The used booster indeed worked just as well as a new one. “We just had an incredible day today,” Mr. Musk said.

Why Is This a Big Deal?

It is cheaper than using a new rocket, potentially much cheaper.

Until now, rockets have almost all been single-use. Once the fuel is expended, a rocket stage plummets to Earth, a quick demise for a complex machine that cost tens of millions of dollars to build.

Mr. Musk has likened that to scrapping a 747 jet after one flight, which would make air travel impossibly expensive.

How Much Cheaper?

We do not know. Ms. Shotwell, the SpaceX executive, has suggested launches with reused boosters could be discounted, to 30 percent off the usual $62 million price tag. SES asked for 50 percent off. Both SpaceX and SES are private companies, and they have not divulged the negotiated going rate, except to acknowledge there was a discount.

Mr. Musk has suggested that rocket launches could eventually be much cheaper since the cost of the rocket propellants are less than 1 percent of the full-price ticket for a launch. So, if a rocket could be simply refueled like a jetliner for another flight, the cost of space travel could drop to a fraction of what it is now.

But the stresses of spaceflight on reused boosters — like the rising mileage on a used car, sometimes called “pre-owned” in today’s parlance — are much greater. The economics will depend on how many times a booster can be flown, and how much the individual expense will be to refurbish the booster — and particularly the engines — each time.