UPDATE, 4:20 pm ET:   The launch is now targeted for 6:10 pm ET, with NASA TV coverage beginning at 5:30 pm ET.   Upper level winds remain a concern.

ORIGINAL STORY:  The chances of favorable weather for the rescheduled launch of Orbital Sciences Corporation’s Antares rocket tonight, Saturday, April 20, is 90 percent.   The launch window is open from 5:00 – 7:00 pm ET.  NASA TV will begin coverage at 4:30 pm ET.

The launch will take place from the Mid-Atlantic Regional Spaceport (MARS) at NASA”s Wallops Flight Facility on Wallops Island, VA.   Weather permitting, a large part of the East Coast may be able to see it as illustrated on this map, courtesy of Orbital.

The first launch attempt was scrubbed on Wednesday when an umbilical detached prematurely.

This is a test launch of Orbital’s new rocket, developed as part of NASA’s Commercial Orbital Transportation Services (COTS) program to facilitate commercial development of space transportation systems to take cargo to the International Space Station (ISS).   If this test is successful, a second test carrying Orbital’s Cygnus spacecraft will take place in about three months to demonstrate the rest of the mission profile to rendezvous and be berthed with the ISS.

Orbital is tweeting updates @OrbitalSciences.  Late this morning, a tweet said that “upper level winds are marginal based on earlier balloon data,” but otherwise the weather looks good.

A very poor weather forecast for tomorrow afternoon means that Orbital Sciences Corporation will wait until Saturday, April 20, before attempting to launch its new Antares rocket again.  The launch window opens at 5:00 pm ET on Saturday.

The first attempt last night was scrubbed 12 minutes before launch when an umbilical providing data between the Transporter Erector Launcher (TEL) and the rocket’s upper stage separated prematurely.   Orbital determined that a “slight hydraulic movement” of the TEL coupled with insufficient slack in the cable caused the cable to pull out.  The company was hoping to resolve the problem and launch tomorrow, but the forecast prevents that.

Weather Underground’s forecast for Wallops Island, VA, where Antares will be launched, calls for 15-25 mile per hour (mph) winds, gusting to 30 mph, and a 90 percent chance of rain and thunderstorms tomorrow night.

This is the first test launch of Antares as part of NASA’s Commercial Orbital Transportation Services (COTS) program to develop commercial space transportation systems to take cargo to the International Space Station (ISS).  Orbital is hoping for a second test launch to send its Cygnus spacecraft to the ISS this summer and for operations to commence soon thereafter.

This article was updated in its entirety at 6:40 pm ET.

The test launch of Orbital Sciences Corporation’s Antares rocket was scrubbed today because of premature separation of an umbilical from the rocket’s second stage.

The launch was scheduled for 5:00 pm ET and the main concern was weather at the launch site at Wallops Island, VA.  As launch time neared, the weather was improving, but at T-12 minutes an umbilical separated from the second stage prematurely.  Orbital says it is gathering data and the mission management team will meet at 8:00 am ET tomorrow morning to review the situation.  One @orbitalsciences tweet said the next most likely attempt would be on Friday, but at a press conference yesterday, officials said Friday was a very poor launch day in terms of weather.

The Antares launch may be viewable along the East Coast and inland as far west as West Virginia.   The map below, courtesy of Orbital, shows where it might be visible — weather permiitting — and the elevation angle above the horizon.

Orbital Sciences Corporation’s Antares rocket is ready for its first test launch tomorrow, but the weather forecast is only 45 percent favorable.

Low clouds and possible showers could force a postponement.  Additional launch opportunities are available April 18-21, but decisions on when to reschedule would be dependent on a number of factors.  At a press conference today, Mike Pinkston, Orbital’s Antares program manager, said he anticipated a “dynamic decision process” if they need to reschedule.  He expects that they would “almost definitely” try again on April 18, but whether they would slip from the 18th to the 19th is less certain.  Not only is the forecast for April 19 especially poor, but the launch crew needs sufficient time to rest.

Assuming a successful test launch, the next flight would be a demonstration mission to the International Space Station (ISS) in about three months.   That would carry the Cygnus cargo spacecraft; the launch tomorrow will carry only a 3,800 kilogram mass simulator.   Antares and Cygnus were developed through NASA’s Commercial Orbital Transportation Services (COTS) program where the agency provided some of the funding for Orbital to develop the system on a commercial basis.  

NASA already signed a $1.9 billion contract with Orbital for eight operational launches.  Frank Culbertson, Orbital’s Executive Vice President and General Manager of the Advanced Programs Group, said they expect to launch the operational missions every 3-6 months after the test program is completed.   Under the contract, NASA and Orbital will jointly agree on when the eight missions take place, but he expects they will be completed around 2016.   

Culbertson stressed that this is a test launch.  Designated the A-One mission, the objectives are to successfully achieve launch and ascent events and collect launch vehicle and payload data.  The target orbit is 250 kilometers by 303 kilometers inclined at 51.64 degrees.  The 3-hour launch window opens at 5:00 pm ET tomorrow.

Orbital Sciences Corporation still hopes to conduct its first test launch of the new Antares rocket on Wednesday despite an anomaly during a wet dress rehearsal yesterday.

Orbital said in a press statement that yesterday’s test was halted at about T-16 minutes.  The company determined “a secondary pyro valve aboard one of the two first-stage engines” malfunctioned and it plans to replace the valve within 24 hours to keep the April 17 launch date.

Antares and its Cygnus spacecraft are competitors to SpaceX for cargo flights to the International Space Station through NASA’s Commercial Orbital Transportation Services (COTS) program.  This test launch will not carry a Cygnus, which has yet to fly in space.  Instead it will carry a mass simulator.  Unlike SpaceX’s Dragon spacecraft, Cygnus is not designed to return to Earth.  Orbital is not competing in the effort to develop a system to transport people to and from space.  Cygnus is a one-way cargo spacecraft only, like Europe’s ATV, Japan’s HTV, and Russia’s Progress. 

Antares will be launched from Wallops Island, VA.  The launch window on April 17 is 5:00-8:00 pm ET.  If the launch is delayed, additional opportunities are available April 18-21.