SpaceX and NASA are moving ahead with the scheduled April 30 launch date of the Dragon spacecraft and its historic docking with the International Space Station after the flight readiness review was approved at the Johnson Space Center in Houston, Texas.

The comprehensive evaluation of the SpaceX mission is one of the last major steps before the company becomes the first commercial carrier to deliver payloads to the ISS. Although SpaceX founder and chief designer Elon Musk was careful to remind everybody that the flight is a test and success is far from guaranteed.

“I think we’ve got a pretty good shot, but it is worth emphasizing that there’s a lot that can go wrong in a mission like this,” Musk told reporters after the review.

The Falcon rocket being used to launch the Dragon has been used twice, once inserting a Dragon capsule into low earth orbit. But Musk and NASA did emphasize that this is not an actual mission. The goal is to demonstrate SpaceX’s capabilities to launch, rendezvous with the ISS and return to earth. And while there will be some cargo on board and some cargo will be carried back from the space station, nothing on board is considered critical or irreplaceable. Musk reminded reporters of the difficulty of the mission, but he remains confident SpaceX will succeed.

“I think it would be a mistake to put too much weight on this flight because there are hopefully going to be two more flights later this year to the space station, which will be almost identical configuration,” Musk said during the press conference. “So if this one doesn’t succeed in getting to the space station, I’m confident that one of the other two will. There should be no doubt about our resolve. We will get to the space station.”

The mission will take roughly four days until the Dragon will dock with the space station in orbit and combines two separate tests into a single flight. The capsule will then remain attached to the station for 18 days. The first demonstration test will be a flight around the space station to test and verify maneuverability, navigation and communication capabilities.

On flight day three, the Dragon spacecraft will begin a series of maneuvers that will take it on a lap around the ISS beginning with a relatively close 2.5-kilometer (1.5-mile) pass underneath the station. During this close pass the Dragon and the ISS will communicate with each other for the first time. “An absolute requirement for proximity operations,” according to NASA flight director Holly Ridings.

The crew on board the space station will send a test command to Dragon to confirm those on the ISS have the ability to control the capsule when necessary. The communication tests are to make sure the crew would be able to command the Dragon to hold or abort if needed when it is in close proximity to the station.

After the 2.5 km pass, the Dragon will move out to 200 km as it continues the lap. It will then again maneuver closer as it passes over the top of the station, this time getting as close as 7 km. The entire lap should take a full day, 22-24 hours according to Ridings.

On flight day four, the Dragon will once again be guided to 2.5 km underneath the station as preparations are made for the final tests before finally docking with the ISS. Once inside this 2.5 km zone, Ridings says the NASA team in Houston has final authority over the mission due to the proximity to the station and the safety of the crew on board.

The next parking spot will be at 1.4 km as the Dragon prepares for the approach initiation. Once everybody agrees for a go, the Dragon will maneuver to a point just 250 meters (820 feet) from the station, which will serve as its hold point for the final tests in very close proximity to the station. According to NASA, the hold point is outside the critical KOS, the acronym-happy agency’s simply named “Keep Out Sphere.”

The next demonstration objectives include interaction with the crew on orbit. From the 250 m hold point, the Dragon team at SpaceX’s Hawthorne, California headquarters will issue a command to slowly begin approaching the station, after which the ISS crew will issue a retreat command as the first test. The demonstration will be repeated, this time with the ISS crew issuing a hold command at 220 m.

“That will be the last of our go/no go objectives in terms of the demonstration objective,” according to Ridings.

After these demonstrations are complete and everybody at NASA and SpaceX are satisfied, the Dragon will then make the final maneuvers towards the station. With all the functionality of the Dragon checked out, the spacecraft will be commanded by the SpaceX team to cross the KOS boundary for the first time.

The Dragon will then stop at 30 m, where the go/no-go decision will be made by everybody on the ground as well as the crew on the station to make the final approach. Though the Dragon is an automated spacecraft and is capable of performing the entire mission autonomously, Ridings emphasized that the station crew will be heavily involved and is there as a safety net, especially on the first flight.

After maneuvering to just 10 m from the station, the Dragon will park in its final hold position, known as the capture point. Once a final decision is made for capture, the station crew takes over the final steps using the robotic arm to reach out and grab the Dragon and move into its berthing spot on the station.

The final step of the mission from the 2.5 km point to docking is expected to take around seven to eight hours. On the following day, the station crew begins the laborious cargo transfer as 521 kilograms (1,146 pounds) are offloaded from the Dragon, and 660 kilograms (1,452 pounds) of cargo from the station are placed into the capsule to be returned to earth. Thankfully, it’s mostly the motions that are laborious and not heavy lifting.

Representatives from the NASA side of the review meeting also reminded reporters of the test nature of the mission. But all sounded confident based on the preparation, simulations and tests that have been completed. NASA space station program manager Mike Sufferdini told reporters there are still some verifications that need to be done, but everything is looking good.

Sufferdini said the past few years has been a positive learning experience for both organizations and he’s excited to see a new vehicle arrive at the station. One of the differences during the flight readiness review he pointed to compared to past NASA missions is the bottom line. “There were no requirements for mission success,” he said. The simple comment in many ways marks NASA’s transition from shouldering the responsibility to deliver payloads to orbit, to a consumer of space delivery services. This point is driven home as today marks the last flight of the space shuttle discovery as it was flown to Washington, D.C., where it will be handed over to the Smithsonian.

SpaceX has one more launch simulation to complete before the launch. Sufferdini said there are some verifications that still need to be completed, but nothing that indicates there should be any issues.

The mission is part of NASA’s Commercial Orbital Transportation Services program to develop a less expensive way to deliver payloads to orbit. As part of the program NASA has invested $381 million in SpaceX. Musk didn’t give a specific number, but he estimated the SpaceX program has cost around $1 billion in total to date. SpaceX and Orbital Sciences are the two companies selected by NASA to provide commercial cargo delivery to the ISS.