Why are astronauts aboard the International Space Station weightless while they are still so close to the Earth?

Great question!

The strength of Earth’s gravitational field as felt by astronauts aboard the Space Shuttle is almost exactly the same as the gravity we feel here on the ground.  So why are the astronauts weightless?

The answer goes back to Sir Isaac Newton in 1687 and a “thought experiment” he performed using just his imagination and his newly-formed Theory of Gravitation.

Isaac Newton - 1689

Newton imagined what would happen if you had a cannon located on a mountain top and aimed at the horizon.

If you fire a cannon parallel to the ground the ball moves horizontally away from the cannon due to its energy from the gunpowder.  At the same time the cannon ball also begins to accelerate vertically toward the ground because of gravity.

The higher the speed of the cannon ball, the farther it travels before it hits the ground.

If you fire the cannon ball fast enough, the ball travels so far from you that the curvature of the Earth begins to curve the ground away from the falling cannon ball.

If you fire the cannon ball at just the right speed – about 17,500 mph, the cannon ball never hits the ground because the speed of the cannon ball’s fall towards the ground is exactly cancelled by the speed of the ground curving away out from under the falling cannon ball.

Newton realized that in the vacuum of space no force would act to slow the cannon ball down, and so a cannon ball that once achieved enough velocity to put it into orbit would remain in orbit indefinitely.  It would be in perpetual free-fall around the Earth.

So even though you are “weightless,” in orbit around Earth, you’re still experiencing almost exactly the same gravity you are right now as you read this.  The difference is that as you’re orbiting the planet at roughly five miles per second, the ground curves away beneath you at exactly the same speed that you’re falling toward the ground.

Nature loves spheres. It can’t get enough of them.

Soap bubbles are spherical because that shape most efficiently balances the outward pressure of the air within the bubble against the surface tension of the soap film.

When water splashes and for a brief instant a droplet of water is neither rising nor falling and is momentarily weightless, what shape does the droplet’s surface tension force the water to take?  A sphere.

Nature loves spheres. In the case of bubbles and droplets of liquid, surface tension creates a sphere to minimize surface area.

Stars are perfect examples of natural spheres.  The mass of a star is mind-bogglingly large and creates an equally mind-bogglingly large amount of gravity. What shape does Mother Nature give to so much mass to minimize its enormous volume?  A sphere.

Stars like our Sun are huge, dynamic, energy-producing concentrations of Hydrogen and Helium, compacted by their enormous gravity into spheres.

The reason planets appear spherical is because gravity compresses the planet into a shape that most evenly distributes the gravitational force among the planet’s mass.

Whether it is shaping water droplets, stars, soap bubbles or planets, nature seeks to minimize the surface area needed to contain a given volume, and the shape that keeps volume at the absolute minimum a sphere.

Any object in weightless space larger than a couple of hundred miles in diameter has enough mass for its gravity to overcome large-scale irregularities and force it into a spherical shape.  This gravitational compression also generates significant amounts of heat at the center of the planet. This heat melts, or at least softens, any solid materials within the planet, facilitating the planet’s collapse into a spherical shape.

Objects in space smaller than about 100 miles in diameter, such as most asteroids, comet nuclei and small moons, lack the mass to create a gravitational field strong enough to compress themselves into spheres.  These little worlds often take on what I call the “sick potato” look.

The 12.5 mile-long, 7.5 mile wide asteroid Gaspra, imaged in October 1991 from a distance of 1,600 miles by the Galileo spacecraft en route to Jupiter.

A really large asteroid, such as Ceres (diameter = 600 miles), has enough mass for its gravity to compress it into a sphere.

The 600 mile-wide asteroid Ceres as seen by the Hubble Space Telescope.

However, “perfect” spheres are hard to find in space.

Pretty much everything is space rotates, and rotating a non-rigid sphere causes it to “bulge” at its equator from the centrifugal forces acting on it.

This spinning distorts large planets into a slightly squashed shape known as an “oblate spheroid.” This means that a planet’s diameter measured through its poles is smaller than the diameter measured through its equator.

Whereas the difference between the polar diameter and the equatorial diameter of Earth is a barely noticeable 0.3%, the oblateness of Saturn, a large, gaseous and rapidly spinning planet,  is greater than 10%.  You can easily see Saturn’s polar flattening through a telescope.

Saturn's polar diameter is 33,700 miles, but its equatorial diameter is 37,360 miles.

There may not be such a thing as a “perfect” sphere in nature, but there is no doubt that spheres, nature’s favorite shape, are perfectly lovely.

