Tag Archives: ISS

Why do Astronauts Float?

NASA Satoshi FurukawaDid you know that astronauts on the International Space Station (ISS) experience about 90% of the gravitational acceleration that we feel on Earth’s surface? That might seem strange, especially since when we look at them, they are always floating around. While this makes it tempting to say that there is no gravity in space, that is quite clearly very far from the truth, so why do they float1?

On Earth, we are pulled down towards the Earth at 9.81 meters per second squared, meaning that if we weren’t pressed up against the ground, our speed would increase by 9.81 meters per second every second! This is known as free fall. This is something that can be experienced during the initial phase of a skydive, during turbulence in an airplane, or while going down a steep part on a roller coaster. It is often accompanied by a weird queasy feeling in the stomach.

By taking a water bottle and drilling some holes in it, we can see why the astronauts are weightless. Both the bottle and the water are being subjected to Earth’s gravity. When the bottle is held still, the water falls out through the holes because gravity is pulling it down. Then, when the bottle is released, they are both pulled down at the same rate. With no difference in acceleration between them, the water floats happily inside the bottle and doesn’t go out the holes at all.

Bottle Microgravity Demonstration

This is actually what is happening to those astronauts. When something is in orbit around the Earth, it is being pulled down by the force of gravity. For the ISS, this is about 9 meters per second squared! If the ISS wasn’t moving above the Earth, it would fall to the surface in less than an hour. However, Astronaut Scott Kelly was just on board for a year and didn’t plummet to his fiery doom. This is because the ISS is not staying still. Not even a little bit.

The ISS goes at about 17,000mph, which is over 10 times the velocity of a speeding bullet. This is entirely necessary, because as gravity yanks the ISS down towards the surface, the space station has moved far enough to miss the surface. This balance is very precarious. If the ISS were going faster, it would instead get farther away from the Earth each time and escape into space. If it were going slower, it would constantly get closer to the Earth, which is not what we want2.

How do astronauts train to get used to this? Let’s go back to the water bottle demonstration again. Take a second to think: What would happen if instead of just dropping the water bottle, we threw it up in the air? Let’s take a look!

Bottle Throw

While you might think that the water will come out on the way up and stay in on the way down, it instead turns out that the water stays inside the bottle from the moment it is free of the hand. This means it is also in freefall while it is going up, which seems strange, but both the water and the bottle are subjected to the same gravity with no other outside forces for the same time. To train the astronauts, they do the same thing, but with an airpZero_gravity_flight_trajectory_C9-565lane. This plane, cleverly and unpleasantly known as the Vomit Comet, flies up and then coasts through the air, allowing it to continue up in a similar trajectory to the bottle. Then, before it hits the ground, the pilots pull up and go skyward for a bit before repeating the process again. As you can imagine, it’s enough to make anyone a little queasy3.

1Image: Satoshi Furukawa floats aboard the ISS. Photo credit: NASA
2This is a real issue. The ISS slows down slightly due to the outer reaches of the Earth’s atmosphere. As a result, they use thrusters to get back up to speed about once a month!
3Image: Trajectory for the Vomit Comet. Courtesy of NASA

Written By: Scott Alton

The Speed of Fire

How fast is fire? We sent a burst of flame through a fluorescent light tube to explore the propagation of an alcohol-burning reaction along an enclosed path. In our experiment, the frontier of flame sped along at over six feet per second. Fire behaves very differently in other environments. In space, combustion becomes almost unrecognizable.


Gif: SpaceFire1

Flames speed along the tube, rebounding at each end.

Fire heats the air around it, causing molecules to speed up and spread out. On Earth, warm air rises as the cooler, thicker surrounding atmosphere sinks towards the base of the flames. This current delivers fresh oxygen to the base of the fire, and it continues burning for as long as the fuel supply lasts. In space, there’s not enough gravity to pull the cold air down, so flames burn spherically. Without the pull of a convection current, oxygen isn’t carried to the source of the reaction; instead, it diffuses into the fire. Random motion of air molecules isn’t a very efficient way to bring oxygen and fuel together, so flames in microgravity burn out much more quickly than we’re used to seeing here on Earth.


