Rare Twilight Shuttle Launch on Monday Visible Along Eastern United States

People in the eastern United States will get a great opportunity, weather permitting, to see the space shuttle Discovery launched into orbit early on Monday morning, April 5.

Should the launch come off on schedule, it will also give viewers a rare opportunity to see a shuttle launch during morning twilight, a very unusual circumstance which has happened very infrequently since the shuttle program began in April 1981.

The shuttle is due to launch Monday at 6:21 a.m. ET from NASA?s Kennedy Space Center in Cape Canaveral, Fla. This viewing map shows the areas where it will most likely be visible to East Coast skywatchers.

This flight (STS-131) will be the 33rd to rendezvous and dock with the International Space Station (ISS). Out of 130 previous Shuttle launches, 30 have occurred in complete darkness.?But only five launches have taken place during morning twilight ? considered to be the interval occurring between the break of dawn and sunrise. One launch, in August 1985, exactly coincided with sunrise at Cape Canaveral, while another in May 1996 came just one minute after local sunup [photos of spaceships spotted from Earth].

And this upcoming launch could also be the very first one where a shuttle will take a track parallel to the Atlantic seaboard while much of the East Coast is experiencing morning twilight conditions.

On those occasions when a shuttle has been launched in the dark of night, its visibility was due primarily to the bright yellow-orange glow of its main rocket engines. But should the upcoming launch of Discovery ? its next-to-last planned mission ? go off on schedule next Monday morning, its visibility might be enhanced by sunlight reflecting off both the orbiter and its orange external fuel tank; a rather unique occurrence!

Originally scheduled to liftoff during early afternoon, Discovery's launch date slipped from March 18 to April 5 because of an unusual cold snap in Florida that delayed launch preparations. In addition, NASA wanted to avoid having a shuttle at the space station when the next Russian Soyuz transport arrives in early April.

The three new crewmembers successfully blasted off from Russia's Baikonur Cosmodrome aboard a Soyuz TMA-18 spacecraft earlier today. Docking is set for early Sunday morning, followed the very next day by the launch of shuttle Discovery. As a consequence of the two and a half week delay, Discovery's original early afternoon launch time was pushed back to before sunrise.

To reach the space station, Discovery must be launched when Earth's rotation carries the launch pad into the plane of the station's orbit. For mission STS-131, this will happen in the middle of a 10 minute launch window at 6:21:22 a.m. ET on Monday, April 5. Although at Cape Canaveral sunrise does not come until 46 minutes later, this is not considered a true "night" launch, since morning twilight will be well advanced and the eastern sky will already be noticeably bright with the approach of sunrise, though NASA officials have said it meets the technical guidelines for a night shuttle launch.

But it still should be dark enough for some of the brighter stars to be visible, as well as the planet Jupiter which will be near the eastern horizon.

What to expect

As has been the case with other launches to the ISS, Monday's liftoff will bring the shuttle's path nearly parallel to the U.S. East Coast. This SPACE.com map shows the area of launch visibility.

If you're positioned near the edge of a viewing circle, the shuttle will barely come above the horizon and could possibly be obscured by low clouds or haze.

In the southeast United States, depending on a viewer's distance from Cape Canaveral, Fla., the "stack" (shuttle orbiter, external tank and solid rocket boosters) can be easily followed thanks to the fiery output of the solid rocket boosters. The brilliant light emitted by the two solid rocket boosters will be visible for the first 2 minutes and 3 seconds of the launch out to a radius of some 510 miles from the Kennedy Space Center ? an area covering more than three times the size of Texas.

Depending on where you are located relative to Cape Canaveral, Discovery will become visible anywhere from a few seconds to just over 2 minutes after it leaves Pad 39-A.  For an example of what all this looks like from Florida, see video of a night launch made by Rob Haas from Titusville, Fla., on Dec. 9, 2006 (the STS-116 mission).

After the solid rocket boosters are jettisoned, Discovery will be visible for most locations by virtue of the light emanating from its three main engines. Based on previous night missions, the brightness should be at least equal to magnitude -2; rivaling Sirius, the brightest star in brilliance. Observers who train binoculars on the shuttle should be able to see its tiny V-shaped contrail.

