With the Space Shuttle Endeavour scheduled to undock from the International Space Station (ISS) on Friday evening, skywatchers across much of the United States and southern Canada are in for a real treat early on Saturday and Sunday morning.

Weather permitting on these mornings, there will be opportunities to see both the Endeavour orbiter and the ISS flying across the sky from many locations.

Appearing as a pair of very bright "stars," the ISS should appear as a somewhat brighter object and will be trailing Endeavour as they move across the sky. The pair will appear only a few degrees apart on Saturday morning, but the gap between the two will likely widen to perhaps 20-degrees or more by Sunday morning (your clenched fist held at arm's length measures roughly 10-degrees; so on Sunday morning expect Endeavour and the ISS to separated by about two fist-lengths).

The sight should easily be visible to anyone, even from brightly lit cities. Considering that after this mission there will be only be four left before the shuttle program ends (tentatively in September) the view of a shuttle orbiter and the Space Station flying in tandem will soon be a sight that will pass into history.

New York's View

The Tri-State Area will get two opportunities to see Endeavour flying with the ISS. The first will come on Saturday morning, just over 9 and a half hours after both vehicles have undocked; so they will still be relatively close to each other. The two spacecraft will emerge from out of the Earth's shadow at 5:21 a.m. EST, at an altitude of 18-degrees above the north-northwest horizon. They will attain a maximum altitude of 25-degrees above the north-northeast horizon at 5:23 a.m. and will drop toward the eastern horizon at 5:25 a.m. If you're familiar with the constellations, you'll see them glide between Cepheus and the "W" of Cassiopeia and pass under the bright star Deneb in Cygnus. Overall, this pass should last about 3 minutes and 50 seconds.

Your other opportunity will come early on Sunday. By then, the two spacecraft will be more widely separated (recall the "two fists" rule), but they will trace a much higher arc across the sky. Emerging from the Earth's shadow, about 15-degrees above the northwest horizon at 5:43 a.m. EST, Endeavour and the ISS will climb to a maximum altitude of 63-degrees above the northeast horizon at 5:45 a.m, then drop down toward the east-southeast horizon by 5:48. Overall, this pass should last 5 minutes 10 seconds! At their closest point (when highest in the sky), they should be about 240 miles away. If you're familiar with the constellations, they will appear to pass between the Big Dipper and Polaris (the North Star), then streak past the lozenge-shaped head of Draco, then, as they descend they will pass close to two of the brightest stars in the sky: Vega in Lyra and Altair in Aquila. Interestingly, the ground track for both vehicles shows that they will appear to pass directly overhead (or very nearly so) as seen from Catskill, NY in Columbia County.

Region of visibility

Generally speaking, the tandem will be visible across southern Canada and most of the 48 contiguous United States (Hawaii and Alaska will not have favorable viewing passes during this upcoming week).

Across southern Canada and the northern half of the United States there will be two or three morning viewing opportunities. For some favored locations, like Chicago and Milwaukee there will be as many as four opportunities. Over the southern United States, the viewing opportunities will be reduced to just one. Much of Florida (save for the Panhandle), central and southern Georgia, and parts of western and southern Texas 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.

For other locations . . .

So what is the viewing schedule for your particular hometown? You can easily find out by visiting one of these three web sites:

• Chris Peat's Heavens Above
• NASA's SkyWatch
• Spaceweather.com

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 where the ISS or Space Shuttle are at any given moment during the day or night.

What to expect

Both vehicles will be traveling across North America on northwest-to-southeast trajectories.

A large telescope is 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 from 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.

On favorable passes, it 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 have even caught a glimpse of the ISS just prior to sunset or shortly after sunrise. And as a bonus, sunlight glinting directly off the solar panels can sometimes make the ISS appear to briefly flare in brilliance.

Other satellites too

The appearance of either the space shuttle or the space station moving across the sky is not in itself unusual. On any clear evening within a couple of hours of local sunset and with no optical aid, you can usually spot several orbiting Earth satellites creeping across the sky like moving stars. 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.

Shuttle Endeavour is expected to undock from the ISS at 7:54 p.m. Eastern Standard Time on Friday. Endeavour will fly around the ISS before finally pulling away from the Station at 9:38 p.m. EST, although it should still remain at a relatively close distance to it until its scheduled return to the Kennedy Space Center in Florida Sunday evening.

On Tuesday (Sept. 8, 2009) and Wednesday (Sept. 9, 2009) the New York metro area will have three great opportunities (local weather conditions permitting) to see the Space Shuttle Discovery and the International Space Station flying in tandem across our evening sky.

Currently, Discovery is docked to the Space Station, but at 3:27 p.m. on Tuesday it will undock from the Station and slowly begin to pull away from it.

