Sky and Telescope - June 2018 - 49
t Vesta's south pole, bottom, saw extensive damage during an impact
event about 1 billion years ago. The tremendous troughs at the asteroid's
equatorial region, which may have been caused by faulting from the
impact, are about 10 kilometers across.
on Vesta (see sidebar at right), 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 neat, to say the least.
Where to Find It
Vesta's in a busy part of the sky this month, traveling through
northwest Sagittarius, not too far north and then northwest of Mu (μ) Sgr. Zero-magnitude Saturn hangs out a few
degrees below Mu, serving as a clear directional beacon.
Vesta's a little, but not much, higher in the south than the
ringed planet, and neither is ideally positioned for observers
at mid-northern latitudes. This is the time to seek out those
open southern horizons at your local star party.
In mid-May, Vesta doesn't clear the horizon until about an
hour before midnight. It stands highest when it culminates
around 4 a.m., reaching around 32°, so plan on some late
nights or very early mornings. Observing (sleeping) conditions get a bit easier as Vesta approaches opposition. On June
19th, it rises a bit before sunset and stands about 21° high by
the arrival of true darkness, about 10:30 p.m. local time. It's
at 30°, its highest for the night, an hour after midnight. It
drops lower as the morning ages, setting just as the Sun rises.
Sorting Vesta from its celestial neighbors might take time
and will certainly be easier with optical aid, even when it's
at its brightest. It will appear stellar in binoculars and small
telescopes; very large apertures, say 10-12 inches or more,
may show hints of the irregular disk. Vesta sports an angular
diameter of 0.69″ for most of June, but seeing and transparency will affect the view, especially when the target is close to
the horizon. Even if you can't resolve the disk, high-aperture
observing can offer its rewards: Many observers report seeing
color in Vesta through large scopes at high power. Descriptions range from pale yellow to pinkish rose.
N ASA / UNIV ERSIT Y OF TENNESSEE
Meet the Neighbors
Around the time of new Moon in mid-May, Vesta is 1° from
the open cluster M18, drawing nearer to the M24 Star Cloud
each night. There are plenty of stars in the area to complicate
ID attempts, particularly as Vesta edges past M24 between
May 15th and 29th (full Moon). But keep in mind Vesta's one
of the brighter objects in the area. About 40′ southwest of
M18, a K2 orange dwarf star (HD 167720) shines at magnitude 5.8. The brightest star in M24 is HD 167356, a 6th-magnitude peculiar A star. So Vesta has plenty of stellar company,
but not much competition in terms of brightness.
In the week before opposition, look for Vesta in the vicinity of the broad open cluster M23. By the end of June and the
next full Moon, Vesta will have crossed into the southern
reaches of Ophiuchus. It remains a naked-eye object in the
opening nights of July.
Ancient Asteroid Bits
The 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-kmwide 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 ½ million cubic miles 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 their infrared and visible spectra were similar
to the asteroid's.
The image above shows three slices of HED
meteorites as viewed through a polarizing microscope. The slices share a common mineralogy, but
their dissimilar textures indicate that they originated
in different parts of Vesta's crust and surface and
crystallized 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 center slice
comes from a cumulate eucrite that fell in Moore
County, North Carolina, in 1913. Cumulate eucrites
are similar to basaltic eucrites, but have oriented
crystals. They're thought to have formed in the upper plutons 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.
sk yandtelescope.com * J U N E 2 018