James E. Byrd shot video of the shuttle from Virginia after a November 2000 night launch. The bright star Sirius briefly streaks through the scene giving a sense of scale and brightness to the shuttle's glow.

A Sunny Bonus

But this flight will have a unique bonus.

Three minutes and 22 seconds after liftoff, Discovery will have moved out to a point 443 miles northeast of Cape Canaveral and will have climbed to an altitude of 283,536 feet (53.7 miles).? It will be here that the shuttle will be high enough to see the sun emerge from beyond the distant curve of the eastern horizon.? For those watching from Florida northeast to the Carolinas, thanks to that reflected sunlight, the orbiter and its orange external tank will be shining against a still semi-dark sky and should suddenly appear to get much brighter, perhaps even surpassing Venus in brilliance as it races quickly across the heavens.?

For those living along the Middle Atlantic and Northeast U.S. coast, the first view of Discovery will likely be of a very bright "moving star" moving up from the southern horizon, that might appear to gradually fade into the background of the brightening twilight sky, since from these regions, the angle of reflected sunlight on the shuttle will diminish as it moves rapidly to the east-northeast.???

If the weather is clear, the shuttle should be relatively easy to see. It will appear to move very fast; much faster than an orbiting satellite due to its near orbital velocity at low altitudes (30-60 mi). Depending upon your distance from the coastline, the shuttle will be relatively low on the horizon (generally 5 to 15 degrees; your fist on an outstretched arm covers about 10 degrees of sky. Most importantly, make sure there are no buildings or trees to obstruct your view.?

Refer to this viewing table for more specific sighting information:

EXAMPLE:? Assuming a launch time of 6:21:22 ET, as seen from Atlantic City, shuttle Discovery will first become visible 5 degrees above the southern horizon at 6:27:44 a.m. ET. Discovery will reach its highest point at 6:29:32 a.m. ET, at an altitude of 8 degrees above the southeast horizon.? At that moment the distance between the shuttle ground track and Atlantic City will be 312 miles.

Shuttle in motion

The point of Main Engine Cut Off (MECO) will come 8 minutes 21 seconds after liftoff, at a point 366 miles southeast of New York City. At that moment, Discovery will have risen to an altitude of 340,500 feet (64.4 miles), while moving at roughly 17,500 m.p.h. (mach 24.6) and should be theoretically be visible for a radius of about 770 statute miles.? Visibility at that point, however, will be highly dependent on your viewing location and how dark your background sky is, as well as how much reflected sunlight is shining on the shuttle.? It is possible that Discovery may no longer be visible at this point for some, while others who might still have it in view may note a "puff" of vapor associated with the jettisoning of the external tank over the Atlantic.? Binoculars might even show the separation of the orbiter from the tank.

And of course, before hoping to see the shuttle streak across your local sky, make sure it has left pad 39-A at the Kennedy Space Center!

In Case of a Scrub . . .

Assuming a good load for its fuel cell system, Discovery should be able to make a number of launch attempts over a nine-day time span, extending to April 14. Should the launch be scrubbed, the liftoff time will occur 22 to 27 minutes earlier per day. If, for example, the launch is scrubbed on April 5, liftoff on April 6 is scheduled for 5:58:45 a.m. ET; on April 7 it's 5:33:03 a.m. ET.

Any delay would push the launch into darker skies. On April 6, liftoff would come just 12 minutes after the start of astronomical twilight for Cape Canaveral, while on April 7 liftoff would come 12 minutes before the first light of dawn, officially making this a nighttime launch.

Should weather or technical issues delay the launch until April 14, liftoff would then be scheduled for 2:45:50 a.m. ET.

What's Left?

After this mission, there will be just three more shuttle flights left before the shuttle program finally comes to a close (tentatively set for September 2010). 

One of these is scheduled to be launched during early afternoon, and one during the midday hours. Interestingly, however, STS-134, which will be the final flight of shuttle Endeavour, is currently scheduled for a July 29 launch just a little over an hour after sunrise. 

But if weather or technical constraints were to delay that mid-summer launch by about a week, then it too would be launched prior to sunrise and the first light of dawn, also making it a night launch.

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