In Programme One the Rough Scientists had to make a Mars Rover which could explore strange new worlds. This week Kathy and Jonathan have to go one better and design an aerial surveyor that can explore much greater areas by floating above land. Just like the rover challenge, they've been given a tiny camera which will record whatever the aerial surveyor sees. Back on Earth, Mike has a very different challenge. Back in 1970 the crew of Apollo XIII faced certain death when an accident damaged their oxygen tanks. To survive they had to build a carbon dioxide filter - and Mike has to do the same. In 1872 California experienced one of the biggest earthquakes ever recorded in the United States. Ellen and Iain have to work out where the epicentre of the quake was, and its magnitude. Kathy and Jonathan decide to make a solar balloon - heated by the power of the sun. Made by sticking black bin bags together, the idea is to hang the camera below it. (An idea, incidentally, that NASA are actively exploring for Mars.) Unfortunately the Rough Science version encounters a series of unexpected setbacks, and on Day Three it is touch and go whether it will get off the ground. For Mike to make his carbon dioxide filter he needs to make limewater. To do that he needs to find limestone, heat it up to make quicklime, and then dissolve it in water. However, making quicklime proves to be more difficult than anticipated, and on Day Three he faces a classic Rough Science test to see if he has succeeded in making a working carbon dioxide filter. Ellen and Iain take to the air to try and find the site of the 1872 earthquake. Their first task is to find the fault line which marks the location of the earthquake. They then set out to measure the amount of movement that occurred along the fault, to work out where the maximum displacement was. This should give them the epicentre of the earthquake, and also provide the information they need to calculate its magnitude. Should - but will it?

• Kate Humble
• Iain Stewart