http://www.skyandtelescope.com.au 59NASA / UNIVERSITY OF TENNESSEE
spherical; it’s missing a good chunk out
of its south pole. It’s also differentiated,
meaning it has a crust, a mantle and
a core, like Earth. This layering was
predicted by spectral analyses pre-
dating the Dawn mission, as well as by
studies of meteorites thought to have
originated on Vesta (see sidebar on the
next page), but spacecraft data helped
confirm these theories. Vesta, mostly
intact, fully differentiated, is unique
among asteroids. That we can see it by
simply looking up at the right part of
the sky is great, to say the least.Where to find Vesta
Vesta’s in a busy part of the sky, travelling
through northwest Sagittarius, not
too far north and then northwest of
Mu (μ) Sgr, and then into Ophiuchus.
Zero-magnitude Saturn hangs out a few
degrees below Mu in mid-June, serving as
a clear directional beacon. By the end of
July, Vesta will be found roughly halfway
between Saturn and the star Antares.The good thing for us is that Vesta’s
declination is around –20°, making
it ideally positioned for observers at
almost all southern latitudes.
Given that it is at opposition in mid-
June, Vesta clears the horizon around
6:00pm local time and culminates
around midnight, reaching an altitude
of around 70°. By mid-July the distant,
miniature world will be well above the
horizon (about 37° altitude) after sunset
and culminating at around 10:00pm.
When observing, sorting Vesta
from its celestial neighbours might
take some time and will certainly be
easier with optical aid, even when it’s
at its brightest. It will appear stellar in
nature through binoculars and small
telescopes; very large apertures, say
20-30 centimetres or more, may show
hints of the irregular disk.
Vesta sports an angular diameter
of around 0.69′′ for most of June and
July, but seeing and transparency can
affect the view. Even if you can’t resolveThe arrival of NASA’s Dawn spacecraft at Vesta revealed
intensive scarring on the asteroid’s surface. Vesta’s not-
quite-spheroid shape is the result of a massive impact
event about 1 billion years ago. The collision produced
an approximately 500-km-wide crater, now named
Rheasilvia, at Vesta’s south pole. About 1% of Vesta’s
volume was displaced, with ejecta deposited in a 100-
km ring around the impact basin and 2 million cubic
kilometres of material sent into space.
About 5% of all meteorites we find on Earth come from
the Rheasilvian impact. The mineralogy of Howardite-
Eucrite-Diogenite (HED) meteorites, which resemble
terrestrial igneous rocks, places them in this group.
HED meteorites were first
connected with Vesta in the
1970s, when scientists noted
that their infrared and visible
spectra were similar to the
asteroid’s.
The image above
shows three slices of HED
meteorites as viewed through
a polarising microscope. Theslices share a common mineralogy, but their dissimilar
textures indicate that they originated in different parts
of Vesta’s crust and surface and crystallised at different
rates. The slice on the left comes from a meteorite
named QUE 97053, which was recovered from the Queen
Alexandra Range of Antarctica. QUE 97053 is basaltic
eucrite that formed in volcanic flows on the surface of
Vesta some 4.4–4.5 billion years ago. The centre slice
comes from a cumulate eucrite that fell in Moore County,
North Carolina, in 1913. Cumulate eucrites are similar to
basaltic eucrites, but have orientated crystals. They’re
thought to have formed in the upper parts of Vesta’s crust
rather than in surface flows. The slice on the right comes
from a diogenite meteorite named
GRA 98108, recovered from
Graves Nunatak, Antarctica.
Diogenites, which formed in
magma chambers deep in
Vesta’s crust, are composed
mostly of orthopyroxene
and hypersthene, with
smaller amounts of olivine,
plagioclase, troilite and chromite.Ancient asteroid bits
the disk, high-aperture observing can
offer its rewards: Many observers report
seeing colour in Vesta through large
scopes at high power. Descriptions
range from pale yellow to pinkish rose.Meet the neighbours
In the week before opposition, look
for Vesta in the vicinity of the broad
open cluster Messier 23. By June 19, the
minor world will be roughly halfway
between M23 and the globular cluster
NGC 6440 (it will right next to the
latter on June 23). By the end of June
and the next full Moon, Vesta will have
crossed into the southern reaches of
Ophiuchus. On June 30 it will about 1°
from the 4.9-magnitude star 58 Oph.
Spending the next couple of weeks
crossing a barer patch of sky, Vesta will
find itself within 1° of the magnitude
5.8 double star, HD 157527 on July- By the end of July, it will located
roughly halfway between that star and
Beta Oph.