Space Journeys: There and back again
D. Indumathi, The Institute of Mathematical Sciences, Chennai
Those of you who like reading will have heard of J.R.R. Tolkien's book called the Hobbit. Its full title is "The Hobbit, or There and Back Again"! So with apologies to Tolkien for stealing his book title, let us discuss why "coming back" is as important as "going there"! Especially when it comes to space travel.
If you have been following the news, you may have read of the return of two American space astronauts, Sunita Williams and Barry Wilmore, on March 18, 2025. Where did they go, and why was their return on the news?

The International Space Station
Where they went, is to the International Space Station (ISS). It is a large space station that orbits the Earth about 400 km above it. It was assembled and is maintained by a collaboration of five space agencies and their contractors: NASA (United States), Roscosmos (Russia), ESA (Europe), JAXA (Japan), and CSA (Canada). It completes about 15.5 orbits of the Earth per day and in fact it is visible to the naked eye from places with clear skies. If you look up, especially at dawn or dusk, the solar panels on the ISS reflect sunlight and it appears as a bright spot moving rapidly in the sky. You can use NASA's "Spot the Station" web-site to find out when the ISS will be visible in your hometown.
The ISS is an amazing piece of construction. Just imagine: the crew took parts of the station module by module, and put it together in space! It has been continuously inhabited by astronauts from different countries for nearly 25 years, since November 2, 2000!
The ISS provides a platform to conduct scientific research, with power, data, cooling, and crew available to support experiments. Small uncrewed spacecraft can also provide platforms for experiments, especially those involving zero gravity and exposure to space, but space stations offer a long-term environment where studies can be performed potentially for decades, combined with ready access by human researchers.
As the largest space station ever constructed, it primarily serves as a platform for conducting scientific experiments in microgravity and studying the space environment. The ISS simplifies individual experiments by allowing groups of experiments to share the same launches and crew time. Research is conducted in a wide variety of fields, including astrobiology, astronomy, physical sciences, materials science, space weather, meteorology, and human research including space medicine and the life sciences.

Freefall
One of the interesting facts that astronauts working on the ISS encounter is weightlessness. Gravity at the altitude of the ISS is approximately 90% as strong as at Earth's surface, but objects in orbit are in a continuous state of freefall, resulting in an apparent state of weightlessness. This makes life on the Space Station both fun and challenging.
Williams and Wilmore went to the ISS on 5th June 2024, nine months ago. They went on a test flight of the Boeing Starliner capsule. It was in fact the first mission of this capsule, which has a reusable crew capsule along with a service module which is replaced after every launch. Unfortunately, the Starliner had a lot of malfunctions and so returned to Earth, leaving its crew behind. So a planned 8 day trip stretched into 9 months!

At the Space Station
At the Space Station, the crew do some experiments, maintain the space-craft, and also exercise regularly. This is because without gravity, the muscles atrophy (become weak) in space. Not just muscles, there is also significant bone density loss in space. For every month spent in space, an astronaut's bone density drops by about one percent. The human heart also becomes weaker in space, as it doesn't need to work as hard to pump blood around the body.
You may have seen videos where the astronaut squeezes water drops out of a water bottle which float in air (there is no gravity to make it "fall"). Then they have fun catching and swallowing the water drop. But the same happens with solid food too: you cannot digest food easily as the routine of food sliding from the mouth through the gullet, into the stomach and to the intestine depends on gravity. So astronauts who spend a lot of time in space also experience significant weight loss. So astronauts go through a very thorough training to ensure that they are both physically and mentally fit for the rigours of space life.

And back again
They were finally brought back by another spacecraft called Dragon 2, which is a series of reusable spacecraft -- the specific capsule was called Freedom -- manufactured by another private company called SpaceX. It was launched by a Falcon 9 rocket also made by SpaceX. Both Boeing and SpaceX work very closely with the National Aeronautics and Space Administration (NASA). It is an independent agency of the US government that oversees space research programmes.
You can see the Dragon Freedom on top of the Falcon 9 rocket in the picture. It also has a reusable space capsule and a trunk module. You can see the various parts of the capsule in the picture, as well as the inside view (Pictures from BBC).

Dragon Freedom
The Falcon 9 rocket launches the Crew Dragon spacecraft into space. This then reaches the ISS and docks with (connects to) it. That is, the spacecraft first alters its speed to match with the ISS (you cannot connect to something that is at a different speed/direction than you: you will simply hit and damage it). The ISS has five docking stations since it was designed as a place for several astronauts to live and work. Once the two spacecraft are close together, a computer controlled process is begun where first a probe on the capsule engages with a drogue (funnel shaped device) on the ISS. Once the docking is stabilised, an air-tight connection is made between the two so that the astronauts can safely go from the inside of the capsule to the inside of the ISS, without being exposed to harsh outer space.

Splash down
The reverse procedure occurs during departure. The astronauts enter the capsule. The space craft detaches itself from the ISS and speeds back to Earth. The spacecraft has to enter Earth's atmosphere carefully, otherwise it will heat up and burn out like a meteor. This is because the spacecraft is travelling at speeds of about 30,000 km/hour! So reverse thrusters are turned on (opposite of what is used when the rocket takes off) to slow down the capsule. More importantly, the spacecraft must enter at a certain angle to avoid large heating (see figure). Even so the spacecraft can experience temperatures of 4000 degrees! Heat shields in the form of tiles are used to keep the inside of the spacecraft cool and safe for humans. Parachutes then open up (six of them) to slow the capsule suffiently. The trunk module is abandoned after each flight while the capsule returns to Earth by splashing down in a water body. After many safety checks, people on boats in the water open the hatch and the astronauts come out, one by one. They are home, again.

Sources: Wikipedia information on ISS, Falcon, and Dragon, NASA: www.nasa.gov, BBC: https://www.bbc.com/news/science-environment-52840482