With a diameter of 525 km, Vesta is the largest asteroid in the main belt between Mars and Jupiter. By surveying Vesta’s surface, Dawn (official mission website) found that it has the most uneven and varied terrain of any similarly sized object in the solar system — even more so than the Moon or Mercury! The space probe discovered two huge impact craters at Vesta’s south pole. The larger of the two, named Rheasilvia, is centred on the pole and measures 505 km across: That’s 90% of the asteroid’s diameter! The mountain that forms its central peak ascends over 20 km high; in comparison, Mount Everest rises only 9 km...
Vesta is a veritable fossil remnant of the early solar system; with its internal structure, and mainly iron core, Vesta resembles a small planet, or the Moon, more than an asteroid. Dawn observed mineral deposits, exposed in deep fissures caused by impacts: These minerals support the hypothesis that Vesta once had a subsurface ocean of molten magma. And lastly, Dawn confirmed that a certain class of meteorites, which represent about 6% of all the meteorites found on Earth, actually originate from Vesta. Traces of pyroxene (a mineral rich in iron and magnesium), found in these meteorites, exactly match those found in Vesta’s surface.
Dawn will leave Vesta in August, for its next destination, the dwarf planet Ceres, where new surprises certainly await. To be continued upon its arrival early in 2015…
Planetary duos and trios
Saturn and Mars are easy to see, early in the evening, above the southwest horizon. The two are currently located in the constellation of Virgo: In fact, Saturn forms a duo with Spica, the constellation’s brightest star. Mars, which is further to the right, gets closer to Saturn from day to day: The two planets will be just three degrees apart by mid-August. From July 23 to 25, the first quarter Moon will join Mars and Saturn, forming a picturesque trio.
An hour before sunrise, look to the east and you’ll see two brilliant planets: Venus and Jupiter. At the beginning of the month, just below Venus, a third object forms a trio with the two planets: It’s the reddish star, Aldebaran, in the constellation of Taurus. Notice how Venus changes position from day to day: On the morning of July 9, the dazzling planet will be less than one degree from Aldebaran. At dawn, on July 15, a thin crescent Moon will be visible next to this “morning trio.” During the days that follow, Venus will continue on its way, gradually leaving Jupiter and Aldebaran behind.
The Milky Way
Mid-summer is the ideal period to see the Milky Way crossing the sky, in all its splendour, like a luminous band. The best time to view it is at the end of astronomical twilight, around 11 P.M. However, the sky must be as dark as possible, and that requires a moonless night: The new Moon will occur in mid-July, making this prime time for observing our galaxy.
The Milky Way extends from horizon to horizon and passes directly overhead. Starting in the south, several constellations lie along its length: The stars of Sagittarius form the shape of a teapot: (one can even imagine that the Milky Way is steam escaping from the spout). A bit higher, Aquila, the eagle, takes flight, followed by Cygnus, the swan, which spreads its wings overhead; and in the northeast, Cassiopeia, sits on her throne, while Perseus, the hero, stands above the northern horizon.
In the country, if you scan the Milky Way with binoculars or a small telescope, you can discover a number of interesting objects: the globular cluster, Messier 22, and the Lagoon nebula in Sagittarius; the North America nebula in Cygnus, next to the bright star, Deneb; and the double cluster in Perseus. With the naked eye, you can even see the Andromeda Galaxy, near the northeast horizon below the Milky Way.