SpaceX and Nasa are moving ahead with the scheduled 30 April
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
emphasising 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 emphasise 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
"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
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 manoeuvrability,
navigation and communication capabilities.
On flight day three, the Dragon spacecraft will begin a series
of manoeuvres that will take it on a lap around the ISS beginning
with a relatively close 2.5 km (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 manoeuvre 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
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 manoeuvre 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
"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
manoeuvres 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 emphasised that the station crew will
be heavily involved and is there as a safety net, especially on the
After manoeuvring 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 kg
(1,146 pounds) are offloaded from the Dragon, and 660kg (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
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 (£240 million) in SpaceX. Musk didn't give a specific
number, but he estimated the SpaceX program has cost around $1
billion (£627.5 million) in total to date. SpaceX and Orbital
Sciences are the two companies selected by Nasa to provide
commercial cargo delivery to the ISS.