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.

If you go out and carefully study the sky near dusk or dawn, and you have relatively dark skies, the odds are that you should not have to wait more than 15 minutes before you see one of the nearly 15,000 satellites now in orbit around Earth. Most of these "satellites" are actually just "space junk" ranging in size from as large as 30 feet, down to about the size of a softball. The Joint Space Operations Center (JSpOC) headquartered at Vandenberg AFB in California, keeps a constant watch on all orbiting debris. And in fact most satellites—especially the bits of debris—are too faint to be seen with the unaided eye. But depending on who's counting, several hundred can be spotted with the unaided eye. These are the satellites that are large enough (typically more than 20 feet in length) and low enough (100 to 400 miles above Earth) to be most readily seen a sunlight reflects off them.

The biggest

The International Space Station (ISS) is by far the biggest and brightest of all the man-made objects orbiting the Earth. On-orbit construction of the station began in 1998, and is scheduled to be complete by 2011, with operations continuing until around 2015. More than four times as large as the defunct Russian Mir space station, the completed International Space Station will ultimately have a mass of about 1,040,000 pounds (520 tons) and will measure 356 feet across and 290 feet long, with almost an acre of solar panels to provide electrical power to six state-of-the-art laboratories. Presently circling the Earth at an average altitude of 216 mi (348 km) and at a speed of 17,200 mi (27,700 km) per hour, it completes 15.7 orbits per day and it can appear to move as fast as a high-flying jet airliner, sometimes taking about four to five minutes to cross 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 can appear as bright as the planet Venus, at magnitude -4.5, and some 16 times brighter than Sirius, the brightest star in the night sky. Some have made estimates as bright as magnitude -5 or -6 for the station (smaller numbers represent brighter objects on this astronomers scale). And as a bonus, sunlight glinting directly off the solar panels can sometimes make the ISS appear to briefly "flare" in brilliance to as bright as magnitude -8; more than 16 times brighter than Venus!

Other things to see

Along with the ISS, you can also look for a space shuttle on those missions when one is approaching or departing from the space station. Also visible to the naked eye is the newly refurbished Hubble Space Telescope. And with good binoculars, you might also try sighting the infamous "ISS Toolbag" which accidentally was allowed to drift into space when veteran spacewalker and astronaut Heide Stefanyshyn-Piper lost her grip on the backpack-sized bag last Nov. 18

Viewing opportunities

From now through the final days of July, North Americans and Europeans will have many opportunities to see the ISS flying over their homes, due chiefly to a seasonal circumstance. Nights are now the shortest and the time that a satellite in a low-Earth-orbit (like the ISS) can remain illuminated by the Sun can extend throughout the night, a situation that can never be attained during other times of the year. Because the ISS circles the Earth about every 90 minutes on average, this means that it's possible to see it not just on one singular pass, but for several consecutive passes. Moreover, because the ISS revolves around the Earth in an orbit that is inclined 51.6-degrees to the equator, there are two types of passes that are visible. In the first case (we'll call it a "Type I" pass), the ISS initially appears over toward the southwestern part of the sky and then sweeps over toward the northeast. About seven or eight hours later, it becomes possible to see a second type of pass (we'll call it "Type II"), but this time with the ISS initially appearing over toward the northwestern part of the sky and sweeping over toward the southeast. During these next several weeks, Type I passes will initially be visible in the morning hours, prior to sunrise. By early July, Type I passes will be visible during the evening hours, just after sunset, while Type II passes will be occurring in the early morning. By late July, visibility of the Type II passes will have shifted into the evening hours. FROM NEW YORK CITY the best ISS passes in July will be Type II. If you are an early riser, there will be two excellent dawn passes of the Space Station. In both cases, it will trace a high northwest to southeast path across the twilight sky. The first opportunity comes on July 6 from 4:43 to 4:48 a.m. There will be another, very similar pass two mornings later on July 8 between 3:56 and 4:02 a.m. If these times are too early for you, then just wait a couple of weeks and the ISS will make two similarly high NW to SE passes during the evening hours. On July 23, it will be visible between 9:44 and 9:48 p.m. and two evenings later you'll see it moving along the same NW to SE path between 8:58 and 9:03 p.m. Of course there will be many other opportunities to see the ISS during July, but at lower altitudes and for shorter durations.

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 four popular web sites: Chris Peat's Heavens Above Science@NASA's J-Pass 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 this one: http://www.n2yo.com/?s=25544 Which provides real-time satellite tracking and shows you at any given moment during the day or night over what part of the Earth the ISS happens to be.

Viewing tips

Some passes are superior to others. If the ISS is not predicted to get much higher than 20-degrees above your local horizon, odds are that it will not get much brighter than second or third magnitude (10-degrees is roughly equal to the width of your fist held at arm's length). In addition, with such low passes, the ISS will likely be visible for only a minute or two. Conversely, those passes that are higher in the sky—especially those above 45-degrees—will last longer and will be noticeably brighter. The very best viewing circumstances are those that take the ISS on a high arc across the sky about 45 to 60 minutes after sunset, or 45 to 60 minutes before sunrise. In such cases, you'll have it in your sky upwards to four or five minutes; it will likely get very bright and there will be little or no chance of it encountering the Earth's shadow. While the ISS looks like a moving star to the unaided eye, those who have been able to train a telescope on it have actually been able to detect its T-shape as it has whizzed across their field of view. Some have actually been able to track the ISS with their scope by moving it along the projected path. Those who have gotten a good glimpse describe the body of the Space Station as a brilliant white, while the solar panels appear a coppery red. For evening passes, the ISS will usually start out rather dim and then tend to grow in brightness as it moves across the sky. In contrast, for the morning passes, the ISS will already be quite bright when it first appears and will tend to fade somewhat toward the end of its predicted pass. This is due to the change in the angle of sunlight hitting the vehicle. Lastly, remember that in certain cases, the ISS will either quickly disappear when it slips into the Earth's shadow (during evening passes) or quite suddenly appear when it slips out of the Earth's shadow (during morning passes). This becomes increasingly more likely for passes that take place more than 90 minutes after sunset or more than 90 minutes before sunrise.