With NASA's space shuttle Endeavour set to undock from the International Space Station late Sunday, skywatchers across much of the United States and southern Canada are in for a real treat: They'll have one last chance to see Endeavour in the night sky before the shuttle retires for good.
The best times to look for Endeavour and the space station will be before sunrise on Monday and Tuesday. Weather permitting, there should be opportunities to see both the Endeavour and space station flying across the sky from many locations. [ Photos of Space Shuttles and Station From Earth ]
The sight should easily be visible to anyone, even from brightly lit cities. Considering that after this shuttle mission there will be only be one left before the program ends (tentatively set for July 8), the view of a shuttle and the space station flying together will soon be a sight that will pass into history.
Endeavour is in the homestretch of its last mission, a 16-day trip to upgrade and resupply the space station. The shuttle launched into space from NASA's Kennedy Space Center in Florida on May 16. Endeavour and its six-astronaut crew are due to return to Earth June 1.
Other satellites visible too
The appearance of either the space shuttle or the space station moving across the sky is not in itself unusual. On any clear night within a couple of hours of local sunset or sunrise and with no optical aid, you can usually spot several orbiting Earth satellites creeping across the sky like moving stars. [ How to Spot Satellites ]
Satellites become visible only when they are in sunlight and the observer is in deep twilight or darkness. This usually means shortly after dusk or before dawn.
What makes the prospective upcoming passages so interesting is that you'll be able to see the two largest orbiting space vehicles in the sky at the same time.
Endeavour will undock from the space station Sunday at 11:53 p.m. EDT (0353 May 30 GMT). Endeavour will fly around the space station before finally pulling away at 4:36 a.m. EDT (0836 GMT).
Monday morning, Endeavour should still be visible at a relatively close distance to the space station until its scheduled return to the Kennedy Space Center in Florida early Wednesday morning. [ Complete Coverage: Endeavour's Final Mission ]
What you can see
Here's what you can expect to see if you have a clear view of the night sky and good weather:
On Monday, Endeavour and the space station will be traveling across North America on southwest-to-northeast trajectories and should appear as a pair of very "bright stars." The space station should appear as the noticeably brighter object and will be trailing Endeavour as they move across the sky.
Across much of the eastern half of the United States, the two spaceships will fly overhead at around 4:46 a.m. EDT, only about 10 minutes after Endeavour has begun maneuvering away from the space station. As a result, the two spacecraft will appear exceptionally close, separated by only about 7 arc minutes — roughly equal to only about one-quarter the apparent width of the moon.
Binoculars will certainly aid in visually separating the two. That narrow gap between the two will have significantly widened when Endeavour and the space station fly over the western United States, just over 90 minutes later. [ N.J. Woman Photographs Shuttle Launch From Airplane ]
And for all observers in the United States and southern Canada, the separation will have increased to 30 degrees or more by Tuesday morning. Your clenched fist held at arm's length measures roughly 10 degrees; so on Tuesday expect Endeavour and the station to separated by about "three fists."
A large telescope would be needed to make out details of the sprawling station. Traveling in their respective orbits at approximately 18,000 mph (29,000 kph), both should be visible anywhere for about one to five minutes (depending on the particular viewing pass) as they glide with a steady speed across the sky.
Because of its size and configuration of highly reflective solar panels, the space station is now, by far, the brightest man-made object currently in orbit around the Earth. [ Infographic: International Space Station: From the Inside Out ]
Astronomers measure the brightness of a sky object in terms of magnitude, a reverse scale in which the lower an object's number, the brighter it appears in the sky.
On favorable passes, the station approaches magnitude -5 in brightness, which would rival the planet Venus and is more than 25 times brighter than Sirius, the brightest star in the night sky. Some skywatchers can even catch a glimpse of the space station just before sunset or shortly after sunrise.
And as a bonus, sunlight glinting directly off the solar panels can sometimes make the space station appear to briefly flare in brilliance.
Region of visibility
Generally speaking, on the mornings of May 30 and 31, the tandem will be visible across parts of southern Canada as well as most of the 48 contiguous United States (Hawaii and Alaska, will not have favorable viewing passes during this upcoming week). [ Photos of Shuttle Endeavour's Final Launch ]
Across the northern half of the United States there will be two or three morning viewing opportunities. For some favored locations, like Philadelphia, Washington D.C. and Richmond there will be as many as four opportunities.
Over the southern United States, the viewing opportunities will be reduced to just one (on May 30). Places below latitude 30 degrees north will unfortunately be denied a view of the "dynamic duo" because they'll appear too low in the sky and too near to sunrise to be easily visible. From Florida, for example, Jacksonville will get a brief view on Monday morning, but the rest of the state will be shut out.
In contrast, some northern localities will be favored with exceptionally good views.
From St. Louis, for instance, the space station and Endeavour will appear to suddenly emerge from out of the Earth’s shadow on Monday morning at a very high altitude of 72 degrees (more than "seven fists") above the southeast horizon during a short 1-minute pass beginning at 3:47 a.m. CDT.
And on Tuesday morning, as seen from Providence, Rhode Island, Endeavour will appear about 30 degrees out in front of the space station as they each emerge from the Earth’s shadow about halfway up in the southwest sky at 3:38 a.m. EDT, taking two minutes to track to the northeast.
At their highest point, the two spacecraft will reach altitudes of 87 and 86 degrees respectively, passing virtually directly overhead.
Europeans will also be favored with views in their pre-sunrise skies.
Both spacecraft will still be docked and will appear as a single very bright "moving star" on Monday morning, but they’ll be widely separated by 25 or 30 degrees on Tuesday morning. Northern locations, such as London will get only one chance (on Tuesday morning), but locations farther south, such as Madrid and Rome, will get up to three opportunities during the Monday/Tuesday interval.
And like Providence, Madrid will have an opportunity to see a nearly overhead pass of both spacecraft on Tuesday morning beginning at 5:03 local time.
When and where to look
So what is the viewing schedule for your particular hometown? You can easily find out by visiting one of these three web sites:
Each will ask for your ZIP code or city, and respond with a list of suggested spotting times. Predictions computed a few days ahead of time are usually accurate within a few minutes. However, they can change due to the slow decay of the space station's orbit and periodic reboosts to higher altitudes. Check frequently for updates.
Another great site is Real Time Satellite Tracking, which shows you what part of the Earth the space station or shuttle happen to be over at any given moment during the day or night.
Endeavour's STS-134 mission is NASA's second-to-last shuttle mission before the 30-year program is retired for good.
Like its sister ships Discovery and Atlantis, Endeavour will eventually be put on public display as a museum piece.
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.