SpaceX will try again to land a Falcon 9 rocket's first stage at sea.
The world watched in awe when SpaceX landed its Falcon 9 rocket’s first stage on solid ground in December. But SpaceX doesn’t expect a successful landing for the Falcon 9 scheduled to launch Wednesday evening and land on a droneship at sea.
For this launch, SpaceX’s Falcon 9 is carrying an SES-9, a commercial communications satellite, for the company SES. The mission itself is already delayed: An original launch was scheduled for last September, but was then pushed back a number of times after one of SpaceX’s Falcon 9 rockets blew up last June. To make up for the delay, the Falcon 9 rocket will travel a different path to help the SES-9 satellite reach geostationary orbit—or 22,000 miles above the equator—much faster. SpaceX will take the satellite part of the way into space, and then it’s up to the satellite to propel itself to its intended orbit.
Get Data Sheet, Fortune’s technology newsletter.
But that’s what makes this mission slightly more complicated and lowers the expectation of a successful sea landing. As The Verge reports, SpaceX “has agreed to drop off SES-9 at a higher orbit than originally planned, cutting down the satellite’s solo trip.” That means a number of things: The Falcon 9 will be weighed down with more fuel, which will be needed for the launch and for taking the rocket with its heavy satellite payload into a higher orbit. The result will be less fuel for the landing. And although less fuel is needed to land at sea, SpaceX has already tried three times, and failed three times, to land the Falcon 9 at sea.
“Given this mission’s unique [geostationary transfer orbit] profile, a successful landing is not expected,” the company said in a press release.
Why the attempt at a sea landing in the first place? As Fortune reported in January, fuel and weight are huge costs when launching rockets: “For rockets carrying heavier payloads—and especially those accelerating to the speeds necessary to reach higher orbits—the economics of a return-to-origin turn less favorable.” This echoes what SpaceX CEO Elon Musk said in January: Ship landings are necessary because, in some cases, a rocket returning to its launch site is not “physically possible.” Such a situation is precisely what’s at play with tonight’s launch.
For more on SpaceX, watch our video.
Of course, a successful sea landing tonight would be a coup for SpaceX, since it would demonstrate the upstart company’s ability to launch and bring home reusable rockets in two different ways.
Falcon 9’s launch today is expected at 6:46 p.m. EST from Cape Canaveral, Fla.