On Tuesday, at 7:42 p.m., both Discovery and the Space Station will rise above the SSW horizon and will track toward the ENE. They will appear as two bright moving "stars." Discovery will appear to lead the Space Station across the sky. The Space Station, however, should appear as the brighter of the two objects. They will be separated by roughly 5 or 6 degrees; your clenched fist held at arm's length measures roughly 10-degrees, so the Shuttle and Space Station will be separated by about a "half fist" as they move across the sky. At their highest point, they will stand 33-degrees above the SE horizon and at 7:44:30, as an added bonus, they will pass directly above the planet Jupiter. This pass should take 5 minutes 17-seconds from start to finish.

Also on Tuesday evening, a second pass will begin at 9:18 p.m. Discovery and the Space Station will appear a little farther apart. They will rise above the W horizon and will track toward the NNW. After 2 minutes 29 seconds, however, they will rapidly fade out, 25-degrees above the horizon, as they pass into the Earth's shadow.

On Wednesday, comes the third and final double pass. The two space vehicles will be much farther apart ... the Shuttle will lead the ISS by about one minute. Watch for Discovery at 8:05 p.m. rising above the WSW horizon, moving toward the NE; the Space Station will follow along the same track about a minute later. At their highest point, they will stand 64-degrees above the NW horizon. This pass should take 5 minutes 42-seconds from start to finish.

Discovery is scheduled to return to the Kennedy Space Center in Florida on Thursday (Sept. 10) at 7:06 p.m.

People in the eastern United States will get a great opportunity, weather permitting, to see the Space Shuttle Discovery launched into orbit Wednesday evening, March 11.

The Shuttle flight (STS-119) will be the 28th to rendezvous and dock with the International Space Station (ISS) and the glow of its engines will be visible along much of the Eastern seaboard of the United States.

To reach the ISS, Discovery must be launched when Earth's rotation carries the launch pad into the plane of the ISS's orbit. For mission STS-119, on March 11 that will happen at 9:20:10 p.m. ET, resulting in NASA's first Shuttle flight of 2009 and its second consecutive nighttime (the previous shuttle flight, last November 14, was also a nighttime launch). This launch will bring the Shuttle's path nearly parallel to the U.S. East Coast.

What to expect

For most locations, Discovery will be visible by virtue of the light emanating from its three main engines. It should appear as a very bright, pulsating, fast-moving star, shining with a yellowish-orange glow.

Based on previous night missions, the brightness should be at least equal to magnitude -2; somewhat brighter than Sirius, the brightest star in the sky, which shines brilliantly in the south-southwest during the evening hours. Observers who train binoculars on the Shuttle should be able to see the rapidly moving shuttle resembling a tiny V-shaped contrail.

In the Southeast United States, depending on a viewer's distance from Cape Canaveral, Discovery will become visible anywhere from a few seconds to 2 minutes after it leaves Pad 39-A. The brilliant light emitted by the two solid rocket boosters will be visible for the first 2 minutes and 4 seconds of the launch out to a radius of some 520 statute miles from the Kennedy Space Center.

No matter where you're located, keep in mind that the Shuttle will not get very high above the horizon. In most cases, it will range from roughly 5 to 10 degrees. To get an idea of how high this is, make a fist and hold it out at arm's length. Place the bottom of your fist on the horizon; the top of your fist is 10 degrees

By location:

Southeast U.S. coastline:

Anywhere north of Cape Canaveral, I suggest viewers initially concentrate on the south-southwest horizon (if you are south of the Cape, look low toward the north-northeast).

Mid-Atlantic region:

Look toward the south about 3 to 6 minutes after launch.

Northeast (Washington, Philadelphia, New York, Boston):

Concentrate your gaze low toward the south or south-southeast about 6 to 8 minutes after launch. Of course, as the shuttle gets closer, its azimuth very quickly swings over to the southeast, where in most cases, the point of maximum altitude occurs. I suspect most people will be scanning the horizon from south-southeast in the final couple of minutes of powered ascent . . . if so, you shouldn't miss out on sighting Discovery.

Discovery will seem to flicker, then abruptly wink-out 8 minutes and 23 seconds after launch as the main engines shut-down and the huge, orange, external tank is jettisoned over the Atlantic at a point about 870 statute miles uprange (to the northeast) of Cape Canaveral and some 430 statute miles southeast of New York City. At that moment, Discovery will have risen to an altitude of 341,600 feet (64.7 statute miles), while moving at more than 17,000 mph and should be visible for a radius of about 770 statute miles from the point of Main Engine Cut Off, or MECO.

Should the launch of Discovery be scrubbed on Wednesday, March 11, the launch will be rescheduled on a daily basis, but the time of the launch will occur roughly 23 minutes earlier for each day the launch is delayed (launch window times through March 16).

Before hoping to see the Shuttle streak across your local sky, make sure it has left the launch pad! Watch a television news outlet to verify that Discovery has been launched, or you can watch the launch on your computer via streaming video from NASA-TV.

Good Luck!