May 6, 2003

May 6, 2003: The planet Mercury never ventures very far from the sun, which makes it remarkably difficult to photograph. Sky watchers on Earth usually strain to see it through the glow of twilight. Sky watchers on the space station have an advantage: they can see it above the glow of twilight.

International Space Station (ISS) science officer Don Pettit captured this picture of the planet Mercury late last month. The sun had just set behind Earth's limb, and Mercury was easy to pick out against the black canvas of space.

On May 7th, tomorrow, Mercury will be so close to the sun that even space station astrophotographers won't be able to separate the two. For the first time since 1999, Mercury and the sun will actually overlap. The planet's silhouette, a tiny black disk, will slowly cross the face our star. The five-hour transit begins at 0513 UT (01:13 EDT). People in Europe, Africa and Asia using properly shielded telescopes will have no trouble observing the close encounter. The crew of the ISS could see it, too, except for one thing: they don't have a telescope onboard. Mercury's black disk will be too small to see without one.

For once, sky watchers on Earth have the upper hand. (Note: Never look at the sun--directly or through a telescope--without suitable eye protection. Bright sunlight can blind you.)