In microgravity, flames are shaped like spheres instead of teardrops. This color image is from the FLEX (Flame Extinguishment) experiment on the ISS. Image credit: NASA/GRC

Fire is a serious hazard on a spacecraft because there’s nowhere to escape to. Reliable fire-extinguishing technology is a vital part of safe, sustainable space travel. Strategies that work well on Earth, however, don’t always do the trick hundreds or thousands of miles above its surface– new tactics are required. In the last several years, combustion experiments on the ISS have begun to reveal the mysterious and fascinating behavior of fire in microgravity.

A few years ago, scientists studying space combustion noticed something phenomenal going on after their experimental flames died out: the fuel kept burning. No visible light was emitted, and the reaction was much cooler — around 700 degrees Fahrenheit instead of the usual 2500 or so — but the fuel burned away nonetheless. This behavior isn’t usually observed on Earth, but if we can find a way to replicate it consistently, it could be used for applications such as low-emission auto engines.


A droplet of heptane fuel burns in microgravity in this false-color time-lapse image. The droplet appears yellow and becomes smaller as it burns. Green areas are initial soot structures. Image credit: NASA

Microgravity research is already yielding exciting knowledge about the nature of fire outside of Earth’s influence, and there’s much more to come. Dr. Forman Williams, UC San Diego, is one of the investigators working on the FLEX-2 experiment to demystify space combustion. In 2013, he summed up the state of the field: “when it comes to fire, we’re just getting started.”

Happy Anniversary, Humans in Space!

November 2, 2000. It was a dark night on our planet. More than 200 miles above Earth’s surface, a Soyuz rocket began a slow dance in the bigger darkness of space. The docking sequence took three hours and forty minutes. At its end, the International Space Station welcomed the first of hundreds of human residents. The outpost has been inhabited ever since.


Expedition One crew members pose after water survival training. From left: Flight Engineer and Russian Cosmonaut Sergei Krikalev; International Space Station Commander and U.S. Astronaut Bill Shepherd; Soyuz Commander and Russian Cosmonaut Yuri Gidzenko. Credit: NASA

Bill Shepherd, Yuri Gidzenko, and Sergei Krikalev passed 4 busy months on the ISS. First, they moved in. Critical life support systems waited to be set up. Supplies needed unpacking. After waking up the station and settling themselves, the astronauts hosted visiting crews, received unmanned spacecraft, made improvements to the station, and conducted research. Their work set the precedent for the next decade and a half of sustained experimentation in low Earth orbit.


Shepherd rehearses an extravehicular activity (EVA) mission in Russia’s Hydrolab training facility. Scuba divers stand by to assist. Aquatic neutral buoyancy practice is a key component of astronaut training around the world. Credit: NASA

Since Expedition One, over two hundred astronauts and scientists have spent time on the ISS. Cool-burning flames have been studied and extinguished in microgravity. Lettuce has flourished in space. Students have peered through digital windows from their classrooms into our planet’s most distant human-operated research lab.

The station has built a foundation of knowledge about space travel’s effects on the human body. That knowledge is expanding even further thanks to identical twin astronauts Scott and Mark Kelly. Scott recently broke the record for most time spent in space, just over halfway through his year of orbital residence. Mark holds down the fort on the ground. The Twins Study includes ten independent investigations of everything from physiology to behavioral health to space-induced genetic changes.


Veggie, the plant growth system used on the ISS, basks in energy-efficient LED light. Credit: NASA/Orbital Technologies

These are just a few of the many imaginative, futuristic ISS experiments bringing home big benefits. Technology driven by space-based farming research keeps produce fresh in supermarkets worldwide and aids food preservation in developing countries. Climate scientists use the unique perspective of low Earth orbit to better predict and respond to natural disasters. The Twins Study deepens our understanding of ourselves as a species. Space poses unique problems that stretch the margins of human creativity and cleverness. Rising to these challenges not only brings us closer to interplanetary travel, it also motivates life-changing science right here on Earth.


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