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Vesta at Its Best: Asteroid 4 Vesta is closest this month, a lustrous light in the southern sky.

Vesta, the fourth-discovered asteroid, reaches opposition (opposite the Sun in the sky when viewed from Earth) on June 19th. Though neither the largest nor the most massive of the main belt asteroids, Vesta's the brightest of them all at opposition. This year No. 4 shines in Sagittarius at magnitude 5.3 and will be visible without optical aid under reasonably dark skies. It's a bit dimmer the rest of the month, ranging from magnitude 5.6 to 5.8, but still within reach of the naked eye under good skies. It's worth tracking during May and July as well, when it will be an easy binocular object.

There's something intrinsically interesting about observing asteroids--they're giant space rocks, after all--but the success of NASA's Dawn mission turned Vesta into a particularly compelling target. While the Hubble Space Telescope resolved some of its largest surface features in the 1990s, it wasn't until the Dawn spacecraft dropped in on Vesta in 2011 that scientists were able to study the asteroid in detail. Vesta's almost, but not quite, 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 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 ([mu]) 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[degrees], 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[degrees] high by the arrival of true darkness, about 10:30 p.m. local time. It's at 30[degrees], 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.

Meet the Neighbors

Around the time of new Moon in mid-May, Vesta is Io 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 1/2 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.

Caption: Vesta's position is marked with a tick at 0h UT every seven days. For North America, this time falls in the early evening (or late afternoon) of the previous date.

Caption: 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.
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Title Annotation:JUNE 2018 OBSERVING: Celestial Calendar
Author:Johnson-Roehr, S.N.
Publication:Sky & Telescope
Date:Jun 1, 2018
Previous Article:Planets Aplenty: The planets are out in full force this month, and they are joined by an asteroid.
Next Article:Saturn Stands (Relatively) High.

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