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Moving in Space

How Do You Move About in Space?

A spacecraft uses most of its energy getting up into space. With no drag to overcome once in orbit, the spacecraft can then “coast” without expending any more energy. But how can you change a spacecraft’s course? Move it from one orbit to another? Or bring it back to Earth?

View of the Space Shuttle Discovery as photographed during the survey operation performed by the crew on the International Space Station.  As part of the survey and part of every mission's activities, the orbiter performed a back-flip for the rendezvous pitch maneuver.
Credit: NASA

How do Rockets Help You Maneuver in Orbit?

Speeding up and slowing down in orbit works just opposite to what you might expect. The larger a spacecraft's orbit, the slower the spacecraft travels.  So if you wanted to pass a spacecraft just ahead of you, you would have to fire a thruster in a forward direction. This would decrease your orbital energy and drop you into a lower orbit, where you would travel faster! The "passing lane" in orbit is always lower.

How do Rockets Help You Maneuver in Open Space?

Once you are far from a planet, say, while flying between Earth and Jupiter, mid-course corrections are fairly straight-forward. To speed up, you fire a rear-facing thruster. To slow down, you fire a forward-facing thruster. To alter your course, you fire a thruster in a sideward direction. To rotate your spacecraft, you fire a pair of sideward-pointed thrusters located near opposite sides of the spacecraft. To stop rotating, you fire thrusters aimed in the opposite direction.

Ask an Explainer

Q:

Why do orbiting spacecraft get so hot during reentry?

A:

To slow down from 28,200 km/h (17,500 mph) in orbit to a safe landing speed, a spacecraft has to convert all its energy into heat.  The heating of the spacecraft and its surroundings results from the formation of shock waves and friction as it moved through the Earth’s atmosphere. ... more

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Did You Know?

Long range exploratory spacecraft, like Voyager 2, use rocket power and gravity to reach their destinations.  Using rockets alone, spacecraft are limited to visiting planets close to Earth (like Mars or Jupiter) because of fuel limitations.  To visit planets farther away, spacecraft use the gravity assist method to speed up and change their course.  Voyager 2 used rocket power to reach Jupiter, but then went on to explore Saturn, Uranus and Neptune using the gravity of each planet it visited to sling it on to the next planet and beyond.  Without these gravitational boosts, Voyager 2’s 12-year journey to Neptune would have taken 60 years.

Pop Quiz

To increase your orbital speed you would need to fire the thrusters at the back of your spacecraft.

A) True
B) False

False: Changing your speed in orbit around Earth works opposite to the way it does on Earth.  The larger your orbit, the slower your speed.  If you want to speed up, you have to fire the thrusters at the front of your spacecraft.  This drops you into a lower orbit and increases your speed.  If you fire the thrusters at the back of your spacecraft you would raise your spacecraft to a higher orbit, and slow down.