Feeling for cold/ Mach 8/ Nerves and speed/ Voyager/ Big dinosaurs

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Q Why does metal feel colder to the touch than plastic when they are both at room temperature?

Your body temperature is about 37 Celsius, and room temperature is about 20C to 25C. This means heat from your body is transported to objects you touch. Metal conducts heat better than plastic, so it conducts that heat away from you into the metal quicker than plastic does. If both materials were at body temperature (or rather, skin temperature) you would probably not feel a difference, since no heat would be transferred.

Q How fast is Mach 8, and can military aircraft travel that quickly?

Mach 8 (measured at 0C) is eight times the speed of sound, about 9,500km/h (6,000mph). The fastest unclassified military aircraft is the SR-71 Blackbird reconnaissance plane, with a sustained cruise speed of a little more than Mach 3.

As Mach 1 is the speed of sound, Mach 1 to 5 is "supersonic flight"; Mach 6 and over is "hypersonic flight". This has been achieved only by the X-15 research aircraft and the Space Shuttle re-entering the atmosphere - at Mach 25! There are persistent rumours of a classified military project, "Aurora", which appears to involve a hypersonic reconnaissance vehicle. However, the US Air Force was recently forced to de-mothball several Blackbirds and put them back into operational service to supplement satellites, so it would seem "Aurora" is not yet in service.

Q How fast does a signal travel down a nerve?

The speed at which a nerve impulse, or action potential, travels is known as its conduction velocity. In human nerve fibres, values range from 1 and 3m/s (2 to 6mph) in unmyelinated fibres (without a fatty sheath), and between 3 and 120 m/s (6 to 270 mph) in myelinated fibres.

Q Where are Voyager 1 and 2 spacecraft now?

Voyager 1, launched in September 1977, is moving at 3.5 astronomical units (AU) per year along a trajectory angled 35 north of the ecliptic. (One AU is the distance between the Earth and the Sun, about 93 million miles). It is moving towards the Solar Apex - the direction of the Sun's motion relative to nearby stars. It is currently about 9.615 billion km from the Sun, with signals taking 17 hours and 51 minutes to make it back to Earth.

Voyager 2 (it in fact preceded Voyager 1) is moving at 3.1 AU/yr along a trajectory angled 48 south of the ecliptic. It is now about 7.628 billion km from the Sun - signals take 13 hr 41 min to reach us.

Both should have enough power to send data back to us until about 2020 - by which time they should have passed the heliopause, the point where the Sun's solar wind dominates and the interstellar wind prevails. They will pass within a few light years of nearby stars over coming millennia, but not close enough to be gravitationally attracted into orbit around them. The Voyagers have nuclear batteries, so they will be dead when their power levels run low; there are no solar panels on the craft to reactivate them if they pass near another star.

Q Why were there such large dinosaurs and mammals years ago?

Organisms tend to become larger as their lineages evolve; this is sometimes known as "Cope's rule". Many factors may interact to influence evolutionary trends in body size. For the largest dinosaurs, increase in size is likely to be correlated with exploiting a new food source: the sauropods literally grew taller with the trees on which they fed. And one way for a carnivore to tackle large prey is to grow large too. It may sound simplistic, but there was nothing to prevent the sauropods (or mammals) from growing large - except, finally, sheer physical constraints.

The reverse is also true. As far as dwarfing is concerned, reduced habitat area is a driving force. Examples are island faunas of dinosaurs (Romania) or Tertiary mammals (Mediterranean islands) where the animals have become smaller as an adaptation to living in restricted areas. It is unlikely that giant lineages will evolve again - we humans have too much influence on habitat and ecology.

Questions and answers provided by Science Line's Dial-a-Scientist on 0345 600444.

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