Sky watchers following the recent arrival of American astronaut David Wolf aboard the Russian space station Mir can look toward the heavens to spot the station in orbit from the Charles River this weekend.

Both the American space shuttle Atlantis and the Russian Mir station, which are 250 miles above the surface of the earth, may still be visible close to each other on Friday evening before Atlantis lands.

Atlantis' five day mission will end on Friday, and both 100-ton vessels will disengage-the shuttle will return to Earth and Mir will remain in orbit.

"By Friday evening they won't be that far apart," said Catherine E. Watson, a public affairs specialist at NASA's Johnson Space Center in Houston.

"The Mir is so big it's really kind of interesting to look at," says Robert P. Stefanik, director of the Oak Ridge Observatory, 30 miles away from Cambridge. "What you sometimes can see is the changing brightness, since as it moves, the panels reflect light in different ways relative to the earth."

Because both spacecraft travel at over 17,000 miles per hour, it takes them only 90 minutes to orbit the earth. As a result, Stefanik says there is a narrow window for stationary observers to spot the spacecraft.

According to Ron F. Dantowitz, an astronomy educator at the Boston Museum of Science, says the bright lights of the big city will not hide the spacecraft above.

"They're brighter than the brightest star in the sky," he said. "If you can see the stars, you can see Mir."

However, it is virtually "impossible" to discern the details of Mir's structure with the naked eye, says Alan M. MacRobert, associate editor of Sky & Telescope magazine.

"[Even] with a pair of binoculars or with a low power telescope you can track it and it'll be a bright point of light," he says.

There are four flybys expected over the next week:

Friday: both vessels will appear above the southwest horizon at 7:21 p.m. and then disappear three minutes later in the East.

Saturday: Look for Mir rising from the southwest at 7:58 p.m. It will rise to the zenith of the sky by 8:02 p.m., then disappear.

Sunday: Mir will appear at 7:03 p.m. on the southwest horizon, then fly towards the northeast before fading four minutes later.

Monday: Mir may be difficult to see in Cambridge-it will fly from the north to north-by-northeast for one minute at 7:42 p.m.