A quick look at “Average Orbital Velocity” in Clark Planetarium’s Solar System Fact Sheet will reveal that the farther a planet is from the Sun, the slower it moves. In contrast, stars and gas in the outer regions of galaxies all have roughly the same speed regardless of their distance from the center. How do astronomers measure their speed? Using the Doppler Effect. Below is an example from a nearby galaxy known as M33. The image on the right is a radio telescope image showing the distribution of hydrogen throughout the galaxy (hydrogen atoms give off radio waves with a wavelength of 21 centimeters). Colors in the image show the Doppler shift of the radio waves. Blue shows hydrogen that is moving toward us. Red shows hydrogen that is moving away.

Right: M33 Galaxy (credit: NOAO/AURA/NSF/T.A.Rector). Left: M33 Galaxy showing Doppler shift (credit NRAO/AUI)

Since Doppler measurements reveal that stars and gas in the outer regions of galaxies all have similar speeds, this implies that mass in a galaxy must increase with increasing distance. But visible matter in most galaxies appears to decrease with increasing distance from the center. So, unless our understanding of the basic laws of physics needs a tweak (as has been proposed by some) most of the mass in galaxies cannot be seen, hence the name dark matter.

A number of possibilities have been suggested for this unseen mass, from dim stars, brown dwarfs and black holes to exotic and not so exotic sub-atomic particles. Recent searches seem to favor sub-atomic particles. However, one of the exciting things about astronomy research into the unknown is that an unexpected discovery or a better observation can revise current thinking.

Hubble composite showing ring of dark matter in the galaxy cluster C1 0024+17

One of the best evidences for the existence of dark matter comes from the Hubble Space Telescope. Astronomers used gravitational bending of light from faint distant galaxies to map the distribution of mass in what appears to be the aftermath of a collision between two galaxy clusters. The blue color in the Hubble image shows the distribution of matter based on these measurements. In addition to the mass of the cluster in the center, a ring of unseen mass surrounds the galaxies. Computer simulations suggest that such a collision, occurring along Earth’s line of sight could produce this ring of dark matter.

What is dark matter? Does it really exist? Stay tuned.

In 1687, Sir Isaac Newton dramatically increased our understanding when he published his law of Universal Gravitation. Newton deduced that anything made of matter is attracted to everything else made of matter. The strength of the attraction depends on the amount of matter in each object (its mass) and how far apart they are. This attraction is easily noticed when one of the objects is very massive, like a planet. The attraction between small objects (like people) is so weak that it can only be seen easily in cleverly designed experiments like those done by Henry Cavendish in 1797-98. I repeated a modified version Cavendish’s experiment long ago in a college physics lab. Our goal was to determine the gravitational constant by observing and measuring the gravitational attraction between small lead balls. Turns out, the lead balls did attract!

Newton’s mathematical description of this attractive force makes possible accurate predictions of the motions of the Moon, planets, comets, asteroids and spacecraft. While we give the name “gravity” to this force, no one understands how gravity works or in other words, why objects with mass attract each other. Isaac Newton wrote, “The reason for these properties of gravity, however, I have not yet been able to deduce from the phenomena . . .  it is enough that gravity really exists, and acts according to the laws set forth by us, and is sufficient [to explain] all the motions of the heavenly bodies and of our sea.”

The gravity of this galaxy cluster acts as a "lens" in space, bending and magnifying the light of galaxies located far behind it, distorting their shapes and creating multiple images of individual galaxies.

Another change in the description of gravity was provided by Albert Einstein in 1915. Previously, Einstein’s Special Theory of Relativity had united three dimensional space with time into the four dimensional playing field of the Universe that we call “spacetime.” It also declared that mass and energy were different forms of the same thing which I shall call “mass-energy.” Einstein proposed that gravity results from the curvature of spacetime. The presence of mass-energy curves spacetime. Curved spacetime controls the movement of mass-energy. One prediction of Einstein’s description of gravity different from Newton’s was the gravitational bending of light. Observations made of bending starlight during the 1919 total eclipse of the Sun confirmed Einstein’s prediction and made him a world celebrity.

Bending starlight now appears on a gigantic scale, with a cluster of galaxies acting like a giant lens, smearing out light into numerous cosmic streaks seen in this image taken by the Hubble Space Telescope.

While Einstein’s description of gravity is more complete than Newton’s, we still do not know how gravity works or in other words, why mass-energy bends or curves spacetime.