SpaceX's upcoming Falcon 9 rocket launch will likely be pushed back from its target date of Saturday, as the company seems to be having some issues with the rocket's propellant. CEO Elon Musk tweeted that they're having trouble keeping the fuel cool enough, which is delaying pre-flight testing of the vehicle. Now, it's uncertain if the rocket will launch this weekend.

The delays add to the heightened tension surrounding this mission — which will be the first launch of a Falcon 9 rocket since June. SpaceX's fleet has been grounded for the past six months, after one if its rockets exploded en route to the International Space Station. This next launch will put an end to the company's flight hiatus, as well as kick off a busy month of Falcon 9 launches for SpaceX, that will carry satellites and cargo into space.

The Falcon 9 being used for this flight is already set up on the launch pad at Cape Canaveral, Florida, and the 11 satellites its carrying are packed on board. But before it can take off, SpaceX wants to do a static fire test on the rocket. That's when the vehicle is restrained and its engines are momentarily ignited, to see if everything is working properly. The static fire test was supposed to happen on Wednesday, December 16th, with the rocket launching three days later. But the test was delayed until Thursday, and delayed again until today, though SpaceX won't say what time it's supposed to occur. Once the static fire test is complete, the launch will happen a few days later. The timing of that is also unclear.

CEO Elon Musk said the Falcon 9's propellant is posing some challenges. For this launch, the company is using an upgraded version of the Falcon 9 — informally called the Falcon 9 v1.1 Full Thrust — which uses cryogenic liquid oxygen as fuel. That propellant has to be kept extremely cold at -340 degrees Fahrenheit, and Musk hinted that the company is having a hard time keeping the temperature down. Maintaining the temperature wasn't a problem at SpaceX's test facility in McGregor, Texas, according to the CEO, so it may be an issue integrating the rocket in the Florida launch pad.

Once SpaceX fix's the fueling problems and executes the static fire, then the company will provide more concrete details about the launch date — and whether or not rocket landing attempt will be made. Earlier this month, a NASA official told press that SpaceX intended to land the Falcon 9 on solid ground after launching it into space. That would be a first for the company, which has been trying to land the rocket on an autonomous drone ship at sea for the past year. It's all part of SpaceX's ultimate goal of reusing its rockets post-launch. However, without a set launch date, SpaceX has yet to confirm if such a landing will take place.

Update December 18th 2:30PM: Patrick Air Force Base, which oversees launches at Cape Canaveral Air Force Station, released a weather report indicating that SpaceX may try to launch on Sunday, with Tuesday as a backup date.