After a long wait, NASA’s Orbital ATK Cygnus is headed up to the space station today, carting 7,000 pounds of gear along with it. Count down with us and watch the whole thing happen right here. UPDATE 6:25 p.m. EST: The launch has been rescheduled due to weather. The new date is tomorrow, Friday December 4th, at 5:33 p.m. (EST). We’ll be watching then!

It’s set to blast off at exactly 5:55 p.m. (Eastern Standard Time)—subject to changes, which we will be updating you on as they unfold. NASA TV is broadcasting the whole thing, and you can watch along here, as soon as they start broadcasting at 4:30:

What makes this launch unusual? Part of it is the 30 minute launch window they’ve allowed for.

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Normally rockets have a to-the-second instantaneous launch window to intercept the International Space Station. Rockets by competitor SpaceX, or national rockets by the Russian, European, and Japanese space agencies all have this restriction. A handful of now-retired rockets have performance to spare to be a bit sloppy, giving them a longer window: both the space shuttle and the Orbital ATK Antares had up to a 10 minute window. But this new United Launch Alliance (ULA) Atlas V configuration is blowing those short timespans away with a luxurious 30-minute launch window.

This extended window isn’t just about time; it’s an indication the rocket is over-powered for the purpose, but that’s part of ULA’s philosophy of maximizing the probability of a first-day launch. Along with more pure juice, the rocket also has better steering algorithms to allow it to catch up in orbit. This means that the launch team has the opportunity to wait for weather to clear up, boats to get out of the clearance zone, a technical problem to resolve, or otherwise pause for more favorable conditions instead of canceling the launch outright.

All of this means that for today, the Cygnus can launch between 5:55pm and 6:25pm EST instead of only blasting off at the very middle of the window at 6:10pm. If it doesn’t go off today, it has additional clear windows on December 4th, 5th, and 6th. Any later than that and the flight team will need to get more creative with their orbital dynamics. Alternately, Cygnus could launch late and loiter in orbit for a few weeks until rendezvousing with the station. The station will be in its annual solar-angle blackout period from December 24th through January 3rd.

Here’s hoping for a safe launch!

Update 6:18pm ET:

The launch attempt was scrubbed before the window was officially closed due to deteriorating weather conditions. This is unusual, but not entirely expected when the probability of Go-for-Launch conditions dropped from 40% to 30% down to 10% throughout the day.

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The next launch window opens at 5:33pm ET tomorrow, again lasting for 30 minutes. The weather forecast remains grim, with only a 30% chance of acceptable conditions. If it also gets scrubbed, the mission has additional windows on December 5th and 6th before mission planners need to get creative.

Top image: Cygnus on the launch pad / NASA TV Bottom image: Orbital Cygnus pre launch / NASA.

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