Skywatcher Dr. Dale Ireland took this photo of the space shuttle Atlantisduring its STS-79 mission as it re-entered the Earth's atmosphere on Sept. 26, 1996 from Silverdale, Washington.

NASA's space shuttle Discovery will attempt areturn to Florida's Kennedy Space Center Monday morning by taking a path acrossthe contiguous United States, giving early-bird skywatchers a chance to see ?and hear ? the spacecraft as it streaks across the sky on the way tolanding.

The spaceshuttle might be visible to keen eyes along the flight path, which runsfrom the northwest toward the southeast. And many people are likely to hear itssonic boom.

It is only the second time since NASA's tragic2003 loss of the shuttle Columbia than returning shuttle will fly over thecontinental United States, and is also expected to be the last time, accordingto entry flight director Bryan Lunney.

NASA's three remaining shuttle missions afterDiscovery's are in the summer, when NASA tends to avoid this specific landingprofile ? called a "descending node" ? in order prevent a shuttlefrom flying through high-altitude noctilucent clouds, which are common in thesummer months, he said. The shuttles usually approach Florida from thesouthwest, flying over Central America on the way.

The shuttle Discovery ? which undocked fromthe International Space Station on Saturday morning -- is aiming for atouchdown at NASA'sprime landing site in Florida. There will be two possibilities for aFlorida landing, on orbits 222 and, if necessary, orbit 223.

If all goes according to plan, Discovery willfire its twin braking rockets at 7:43:20 a.m. EDT over the Indian Ocean,just to the south of the French Southern and Antarctic Lands (a group ofvolcanic islands southeast of Africa), for 3 minutes and 11 seconds, slowingthe shuttle down for its de-orbiting and a gradual slide back to Earth.

At 8:16:59 a.m. EDT, the orbiter will arriveat that point where it will begin to encounter the first effects of the Earth'satmosphere at an altitude of 399,800 feet.

Where to look

On Orbit 222, Discovery will cross thewestern coast of North America at 8:25 a.m. EDT (5:25 a.m. PDT), south ofthe Queen Charlotte Islands of western Canada. [Graphic:Discovery's landing paths across the U.S.]

From Vancouver, British Columbia prospectiveshuttle watchers will see Discovery reach a maximum altitude of 18-degreesabove the northern horizon at 5:25:31 a.m. PDT. As a reference, yourclenched fist held at arm's length measures roughly 10-degrees in width, so asseen from Vancouver, the Shuttle will appear to reach nearly "twofists" above the northern horizon.

Traveling on a southeast trajectory, theorbiter will then pass over northeast Montana.

From Glendrive, viewers will seethe shuttle reaching a maximum altitude of 50-degrees above the northeasthorizon at 6:28:51 a.m. MDT. Just over a minute later at 7:30:08 a.m. CDT,Discovery will be passing almost directly over Pierre, South Dakota.

From there, the shuttle will reach a maximumaltitude of 82-degrees up in the northeast. After about another three moreminutes, it will be racing over the heart of Missouri, between Kansas City andSt. Louis. From Kansas City Discovery will be 25-degrees high above thenorth-northeast horizon at 7:32:11 a.m. CDT, while from St. Louis it willpassing 21-degrees above the southwest horizon at 7:33:12 a.m. CDT.

About a minute later it will be over theArkansas-Tennessee border to the east of Memphis, where Discovery will reach analtitude of 33-degrees above the north-northeast horizon at 7:34:13 a.m. CDT.

At 8:44:55 a.m. EDT, the shuttle willdecelerate to two and a half times the speed of sound (mach 2.5), droppingto an altitude of 82,200-feet just to the northwest of Cape Canaveral.Touchdown is scheduled for 8:51:22 a.m. EDT.

Should Discovery be waved off on this firstattempt, a second attempt will be made on orbit 223.

On this track, the shuttle would reach theWashington coast south of the Seattle-Tacoma area shortly after 7:00 a.m.PDT.

From Seattle, viewers should look 44-degreesabove the southwest horizon at 7:00:44 a.m. PDT. Discovery will then passover Southwest Wyoming, reaching an altitude of 48-degrees up in thenorth-northeast at 8:03:48 a.m. MDT as seen from Rock Springs. It will thenpass to the north of Denver, Colorado at 8:04:49 a.m. MDT, at an altitude of45-degrees and four minutes later it will be over Arkansas, making a flyoveralmost directly above Little Rock at 9:08:40 a.m. CDT, 87-degrees above thewestern horizon.

