Our final set of Rough Science space challenges are all about rockets. Mike, Jonathan and Kathy have to make three different rockets, but there's a catch; they're only allowed to use one thing as a fuel - and that's water! They've also got to design their rockets to carry a "passenger" - a (raw) egg. And Ellen and Iain have to find a way of returning the egg safely to Earth. Each of our 3 rocketeers designs a rocket that fits their science background. Jonathan's is the most ambitious - his physics background inspires him to build a steam powered rocket. Kathy - also a physicist - decides to use pressurised water, and Mike takes a chemical approach, using electrolysis to split water into hydrogen and oxygen gases, and then recombining them to form an explosive mixture. (Hydrogen and oxygen are what NASA uses to fuel its rockets - but the difference is that NASA uses a liquid fuel which can compress far more energy into a much smaller space.) What develops is a Rough Science space race, as the scientists compete to see who can get to the launch pad first, and whose rocket will be the most effective. Meanwhile, Ellen and Iain have to find a way of putting an egg on each rocket and returning it to Earth. They opt for a parachute made out of bin bags, and design a detachable nosecone for the rockets. However, the "detachable" part proves harder to achieve than first thought...

• Kate Humble
• Iain Stewart