NASA plans to undock the shuttle Atlantis from the International Space Station at 10:42 EDT this morning. If so, skywatchers under clear skies in Maryland this evening could be treated to a double flyover by the shuttle and the space station. The weather prospects are not great. Here's the forecast. But it's a rare enough event to be worth notice here, in case the clouds part somewhere, for someone.

The timing will be a bit uncertain, dependant on when the two craft undock, and how quickly the shuttle moves away. But here's the most likely scenario, at least as of this morning:

At 10:33 p.m. this evening, the space station will appear over the northwest horizon as it passes over Lake Michigan. It will zip toward the southeast, passing just southwest of Baltimore at 10:36 p.m. At that point - 70 degrees above the southwest horizon, it will move into the Earth's shadow and disappear. This is NOT a pass that will be visible from horizon to horizon. The good new is the ISS is, by all reports, brighter than it has ever been, thanks to the new solar panels added by the shuttle crew.

But here's the bonus. The shuttle Atlantis, having undocked from the station, will have begun to lower its altitude to separate itself from the station and get into position for landing. That drop in altitude will cause it to speed up relative to the station, and move ahead. It's not certain at this point how far ahead of the space station Atlantis will be by 10:30 p.m. tonight, so it would be wise to step outside 5 or 10 minutes early, just in case.

Keep your eyes peeled (bring the kids; they're great at this) for a steady, star-like object moving at a fair clip from the northwest. If it blinks or has multiple, colored lights, it's an airliner. Keep looking. If you spot something before 10:33, it's probably the shuttlle. The ISS should follow, on time, at 10:33 p.m. You should expect both craft to vanish into the Earth's shadow in the same spot - almost directly overhead. So, Atlantis first, followed at 10:33 by the ISS, which should look much the brighter of the two.

I have witnessed this kind of double flyover by a shuttle and the ISS only once before. The shuttle was only 10 or 20 seconds ahead of the station, but the tandem flyover was pretty amazing. That's 10 people soaring 220 miles overhead at 17,500 mph - three in the ISS and 7 in the shuttle.

And while you're out there, don't forget to look for Venus, Saturn and the moon, all lined up - right to left in that order - in a row in the western sky. In the southeast, you'll find brilliant Jupiter.