For Montgomery, Alabama it would appearhighest (66-degrees) above the east-northeast horizon at 9:11:13 a.m.CDT. Slowing to mach 2.5 at 10:20 a.m. EDT, Discovery would be scheduledto land at KSC at 10:26:26 a.m. EDT.

If you would like to calculatespecific viewing circumstances for your location, NASA recommends that you usetheir Skywatch 2.0 applet that you can access at the space agency's spacesightings website.

When selecting a satellite, scroll to eitherKSC 222 (ENTRY) or KSC 223 (ENTRY) for the options for Monday, April 19. If there is one-day delay in the landing because of weather concerns, there arealso options for Tuesday, April 20 for a landing at either the Kennedy SpaceCenter: KSC 237 or 238 or for Edwards Air Force Base in California: EDW 238 or239.

Can you see it?

If youlive very near or directly under the re-entry track, you might want to try andview Discovery as it comes down through the atmosphere on itsnorthwest-to-southeast path. A reentering shuttle usually appears as avery bright "star" leaving a long contrail in its wake.

In adark or twilight sky, the view of this reentry fireball can be a spectacularsight. John A. Dormer, a Texas observer, describes the golden plasma trailtrailing behind the shuttle as "...stunningagainst a rapidly-bluing sky; it looks like liquid gold."

If Discovery reenters on the first attempt (Orbit 222) it willcome just minutes before sunrise for British Columbia and Washington, meaningre-entry comes in a bright twilight sky, but still sufficiently dark so that itstill should be a spectacular sight.

Discovery will then cross into daylight north of Idaho, where theremainder of the reentry over the U.S. occurs during the daytime. Is it stillpossible that the shuttle will be seen?

"Yes," said veteran satelliteobserver, Dan Laszlo, a skywatcher based in Fort Collins, Colo. "I havealso seen shuttles on entry in daylight, appearing about as bright as Venus ataround magnitude minus 4, so it's possible, though not the spectacle of a nightentry."

Another assiduous skywatcher, Dale Ireland ofSeattle, Wash., agrees with Laszlo.

"It would be hard to spot in daylight,but not impossible," Ireland said.

Certainly, the clearer your sky with few orno clouds and little or no haze will improve your chances of making a sighting.

Can you hear it?

If you can't see the shuttle, perhaps you'llhear it. As it races toward Earth, a re-entering shuttle produces adouble sonic boom.

Sonicbooms are created by air pressure. Much like a boat pushes up a bow wave asit travels through the water a vehicle pushes air molecules aside in such a waythey are compressed to the point where shock waves are formed. The reason fortwo booms is that the shock waves form two cones, at the nose as well as at thetail of the vehicle.

The shock waves move outward and rearward inall directions and usually extend to the ground.

Those who live near and around the KennedySpace Center are accustomed to hearing the double booms of a returning shuttle,but those located under and close to the Shuttle path, perhaps all the way backto the Pacific coast, may also hear the booms as well.

From a location in the nation's midsection orover the northwest United States, where the altitude of the shuttle will be inthe range of 100,000 to 200,000-feet, it will take time for the shockwave to propagate down to the ground. Sound travels at roughly1,100-feet per second, so depending on where you live relative to the track, itcould be anywhere from 90 to 180 seconds after the shuttle has passed onby before you hear anything.

"You will definitely be able to hear thesonic boom a few minutes after it passes directly overhead if that is where youwill be watching from," writes Texas amateur, Jeff Umbarger. "AndI once heard three booms in short succession. I can explain two ofthem (the over and then under pressure wave) but not three (probably a bounceoff some thermal layer way up)."

"In 1999, one of the shuttles came injust after sunset, on a ground track between Waco and Austin IIRC," addedDormer. " I figured it'd be about 12 minutes before the boom hit Kyle,Texas, which is south of Austin by 20 or so miles. I heard a"Whump-wuh-whump" within about thirty seconds of when I thought itwould happen. There were uncertainties in my understanding of the atmosphere,but I knew my prediction would be early."

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Joe Rao serves as an instructor and guestlecturer at New York's Hayden Planetarium. He writes about astronomy for TheNew York Times and other publications, and he is also an on-camerameteorologist for News 12 Westchester, New York.