SpaceX is launching its last mission of 2017 later today, with a planned launch window of 5:27 PM PST (8:27 PM EST). The mission is for client Iridium, and will send a fourth set of 10 satellites into low Earth orbit for Iridium’s NEXT constellation, which will eventually consist of 75 satellites launched by SpaceX in total.

SpaceX is launching the Falcon 9 for Iridium-4 from California’s Vandenberg Air Force Base, where it has its Space Launch Complex 4E. It’ll be the 18th launch of a Falcon 9 for SpaceX this year – but it won’t include a recover attempt for the first stage, as the launch requirements include configuring the Falcon 9 used to be expended during the launch process.

The Falcon 9 first stage booster being used for the mission has been used before, however – it was flown during Iridium-2, the second mission that Spacex flew for this client, which took place in June. That means this rocket was turned around and ready for re-use in just six months.

SpaceX launched an International Space Station resupply mission just last week, using both a reflown Dragon capsule and a reflown Falcon 9 first stage. It’s also preparing for its first ever Falcon Heavy launch, which should take place early in 2018.

If the mission today is scrubbed for any reason, SpaceX has a backup launch window on Saturday, at 5:21 PM PST.