Astronauts on the International Space Station look weightless because they float beside their tools, food, and equipment. This does not mean they are beyond Earth's gravity. At the station's altitude, Earth's gravity is still strong, about 90 percent of its surface value.
The correct idea is microgravity, a state where astronauts and their spacecraft are falling together around Earth.
An orbit is a balance between forward speed and downward acceleration due to gravity. The spacecraft moves sideways fast enough that as it falls, Earth's curved surface drops away beneath it. Since everything inside the spacecraft accelerates together, there is almost no support force pushing on the astronauts.
That lack of a normal force is why they feel weightless, even though gravity is still acting.
Key Facts
- Zero gravity means g = 0, but this is not the usual condition in Earth orbit.
- Microgravity means the apparent weight is very small, but gravity is still present.
- Gravitational field strength is g = GM/r^2.
- At the International Space Station, g is about 8.7 m/s^2, not 0 m/s^2.
- Circular orbital speed is v = sqrt(GM/r).
- Apparent weight depends on support force, not directly on gravitational force.
Vocabulary
- Microgravity
- A condition in which objects appear nearly weightless because they are falling together with their surroundings.
- Zero gravity
- An ideal condition where the gravitational field strength is exactly zero.
- Free fall
- Motion in which gravity is the only significant force acting on an object.
- Orbit
- A curved path around a planet or other body caused by forward motion and gravitational acceleration.
- Apparent weight
- The weight an object seems to have based on the support force acting on it.
Common Mistakes to Avoid
- Saying astronauts float because there is no gravity is wrong because Earth's gravity is still strong at spacecraft altitudes.
- Confusing microgravity with zero gravity is wrong because microgravity means very small apparent weight, not no gravitational force.
- Thinking orbiting spacecraft are not falling is wrong because orbit is continuous free fall around a curved planet.
- Using surface gravity g = 9.8 m/s^2 at all altitudes without checking distance is wrong because gravity decreases with distance from Earth's center.
Practice Questions
- 1 Earth's radius is 6.37 x 10^6 m and the ISS orbits about 4.00 x 10^5 m above the surface. Using g = g0(R/r)^2 with g0 = 9.8 m/s^2, estimate g at the ISS.
- 2 A spacecraft in low Earth orbit travels at about 7.7 km/s. How far does it move along its orbit in 10 minutes? Give your answer in kilometers.
- 3 Explain why an astronaut and a wrench released inside an orbiting spacecraft appear to float next to each other even though Earth's gravity pulls on both objects.