Vesta Observable at Dawn

by Lucy McFadden, Dawn Co-Investigator, University of Maryland


As the Dawn spacecraft is tested and readied for launch in September, 2007, scientists around the world are preparing to take advantage of Vesta’s position in the night sky to augment our understanding of this large asteroid, or small protoplanet. Professional and amateur observers with both large and small telescopes have an opportunity to contribute to knowledge of Vesta and to enjoy its appearance in the night sky. Vesta  hasn’t been so close to Earth since June 1989. It is the Dawn mission’s first target scheduled for orbital study beginning October 2011 through May 2012.

Members of the science team and their colleagues have telescope time on Hubble Space Telescope to map the surface of Vesta at the highest spatial resolution currently possible. Other scientists at large ground-based telescopes are studying Vesta at different wavelengths across the electromagnetic spectrum.  Small telescope observers are invited to contribute photometric measurements to build light curves throughout this apparition. Our friend, Gary Emerson, will be observing with a 20 cm (8”) telescope and four different filters to compare the light scattering properties at different wavelengths.



Vesta night sky in March
    Location of Vesta in the night sky through the spring and summer. Charts generated in SNP+6.
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Vesta’s appearance in the sky

Enthusiastic observers have a chance to see Vesta this spring and through the summer. Beginning in January 2007, it can be seen in the morning sky. It rises earlier and earlier throughout January and February. Located above and to the west of Jupiter in the southeastern sky, by mid-March it is above the horizon long enough before sunrise for a complete rotation to be observed (5 hours and 20 min). Each day, Vesta’s magnitude grows by about 0.1 magnitude, getting 10% brighter every day. This is due to three factors: Vesta is getting closer to the Sun as it approaches its perihelion point on June 16, 2007 (2.151 AU); the distance between Earth and Vesta is also decreasing, as is its phase angle (the angle formed by the sun, Vesta and Earth). As Vesta approaches the Sun, its angular diameter increases reflecting a larger amount of sunlight. Furthermore, as Vesta and the Earth approach each other, observers on Earth see more reflected light from its surface. And finally, as the phase angle gets smaller, the percentage of the surface that is illuminated grows larger, just as the waxing Moon grows brighter. Vesta’s closest approach to Earth is May 31, 2007 (1.1427AU), and it is almost directly in line with the sun and Earth on May 30, 2007 (phase angle=3.65°). At its brightest, it will be 5.4 magnitude, and theoretically bright enough to see without a telescope or binoculars. But binoculars and a good sky chart will be necessary to find it. Vesta starts out in Scorpius in January, travels into the constellation Ophiuchus, the Serpent Holder, where it stays until the end of June. It then moves into Scorpius again.

While seeing Vesta itself is pretty neat, observers may want to make note of brightness variations due to light and dark spots on its surface. On any given night as it rotates on its axis, its brightness varies by about 15%.  This is due mostly to bright and dark surface spots with some contribution due to shape.  Simpler observations would include watching Vesta move against the background stars.

To learn more about asteroids and how to observe Vesta, check out the Amateur Observers’ Program at

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