The Axiom-4 space paradox: Why does it take 28 hours to cover just 400 km?

Axiom-4 Mission: India’s Group Captain Shubhanshu Shukla joined a historic 28-hour journey to the ISS aboard SpaceX's Dragon capsule, showcasing the precision and science behind space docking.

Axiom-4 Mission: At 12:01 PM IST on June 25, a spacecraft carrying four astronauts — including India’s Group Captain Shubhanshu Shukla on his first journey to space — took off from NASA’s launch site in Florida.

The Falcon 9 rocket lifted off smoothly, marking the start of the long-awaited Axiom-4 mission. After the rocket’s two stages completed their job, the Dragon crew capsule separated and began traveling at thousands of kilometers per hour.

The capsule’s destination was the International Space Station (ISS), which orbits the Earth about 400 km above ground. At launch, Axiom Space — the company behind the mission — said it would take the Dragon capsule around 28 hours to reach and dock with the ISS.

This raised an obvious question:

Why does it take 28 hours to travel just 400 km — a distance similar to that between Chennai and Bengaluru?

The Race Track Analogy

Think of the ISS and Dragon capsule like two race cars on different lanes of a circular track. The Dragon capsule can’t go straight up to the ISS. It must join it in orbit, matching its speed and path.

So, instead of heading straight for it, the spacecraft must slowly adjust its position and speed until it's ready to safely dock.

The Stadium and Bicycle Example

Imagine the Earth as a giant stadium. The ISS is like a fast-moving bicycle going around the top level. The Dragon capsule is another bike trying to join in.

Even though the ISS is only 400 km above — like a balcony in the stadium — the second bike can’t just go straight up. It has to:

• Move sideways, like the ISS
• Speed up gradually to avoid crashing
• Match both the height and direction of the ISS

This step-by-step process takes time — about 28 hours — even though the height difference is small. In space, everything moves very fast, and safety is key.

The Two Critical Requirements

To dock with the ISS, Dragon must:

• Reach the same height (around 400 km above Earth)
• Match the speed and direction of the ISS — called the velocity vector

This needs precise timing and control, not just speed. The science behind this is called orbital mechanics, like traffic rules in space.

The Moving Train Example

Imagine trying to jump onto a moving train. You can’t just run straight at it. You must:

• Be on the same level
• Run next to it, at the same speed and direction

If you’re too slow, you miss it. Too fast, and you overshoot. Wrong direction? You crash.

Dragon must do the same — adjusting its speed and height to safely match the ISS, which is orbiting Earth at about 28,000 km/h.

The Flyover and Bus Analogy

Think of the ISS as a bus on a flyover, and the Dragon capsule as a car trying to join it.

The car can’t just drive straight up. It has to:

• Reach the same height as the flyover
• Match the bus’s speed and direction
• Then safely merge and drive alongside it

Dragon does the same in space, using carefully planned moves to match the ISS’s orbit.

Why Sideways Speed Matters Most

Most of the rocket’s energy pushes the Dragon capsule sideways around the Earth, not just upward.

If it only went straight up 400 km without sideways motion, it would have no orbit speed — and would fall back to Earth, like a ball tossed in the air.

To stay in space, Dragon must move sideways very fast — that’s how it keeps circling Earth without falling.

The Starting Point: Parking Orbit

After liftoff, Falcon 9 places Dragon in a parking orbit — a slightly oval path about 200 km above Earth.

Here, Dragon travels at about 27,000 km/h, fast enough to stay in orbit and avoid falling back. It doesn’t head directly to the ISS right away.

The Chase Begins: Different Heights, Different Speeds

The ISS orbits higher — around 400 km — and moves slightly slower than Dragon.

The ISS takes 92 minutes to orbit Earth once. Dragon, starting lower, takes 88 minutes per orbit. Since it’s moving faster, it slowly catches up from behind — closing the distance with each lap.

Climbing Up: Phasing Burns

Once system checks are done, Dragon uses small engine bursts called phasing burns.

These short thruster moves:

• First raise the highest point of Dragon’s orbit (called apogee)
• Then lift the entire orbit gradually until it matches the ISS's height

This slow climb ensures a safe and smooth approach.

The 18-Orbit Journey

The mission team planned a path that took Dragon 18 orbits to reach the ISS — about 26.5 hours, since each orbit takes 88 minutes.

The remaining time is used for final adjustments and careful timing to arrive when the ISS crew is awake and ready for docking.

The Final Approach: 30 km to 1 meter

Once Dragon gets about 30 km from the ISS (in the same orbit, not height), it enters a special approach corridor aligned with the station.

• From here:
• Dragon must slow to just a few meters per second
• When within 20 meters, it slows down further — to a few centimeters per second

This slow approach helps avoid any collision.

Multiple Safety Checkpoints

Dragon stops at several holding points: 400 m, 220 m, 20 m, and 1 m from the ISS.

At each stop, the crew — including Group Captain Shubhanshu Shukla — performs safety checks like:

• GO/NO-GO decisions
• LIDAR scans (laser-based distance measuring)

If even one reading is off, Dragon must back away using a pre-programmed path to keep everyone safe.

Why the Extra Time Is Worth It

This careful, step-by-step approach adds a few extra hours to the mission.

But it's worth it. These slow moves, safety checks, and pauses make sure that the spacecraft docks without any risk to the crew or the ISS.

The Dragon Advantage

Using SpaceX's Dragon capsule made this safe approach possible.

Dragon allows a fuel-rich, slower plan, giving astronauts time to:

• Complete all checkouts
• Eat and rest
• Be fully prepared for the final docking phase

The 28-hour journey may seem long for just 400 km, but it’s a carefully designed process based on science, safety, and precision.

Proud Moment for India

Every minute of this 28-hour mission was planned with care — to make sure Group Captain Shubhanshu Shukla and his team reached the ISS safely.

It marks a proud and historic step forward in India’s growing space journey.

(Girish Linganna is an award-winning science communicator and a Defence, Aerospace & Geopolitical Analyst. He is the Managing Director of ADD Engineering Components India Pvt. Ltd., a subsidiary of ADD Engineering GmbH, Germany. Contact: girishlinganna@gmail.com )

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