Science: Vertical takeoff/ Leg counters/ Food for thought/ Icy Europa

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Q Why are rockets launched vertically, not horizontally?

In the past, the simplest way of getting into space was by using a staged, vertically-launched rocket. It had to be staged because we didn't have materials lightweight enough or engines powerful enough to build a single- stage-to-orbit (SSTO) rocket. Vertically-launched SSTO rockets are now theoretically possible with powerful new engines and super-lightweight, durable, materials for the propellant tanks and structure. But 90 per cent of the take-off weight of such a SSTO rocket is propellants.

AeroSpacePlanes are also theoretically possible. They take off and land horizontally, use wings to get the benefit of lift from the air they are flying through and can be "just" 70 per cent propellants at take-off. Their drawback is that they work by using atmospheric oxygen, which is sucked in through air intakes while in the lower atmosphere to burn the onboard fuel. They only switch to onboard oxygen when the air gets too thin. The engines are very complicated: projects like the British HOTOL and NASA X-30 (NASP) have always floundered owing to expensive development.

Q How many legs do centipedes and millipedes have?

It's not 100 and 1,000. Centipedes have one pair of legs per body segment and millipedes have two pairs per body segment. As the number of body segments varies, it is impossible to give a definite answer.

Q How long does it take to digest food?

Unless you are ill, it takes hours or even days. The complete passage from mouth to anus is called the "intestinal transit time" and is about 24 hours to three weeks, depending mainly on the fibre content. This is measured nowadays using X-ray opaque shapes in the food, or dyes. The 24-hour figure is from rural Africans and the three weeks is US college students.

Early work on this topic was reported in one of the first editions of The Lancet, by tying strings to bits of food, swallowing them and pulling them back up to see how they were getting on! It is important to realise that the stomach acts as a hopper to try to ensure a steady stream of food passes through the gut.

Sloppy, bland foods like porridge will pass quicker into the gut while foods with stimulating ingredients, like Bovril or meat, stay longer, as do fatty foods. The job of the stomach, and the rest of the alimentary canal, is to mix everything up as much as possible: nothing passes through separately, and it will be absorbed over a period of time. Our digestion process is very efficient - the Victorian concept of indigestible food is outdated.

Q Of Jupiter's moons, why is Europa made of ice and not Io?

The Sun and other planets formed from a huge cloud of dust, ice and gas billions of years ago. Close to the (hot) Sun that was forming, temperatures were so high that only rocks could exist. Further out, ices could exist as well.

The best current theory for planet formation claims that dust/ice grains collided and grew in size to eventually form "planetesimals" (little planets). At some locations in the solar system there was a local "clump" of these planetesimals - the points at which planets grew. As other planetesimals were attracted to these locations by gravity the planets grew in size to form the bodies we see today. So, planets near the Sun are rich in rocks and metals - especially Mercury. The planets further out are "gas giants" with icy interiors. Their moons are mainly icy with some rock mixed in; less so further out in the solar system.

If you look at Jupiter and its moons as a mini solar system forming out of this huge cloud of gas, ice and dust, then near the hot Jupiter only rocks could exist. So Io, being closer, is mainly made of rock; as you get further from Jupiter there are a lot more ices mixed in with the rock - like on Europa.

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