The Axiom-4 Mission to the International Space Station (ISS) will launch from the Kennedy Space Centre in Cape Canaveral, Florida, on Wednesday.

The mission will take a crew of four astronauts — American Peggy Whitson, Indian Shubhanshu Shukla, Polish Slawosz Uznanski-Wisniewski, and Hungarian Tibor Kapu — to the ISS aboard SpaceX’s Crew Dragon spacecraft which will be launched by the company’s Falcon 9 rocket. Shukhla is set to become only the second Indian to venture into space.

What will his journey to the ISS look like? How much time will it take to reach the space station? And once there, how will the Dragon capsule dock with the ISS?

Story continues below this ad

Planning the flight

Before launching any mission to space, scientists first have to first select a launch window, that is, a time slot in which the spacecraft must be launched so it can reach its intended destination, be it a space station like the ISS or a celestial body like the Moon or Mars.

Since everything in space — including the ISS — is in constant motion, it is not viable for a mission to be launched at just any time. Celestial alignment is essential for any mission to be viable. Scientists make complex calculations to ensure that the trajectory of the spacecraft aligns with the trajectory of the intended destination.

In case of missions to the ISS, spacecraft orbit around Earth multiple times to align with the orbit of the space station. Such a trajectory also makes any mission viable in terms of the fuel needed.

If a spacecraft were to travel to its destination in a straight trajectory, it would have to continuously accelerate to counteract gravitational forces, which would be inefficient in terms of the fuel needed.

Story continues below this ad

Spacecraft usually travel in a curved trajectory upon reaching a certain altitude and velocity, which minimises the energy they need to expend to counteract the force of gravity.

Also Read | Axiom-4 mission carrying Shubhanshu Shukla postponed due to technical snag

The rocket & capsule

Falcon 9 is a partially reusable rocket designed and manufactured by Elon Musk-owned SpaceX. It is used to transport satellites, cargo and the Dragon spacecraft to low Earth orbit (an altitude of 2,000 km or less) and beyond.

The rocket has two stages. The first stage or booster stage comprises nine Merlin engines (a family of rocket engines developed by SpaceX), and aluminium-lithium alloy tanks containing liquid oxygen and rocket-grade kerosene propellant. The second stage consists of a single Merlin engine.

After the lift-off, as Falcon-9 reaches the edge of the atmosphere, it typically cuts off its main engines. Once the rocket is beyond the atmosphere, the first stage separates from the second stage.

Story continues below this ad

While the first stage re-enters the atmosphere and lands vertically, the second stage continues its journey towards the targeted orbit with the help of its Merlin engine. Soon after, the Dragon capsule separates from the second stage.

Dragon’s path to the ISS

Given that the ISS is 400 km above Earth and is a moving target at the speed of about 28,000 kmph, the Dragon spacecraft has to raise its altitude gradually, and align its trajectory with the space station.

The spacecraft does so by performing a series of phasing manoeuvres — they enable the Dragon to change its orbit — with the help of 16 Draco thrusters. Each thruster is capable of generating 90 pounds of force in the vacuum of space.

Note that while the Dragon spacecraft typically takes 28 hours to reach from the launchpad to the ISS, other spacecraft such as Russia’s Soyuz take up to just eight hours to cover the same journey.

Story continues below this ad

One of the reasons why the Dragon is slower is that it is a relatively newer spacecraft (it was first launched in 2012) compared to, let’s say, Soyuz, which has a long and proven flight history. SpaceX is still in the process of developing mathematical models for optimal start time and phasing manoeuvres.

Due to this, astronauts aboard the Dragon conduct multiple tests of all the spacecraft’s systems and collect and transmit data about its operation to Earth which makes its rendezvous time longer.

The docking

When the Dragon capsule gets close enough to the ISS, it establishes communication with the space station and performs its final phase manoeuvre. Then, the spacecraft enters an imaginary 200 metres bubble around the ISS known as the “keep-out sphere”, and aligns with the space station’s docking port. At this point, the Dragon capsule initiates its autonomous docking system and slowly moves towards the ISS to finally dock with it. This happens while both are moving at great speeds but are almost at rest relative to each other.

The spacecraft carries out autonomous docking with the help of GPS sensors, cameras and imaging sensors such as Lidar (laser ranging) on its nosecone. All these sensors feed data back to the flight computer which then uses algorithms that determine — based on this information — how to fire the thrusters to most effectively get to the docking target. If needed, the astronauts on board can also take over manual control of the spacecraft.

Story continues below this ad

After the docking is done, it takes between one and two hours for the spacecraft to stabilise and carry out safety checks. Subsequently, transfer gates are opened, and astronauts enter the ISS.