EXPLAINED | Why does it take 28 hours for Axiom-4 astronauts to reach space station?

Axiom-4 astronauts aboard the Crew Dragon capsule, nestled atop the SpaceX Falcon 9 rocket, blasted off at 12:01 pm Indian time, 25th June, Wednesday. Barely 13 minutes after liftoff, the SpaceX rocket placed them in orbit around the Earth, and the crew began feeling microgravity-induced weightlessness. However, their journey to the International Space Station would take as much as 28 hours. Here are the technical and scientific reasons behind why the four astronauts would reach the International Space Station only by 4:30 pm Indian time, 26th June, Thursday.

Energy and fuel-efficient spaceflight

There are space missions where the crew reach the space station within seven hours of launch, this is known as "fast rendezvous". However, that requires highly precise planning, perfect alignment between the launching spaceport and the space station. Approaching the space station in such a short duration also requires a higher number of manoeuvres and engine firings. This would mean that more fuel is consumed in the process. To cater for this, the spacecraft must carry adequate fuel, and the rocket must be capable of lifting the heavier craft.

Instead, the Crew Dragon capsule is taking the 28-hour option. This allows adequate time to perform various system checks before gradually pushing the craft into orbit where the Space Station is. During this 28-hour journey, the Crew Dragon craft works fully autonomously, with the four astronauts only monitoring all the crucial parameters. In extremely rare cases, the Mission Commander(leader) would have to use the touch screen controls and take control of the touchscreen interfaces to manually manoeuvre the craft. However, such occasions are very rare with the Crew Dragon.

Crew Dragon and Space Station - Both orbits need to intersect

Initially, the Crew Dragon Spacecraft is launched into an orbit (circular path) that is 200 km above the Earth. The Space Station is about 400 km above the Earth. The way space works, it is not possible for Crew Dragon to travel 200 km straight upwards and meet up with the space station immediately. Therefore, the Crew Dragon gradually adjusts its orbits via a series of engine burns. Each of these engines burns pushes the craft into a higher circular path (orbit), and eventually brings it into a path that matches that of the International Space Station. It must be remembered that objects in low Earth orbit travel at 28,000kmph, and extreme care must be taken during manoeuvres.

The final runup to Docking

The final approach is done in stages, once the Crew Dragon spacecraft gets barely 400 meters from the Space Station. Then, the Crew Dragon gradually moves itself towards the Space Station, while maintaining constant communication. The 400 metres is brought down to 200 meters, and eventually 30 meters and then the docking takes place. At each of these distances, the spacecraft may hold to carry out system checks, communicate with the space station and ground controllers, and monitor other technical parameters. When within 30meters of the space station, the Crew Dragon uses its cameras, Light Detection and Ranging(LIDAR) sensors.

Docking

While a few centimetres away from the Space station, the docking ring of the spacecraft makes contact with the space station's docking port. Then, once aligned, the latching mechanisms align and create a pressure-tight seal. The electrical and data connections are made, and the Crew Dragon becomes a part of the ISS. After various checks are carried out, the hatches on both sides are opened, and the Crew Dragon astronauts enter the space station.