[link to news.yahoo.com]

The final voyage of NASA's space shuttle Atlantis is part of a rare weekend treat for skywatchers on Earth – a cosmic line up with incredibly bright Venus, the moon, the shuttle — and even the International Space Station thrown in for good measure.

Atlantis launched on its final planned mission today at 2:20 p.m. EDT (1820 GMT) from NASA's Kennedy Space Center in Florida to join the space station, Venus and the moon in the sky. The shuttle will dock at the space station on Sunday, but until then the two spaceships can be easily seen with the unaided eye (weather permitting) over North America.

NASA has called it "a rare four-way meeting of spaceships and planets over many locations" in a skywatching alert. This shuttle mission is likely the last chance to see Atlantis in space since the orbiter, and NASA's two other space planes, are due to be retired later this year. [Spotting spaceships from Earth.]

In addition to this weekend's sighting opportunities, there is one more chance to see Atlantis with your own eyes. That is slated to come between May 24 and May 26, between Atlantis' departure from the space station and its planned landing in Florida.

First find the moon

The celestrial sky show will begin tonight at sunset, when Venus and the moon begin to outshine the twilight from the setting sun. Venus will appear as a bright object near the moon.

The moon is currently a slender crescent, appearing just 5 percent illuminated by the sun. By Sunday, docking day for Atlantis and the space station, the moon will grow fatter to be 10 percent full.

Until then, the effects of light reflected off of Earth, called "Earthshine," may be easily seen. Earthshine light can fill in the darkened portion of a crescent moon, giving it a ghostly look half-lit orb with a crisp, bright smile.

Enter the spaceships

Stunning as they may be, Venus and the moon are just the backdrop for the orbital chase between Atlantis and the space station as the two spacecraft draw closer for Sunday's planned 10:27 a.m. EDT (1427 GMT). docking.

The International Space Station will be flying over much of the United States over the next week and where it goes, Atlantis will be nearby.

The International Space Station (ISS) is the largest man-made object in the night sky. It is as wide as a football field and has four giant arrays of solar panels that reflect light back at Earth, rivaling Venus in intensity.

"Some have even caught a glimpse of the ISS in the daytime just prior to sunset or shortly after sunrise," explains SPACE.com skywatching columnist Joe Rao. "And as a bonus, sunlight glinting directly off the solar panels can sometimes make the ISS appear to briefly flare to super-brilliance."

The shuttle Atlantis is smaller than the space station, but still a bright object in the night sky. It is about 122 feet (37 meters) long and has a wingspan of about 78 feet (nearly 24 meters).

Amateur astronomers with telescopes built to track objects may even get a better view of the station (and perhaps even a docked shuttle) when they zoom overhead at 17,500 mph (28,163 kph).

Region of visibility

Generally speaking, Atlantis and the ISS will be visible across parts of southern Canada and most of the 48 continental United States.

Many locations will be favored with as many as three or four viewing opportunities. In contrast, Hawaii and Alaska will not have any viewing opportunities, nor will there be any views in Canada roughly north of latitude 50 degrees.

Over the southernmost United States, the viewing opportunities will be reduced to just one or none at all.

Some localities will be favored with exceptionally good passes. On Friday evening, the ISS will appear to soar to a point virtually overhead as seen from Toronto (9:10 p.m. ET) and New York (9:11 p.m. ET).

A similarly high overflight of the ISS will be visible from Chicago on Saturday evening (8:33 p.m. CT), and the two docked spacecraft will pass almost directly over Flagstaff, Arizona on Sunday evening (8:30 p.m. MT).

When and where to look

So what is the viewing schedule for your particular hometown? You can easily find out by searching for 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, which provides real-time satellite tracking and shows what part of the Earth the ISS or shuttle happens to be passing over at any given time.

NASA is retiring its three-orbiter fleet after nearly 30 years of shuttle flight. The move is aimed to make way for new technologies, rockets and spacecraft for more ambitious missions to an asteroid (by 2025) and to Mars in the 2030s.