1. Satellites aren't floating in space. They're falling, forever.
2. Almost all of the weight of a rocket on the launchpad is just fuel.
3. When a spacecraft uses a planet to speed itself up, the planet slows down a bit.
4. If the Earth was the size of a football, then no humans would have gone more than about 2cm from its surface since we last went to the moon, 44 years ago.
5. By the way, if Earth were the size of a football, the sun would be about 25 metres across. The sun is quite big.
6. But we're in a golden age for space robots. There are two rovers crawling around on the surface of Mars right now.
7. And there have been a few others that have fallen in service on the red planet.
8. There's a space probe that has been orbiting Saturn for the last 12 years as well.
9. And there's another one which has entered the orbit of Jupiter just last week.
10. New Horizons, which flew past Pluto last year, is so far away that it takes 4 hours, 25 minutes for its radio messages to reach Earth.
11. Then there are the Voyagers. Voyager 1 is travelling at 17,000m/s, or 38,000 miles per hour. That means it would travel from New York to London in five minutes.
12. Getting Voyager 1 to reach Neptune accurately was the equivalent of sinking a 3,630km (2,260 mile) golf putt.
13. Even though it's going so fast, for part of the year, Voyager is actually getting closer to Earth.
14. And after 37 years, Voyager 1 has still only just made it out of the solar system.
15. And that's just the start. It would take Voyager 73,000 years to reach the nearest star. If it was going in the right direction.
16. But it's OK. We're not going to get to the other stars very soon, but our robots can take incredible images of them…
17. …and learn more about them by the day.
Go humans! And also robots.
If the Earth was the size of a football, the sun would be about 2,700 metres away; the mass of the Saturn V is 2,900 tonnes; and the velocity needed to reach orbit is a little less than 8km/s. An earlier version of this piece misstated these numbers.