 he
made the first recovery of 1 Ceres in Virgo (thanks to calculations
by mathematician Carl F. Gauss), a year to the day after its discovery.
A few months later he discovered Pallas in the northern part of Virgo. Five
years and one day after that, he likewise spotted Vesta in Virgo,
close to its current location. Vesta's track among the stars this year
is rather similar to the course it took 196 years ago.

Why So Bright?

Vesta is not unlike other asteroids, and indeed the major planets, in
that its brightness is not the same from one opposition to the next.
For example, if we look at the period 1990-2020, Vesta's peak brightness
ranges between 5.3 (June 2018) and 6.5 (November 1990). Over the next
few years Vesta will shine as brightly as 6.1 in September 2004, 6.2
in January 2006, and 5.4 in May 2007 - when, during the bicentennial
year of its discovery, it will again be brighter than Uranus.

Obviously, the position of Vesta along its elliptical orbit is a
contributing factor to the variation in visibility. The closer Vesta
is to the Sun in its 3.6-year circuit, the closer it is likely to be to
Earth. In 2003 Vesta doesn't reach perihelion, the point nearest the Sun,
until October 28th, seven months after opposition. Then it will be just
2.15 astronomical units (Earth-Sun distances) from the Sun, compared to
2.57 a.u. at aphelion, the far point.

Size is also a factor in Vesta's prominence, though not as much as you
might think. According to the 2003 Astronomical Almanac, Vesta is
believed to be 530 kilometers in diameter (though not a perfect sphere),
edging out Pallas's width of 524 km. These values stand well in the
shadow of Ceres' 933 km. In the main belt of asteroids between Mars and
Jupiter, 10 Hygiea ranks fourth place in size at 429 km. But no longer is
it correct to say that these are the four largest asteroids. Several
newly discovered objects in the far-flung Kuiper Belt - 20000 Varuna,
28978 Ixion, and 50000 Quaoar - may outrank even Ceres. Needless to say,
those in the main belt are much easier to hunt down than those lying far
beyond Neptune.

Given that Vesta is neither the largest nor the closest of the asteroids,
you might wonder what distinctive characteristic makes it so readily seen.
The answer lies in its albedo, or surface reflectivity. Asteroids like
Ceres reflect around 11 percent of the sunlight reaching their surfaces,
and others like 18 Melpomene have albedos as high as 22 percent. But
Vesta is geologically quite different and reflects a substantial 42
percent, much like a pink grapefruit! That's why it can become so bright
in our night sky.

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Contributing editor Greg Bryant writes a monthly column for Sky & Telescope about the
sky seen from south of the equator.
Received on Sun 16 Mar 2003 12:42:18 AM PST