The word from Houston is that NASA managers have settled on April 29th for Space Shuttle Endeavour's final launch. That's timed so that the orbiter can easily chase down the International Space Station and deliver its billion-dollar cargo, the Alpha Magnetic Spectrometer (a story for another time).Not coincidentally, these shuttle launches also coincide with periods when the ISS repeatedly passes over North America in the hours just after sunset. So not only do you get a chance to see the mammoth, football-field-size assembly glide across the evening sky, but you also might glimpse the shuttle in hot orbital pursuit.
I'm always a little surprised when I run into skywatchers who've never seen the ISS pass overhead, because it's slam-dunk easy to spot — if you know where and when to look. Due to its relatively low orbit, 225 miles (360 km) up, the spacecraft circles Earth about 16 times each day. We can see it whenever we're in darkness on the ground and the spacecraft is still in sunlight — which means the hours just after sunset or just before sunrise.As with all artificial satellites, the Space Station's orbit is defined mathematically by its altitude, inclination to Earth's equator, and so forth, values that are routinely expressed in a cryptic shorthand called two-line elements. But don't worry: you won't need a crash course in spherical trigonometry to figure out when it's the Space Station. Plenty of websites do that for you. If you've registered (free! easy!) here at SkyandTelescope.com, sighting predictions for your location are just a click away.
Whenever the Space Station cruises overhead, it's generally traveling west to east, the direction of its motion around Earth. However, depending on the specific pass, it could be going northwest to southeast, southwest to east, and so forth. Even though it's zipping along at 5 miles (8 km) per second, you'll see that it takes a few minutes to cross the sky. Look for a bright steady beacon that's gliding along a smooth, stately path — not a quick flash like a meteor.Y'know, when the first module reached orbit way back in 1998, I pinged some of my NASA contacts to see if anyone had calculated how bright the space station would appear once all the pieces came together. Surprisingly, no one knew. But now that it's completely tricked out (pending Endeavour's final additions), the answer seems to be about magnitude -3.8. That outshines every nighttime star and every planet except Venus.
So sometime in the next week or so, head outside and wave to the station's six cosmonauts and astronauts as they coast overhead. Grab your neighbors too — it's a fun, easy way to introduce them to evening skywatching. The ISS is now so huge — the size of a football field, including the end zones — that you can detect its shape through a decent pair of binoculars.