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EXPLAINED: Why It Takes A Rocket 22 Hours To Cover The 400Km To The International Space Station

A SpaceX Falcon 9 rocket, with the Crew Dragon capsule, is launched carrying three NASA and one ESA astronauts on a mission to the International Space Station at the Kennedy Space Center in Cape Canaveral, Florida, U.S. November 10, 2021.

A SpaceX Falcon 9 rocket, with the Crew Dragon capsule, is launched carrying three NASA and one ESA astronauts on a mission to the International Space Station at the Kennedy Space Center in Cape Canaveral, Florida, U.S. November 10, 2021.

The SpaceX rocket can hit top speeds of close to 40,000kmph. But flying to space destinations is not a simple case of going from point A to B in a straight line

Led by Indian origin astronaut Raja Chari, a Nasa mission blasted off for the International Space Station (ISS) earlier this week atop a SpaceX Falcon 9 rocket. The rocket is capable of speeds of more than 39,000kmph, yet it will take the travellers about 22 hours to reach humanity’s home in the sky. So, why does it take so long to cover the distance that an average passenger car can cover on Earth in a matter of hours?

Why Can’t Rockets Head Straight Up To The ISS?

The traffic to space is thicker than ever before and the queue to heavenly destinations is only going to get longer. Thankfully, you’d say, we have rockets that zip us beyond earth’s atmosphere at supersonic speeds. The land speed record is of over 1,200kmph while the fastest fighter jets can hit up to 3,500kmph. Space-bound rockets on the other hand habitually travel at above 30,000kmph. Which is par for the course given the stellar distances involved in travels out there.

But it is just that space travel times are not so straight as we might like to think and arriving anywhere up there is not solely dependent on how fast you travel. That’s because things in space are not stationary, remember. And things like planets, satellites, and the ISS, are moving quite fast. The Earth, for example, revolves around the Sun in an almost circular orbit at a speed of about 108,000kmph.


The ISS orbits the Earth at a speed of about 28,000 kmph. It needs to travel that fast because it is in low Earth orbit (LEO) and moving any slower would drag it back down to Earth. That is, while it is falling towards the Earth, it is laterally moving fast enough so that its path matches the curvature of the Earth. But that still does not explain why it takes spacecraft almost a day to reach the ISS?

How Do Spacecraft Reach ISS?

To get to the ISS, a rocket first has to move to the elevation where the ISS is perched and then get into the orbit it is moving in. As experts point out, given the circular route it is travelling around Earth, when the docking finally happens, it is a case of the ISS catching up with the spacecraft and not really of the latter arriving at the space laboratory.

There are several factors involved in getting a spacecraft to dock with the ISS, or landing a probe on Mars or the Moon. Key among them is the need for speed. That is, the speed required to escape the gravitational pull of the Earth. For this a rocket needs to employ great thrusting power. But such power is a function of the fuel it is carrying and that fuel in turn adds to the weight of the vessel where every additional kilogram just makes the job of lifting a vehicle to space that much tougher.

So, what scientists aim for when launching a spacecraft is to find a low energy mode to reach it to the destination. For this they rely on a special type of orbit, called the Hohmann Transfer orbit. The Hohmann transfer is described as “an orbital manoeuvre that transfers a satellite or spacecraft from one circular orbit to another" and is named after the German scientist who hit upon the strategy in 1925.

While it is considered to be the most fuel efficient way to get from one circular orbit to another circular orbit, “it is a fairly slow process". Which explains why it takes 22 hours to reach the ISS or eight months to get to Mars, when the straight line journey (if these space bodies had been stationary) would have taken a matter of minutes or about a couple of months, respectively.

Once a rocket lifts off, it has to first aim at escaping Earth’s gravitational pull and reach orbital space. Fly too fast though and you could overshoot the ISS, and going too slow is out of the question since the rocket would fall right back to Earth. Instead of heading to ISS, or any celestial body for that matter, in a straight line, a rocket needs to find an orbital path by employing the Hohmann transfer method.

Thus, to complete the Hohmann Transfer, the rocket first uses its thrusters to get into space and, once there, it again fires up its engines to attain the second, circular orbit in which the destination is placed. This involves the spacecraft moving around orbit itself before it finally uses a series of brief correction burns to get into the correct place for docking with the ISS.

But if both the ISS and the spacecraft are moving at dizzying speeds, then why do videos from space look like things are happening in ultra slow time? That is explained by relative speed. When you’re moving fast on Earth, things around you are standing still. But if everything was moving at the same speed, then you wouldn’t notice how fast they are moving.

Is There No Simpler Way To Reach Space?

Right now, the Hohmann Transfer is the best route to heavenly objects, but scientists and space enthusiasts have thought of building a ‘space elevator‘ to facilitate future forays into the deep skies.

According to US space agency Nasa, what a space elevator would involve is dangling “a long cable extending from our planet’s surface into space with its centre of mass at geostationary Earth orbit (GEO)", that is, at a height of over 35,000km. “Electromagnetic vehicles travelling along the cable could serve as a mass transportation system for moving people, payloads, and power between Earth and space," it says.

However, building one is not going to be as simple as it sounds, since such a cable would need to be ultra strong and, to prevent it from falling to Earth, would need to be tethered to a large counterbalance mass out there in space, likely an asteroid “moved into place for that purpose", or the Moon. While the idea for a space elevator is more than a century old, it appears that there is still some ways to go before something like it can be realised. Till that time, space travel will hinge on eleborate calculations and deployment of massive rockets burning tonnes of fuel.

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first published:November 12, 2021, 10:45 IST