In a gravity assist, what does a spacecraft do as it flies past a planet?

• Uses its rocket engines to speed up
• Borrows a small amount of speed from the planet
• Gets pulled into orbit around the planet permanently

What is a satellite in orbit actually doing, according to Newton's idea of orbit?

• Continuously falling but moving sideways too fast to hit the ground
• Floating because there's no gravity that high up
• Held up by air pressure like a balloon

True or false: a rocket that needs to gain a lot more speed only needs a little more fuel.

A spacecraft speeds up after flying past Jupiter during a gravity assist. What happens to Jupiter?

• Jupiter slows down by an amount too tiny to ever measure
• Jupiter speeds up by the same amount the spacecraft gained
• Nothing happens to Jupiter, only the spacecraft is affected

The Moon is falling toward Earth just like a dropped ball falls toward the ground. Why doesn't the Moon crash into Earth?

• It stopped feeling Earth's gravity a long time ago
• It is too light for gravity to affect it
• It's moving sideways fast enough that it keeps missing Earth as it falls

Look at the bar chart. Out of every 100 parts of the rocket's launch mass, about how many parts are fuel?

Two missions each use a gravity assist, one flying past tiny Mercury and one flying past giant Jupiter. Which planet can hand off more speed to the spacecraft, and why?

• Both give exactly the same boost, since gravity assists always add the same speed
• Mercury, because it's closer to the Sun and moves faster
• Jupiter, because it has far more mass and momentum to share

Zoe says the ISS doesn't fall because it's above Earth's atmosphere, where there's no gravity to pull it down. What's the mistake in her thinking?

• She's right, there's no gravity in space
• Gravity is still there and pulling; the ISS is actually falling, just moving fast enough sideways to keep missing
• The ISS is held up by rocket engines that fire constantly