5:00 p.m. update: Officially, the 5-day launch window for the Antares rocket continues through April 21, so another attempt could occur in the coming days.  However, a new launch date has not been selected. According to NASA TV “quite a bit has to happen before another launch is attempted.”  For its part, the weather is unlikely to cooperate for a launch through Friday, but the weekend may offer an opportunity provided the rocket’s technical issues can be overcome. (5:30 p.m. update: Meteorologist Dan Satterfield tweeted he spoke to a NASA official who said launch will not be tomorrow, Thursday. 6:00 p.m. update: Orbital Sciences tweeted the next likely launch attempt will be Friday at 5 p.m.).

4:50 p.m. update: The Antares rocket launch from NASA’s Wallops Island flight facility was aborted due to premature separation of one of the umbilicals from the second stage of the rocket. The umbilical is a connecting cable between the launch vehicle stage and the telemetry and power on the ground.

4:15 p.m. update: NASA is preparing the Antares rocket for launch but could suspend it based on a final evaluation of weather conditions. Watch live coverage on NASA TV. Skies remain mostly cloudy in D.C.  Although the cloud shield overhead is sinking south, it may not clear in time to view the launch locally (should it occur).

3:20 p.m. update: Even if Wallops Island proceeds with launch, clouds may make viewing difficult in the D.C. area. It’s currently more cloudy than not, though it’s possible there could be sufficient breaks to catch a glimpse of the rocket come launch time. Stay tuned for updates, and keep an eye on the satellite loop for the latest cloud cover.

2:45 p.m. update: NASA has increased the likelihood of launch to 55 percent (from 45 percent earlier).

Blast off!

Assuming good weather, Washingtonians will be able to catch a view of the most spectacular rocket launch from NASA’s Wallops Island, Va. flight facility since its inception.

“…it’s going to be the biggest and loudest and brightest thing that’s ever launched from Wallops! So it’ll be visible up and down the coast,” Frank Culbertson, an executive with Orbital Sciences Corporation, the rocket builder, told CBS News.

The unmanned rocket is scheduled to lift off at 5 p.m.,with the launch window extending until 8 p.m.

The big wild card for hopeful skywatchers is the weather.  NASA says the chance of launch is just 45 percent due to the possibility of low clouds and showers.

If it’s determined the mission is a go, look east and the rocket will be viewable about 10 degrees above the horizon in the Washington area.

The rocket is 133-feet tall, the largest ever launched from Wallops Island, CBS News reports.

Designed “to provide responsive, low-cost, and reliable access to space“,  the Antares rocket is being launched as part of an Orbital Sciences test mission with the goal of transporting cargo to the International Space Station (ISS).  No delivery to the ISS will be made on this mission but, in a second test mission planned for June,  the rocket will eject a capsule – named Cygnus – that will hook up with the ISS.

“It will demonstrate the ability of Cygnus to rendezvous and berth with ISS and then destructively reenter Earth’s atmosphere,” NASA says.

Watch: Animation of the launch, flight, and capture of an Orbital Sciences Cygnus capsule on a demonstration mission to the International Space Station

Assuming this second demonstration is successful, Orbital Sciences is contracted by NASA to complete eight cargo delivery missions to the ISS – the first tentatively scheduled for September.

Additional viewing information

* Real time launch updates will be available on Facebook and Twitter.

* Launch status is available by phone by calling 757-824-2050.

* The launch will be broadcast on NASA TV.

* For those living in or visiting the local area, there are two primary viewing locations for Wallops launches: (1) The Wallops Visitors Center located along Virginia Highway 175. Visitors are advised to arrive early as parking is limited. More info: http://sites.wff.nasa.gov/wvc/. (2) The beach on Assateague Island (Virginia-side). Capacity is based on a number of conditions, such as time of year and state of the beach. More info: http://www.fws.gov/northeast/chinco/. Finally, visitors to the area may also inquire with local tourism officials on alternate viewing locations as well as any local business events tied into Wallops launch operations.

* Information on upcoming missions and updates during launch countdowns are available in the Wallops area on radio station 760 AM.