This afternoon, Russia is launching an otherwise routine cargo spacecraft to the International Space Station — except this vehicle’s trip is meant to be the speediest trip to the ISS ever. Following its launch, the spacecraft, known as Progress, is supposed to rendezvous with the ISS within just a little under four hours. That’s a speedy jaunt compared to other cargo and crew spacecraft, which can take between six hours and two days to reach the ISS after launch.

However, it’s possible the quick journey may not happen. The Progress has to launch at a very specific time to get to the ISS in under four hours. Otherwise, it will have to take a much longer route to its final destination.

Four hours is a speedy jaunt compared to other spacecraft, which can take between six hours and two days to get to the ISS

Meeting up with the space station is a fairly complicated process, which is why it can take a while to get there after takeoff. Spacecraft that are headed to the ISS are initially put into a much lower orbit than where the space station lives, known as an insertion orbit. They then slowly raise their altitude to dock with the orbiting lab. To do this, vehicles have to ignite their onboard engines at precise times to get into higher and higher orbits. These spacecraft also have to wait a little while to catch up with the space station’s position, since they’re moving at different speeds than the ISS. Additional adjustments also need to be made sometimes to adjust the plane of the spacecraft’s orbit. It’s all a complex dance of orbits and engine burns to get the two vehicles to meet up at the same altitude and make sure they don’t slam into each other at the same time.

For Russian spacecraft, like the Progress or crew-carrying Soyuz vehicle, these rendezvous can take 34 orbits (two days) to complete. However, Russia has also pulled off much shorter four-orbit (six-hour) meetups with the ISS before. Today’s trip is meant to get the Progress to the ISS in just two orbits, making the journey a total of just three hours and 48 minutes.

This fast rendezvous is tricky to pull off, though, as the timing and position of the ISS has to be just right. In fact, NASA and Russia had to prepare weeks in advance for this trip by boosting the orbiting of the ISS on June 23rd. This orbital maneuver ensured the path of the ISS would line up today with that of the Progress’ launch site, the Baikonur Cosmodrome in Kazakhstan. That means that the Progress, which rides to space on a Soyuz rocket, has to launch at a very specific time during this alignment. If it misses this instantaneous deadline, it will have to perform a typical two-day trip instead.

Russia has wanted to try out this quick four-hour trip on two Progress missions before, but was forced to perform longer trips with them after missing the precise launch times. However, if today’s trip works out, then Russia may consider trying this on future Progress flights as well as Soyuz flights that carry humans to the ISS. That way, astronauts wouldn’t have to wait as long in the cramped confines of the Soyuz before getting to roam around inside the spacious living quarters of the ISS.

Today’s Progress vehicle is loaded with nearly three tons of supplies for the crew members of the ISS, and the spacecraft is slated to remain at the station until January 2019. Today’s strict takeoff time is scheduled for 5:51PM ET. NASA’s coverage begins at 5:30PM ET, so check back then to watch this mission live.