June 1, 2007


In the coming evenings, sky watchers can acquaint themselves with the MESSENGER spacecraft mission to Mercury. Late afternoon on Tuesday, June 5, 2007, MESSENGER will fly within about 210 miles (340 kilometers) of the surface of the planet Venus, and get a gravity kick toward its ultimate destination, the sun-baked planet Mercury.

Both Venus and Mercury will be well-placed for viewing during dusk the week before MESSENGER's encounter with Venus. Go to a location away from bright lights with a good view of the western and northwestern horizon. Venus is the brilliant "evening star" fairly high above the western horizon. Forty-five minutes after sunset (or between 8:45 p.m. and 9:45 p.m. Daylight Time, depending on your location within your time zone), you should start to see other bright stars and planets as the sky darkens.

To the right of Venus is the star Pollux, and an equal distance farther to the right is Castor. These "twins" are the brightest stars in the constellation Gemini. The planet Mercury will be similar to Pollux in apparent brightness, located close to the horizon almost directly below Castor in the west-northwest. Another way to estimate the location of Mercury is to find the planet Saturn to the upper left of Venus. Mercury is a similar distance away from Venus but to the lower right. Find it quickly, before it sets. (These instructions are roughly correct for the evenings of June 1-5, 2007).

MESSENGER (which is much too small and faint to see) is approaching Venus from the right. You can imagine MESSENGER about a third of the way from Venus to Pollux (about four Moon diameters from Venus) tonight; the Moon will be rising in the east. By the evening of June 4, MESSENGER will be closing in on Venus, just two-thirds of the Moon's diameter away. MESSENGER flies by Venus before sunset as seen from the United States on June 5. But two hours later, as the sky darkens and Venus gleams in the west, the spacecraft will still be an imperceptible distance to the left of Venus, with its motion slightly changed so that it is accurately on course for its first flyby of Mercury in January 14, 2008.

This is a good time to try to see MESSENGER's elusive target, Mercury, which is always a challenge to spot since it stays so close to the Sun. Look for it on the first clear evening, because it fades rapidly during the first two weeks of June, moving ever closer to the Sun. (Through a telescope, one can see Mercury's phase changing from half-phase to crescent.)

MESSENGER will be making scientific measurements of Venus, as well as testing out its instruments for Mercury, as it zooms by Earth's sister planet late Tuesday afternoon.

Clark R. Chapman,
Member, MESSENGER Science Team

MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) is a NASA-sponsored scientific investigation of the planet Mercury and the first space mission designed to orbit the planet closest to the Sun. The MESSENGER spacecraft launched on August 3, 2004, and after flybys of Earth, Venus, and Mercury will start a yearlong study of its target planet in March 2011. Dr. Sean C. Solomon, of the Carnegie Institution of Washington, leads the mission as principal investigator. The Johns Hopkins University Applied Physics Laboratory built and operates the MESSENGER spacecraft and manages this Discovery-class mission for NASA.