Orbital ATK plans to launch the company’s Antares rocket tonight at 7:40 pm EDT from Wallops Island, Virginia. It will send a Cygnus cargo spacecraft to orbit and the Cygnus will proceed to a rendezvous and docking with the International Space Station on Sunday. (Has to wait for a Soyuz that will launch from Russia [on Wednesday morning at 4:05 am EDT. 0805 GMT] with a crew of three  to dock with the ISS first.)

Update: Here is a video of the NASA TV webcast of the launch:

Update 2: Here is the post-launch briefing: