Yesterday, SpaceX re-ignited the engines on the Falcon 9 rocket the company famously launched and then landed in December. The idea was to see if the rocket's first stage booster is in suitable condition to fly again after its trip to and from space. CEO Elon Musk said that overall, data from the test looks good, but one of the outer engines showed "thrust fluctuations" — without going into detail about what that means. Musk said it's possible there is some debris inside the engine, and the company will investigate more closely.

Maybe some debris ingestion. Engine data looks ok. Will borescope tonight. This is one of the outer engines. — Elon Musk (@elonmusk) January 16, 2016

Musk has noted that this particular Falcon 9 won't return to space; SpaceX considers it too special to launch again, since it's the first rocket the company has ever landed. But SpaceX still wants to know if the booster could perform another mission if warranted — and how much refurbishment would theoretically be needed to launch it a second time.

SpaceX wants to know if the booster could perform another mission

The whole point of landing and reusing rockets is to save money on manufacturing costs. Right now, an entirely new rocket must be made for each flight. By launching a used first stage booster for certain missions, SpaceX could potentially cut down on the $60 million it spends on making each new rocket. Instead, the company would only need to spend $200,000 on refueling the vehicle. But if the inspections and refurbishments of a rocket between flights turn out to be too costly and lengthy, then that will erode the economic benefits of reusability. SpaceX has stated previously that it's confident its rockets won't require too many repairs, if any, after they have landed.

Meanwhile, SpaceX is gearing up for its next rocket launch and landing attempt on Sunday. The company's Falcon 9 is scheduled to take off from Vandenberg Air Force Base in California on Sunday at 1:42PM ET, sending NASA's Jason-3 ocean-monitoring satellite into orbit. Afterward, the rocket will try to land on a drone ship at sea — not at a land-based spaceport like it did in December. It's a harder target to stick, but if SpaceX lands the vehicle without crashing it, that rocket could be the first Falcon 9 to be launched into space again. It all depends on what the company decides to do with it. Check back on Sunday to follow along with our liveblog of the launch.