Heat reduction
Question: Spacecraft returning to Earth from orbit are subject to the intense
heat that is generated by the friction from re-entry into the atmosphere. Why is
it not possible for them to employ an opposing thrust to counteract the pull of
Earth鈥檚 gravity?
Answer: It is quite possible to employ an opposing thrust to counteract the
pull of gravity.
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However, in practice this would need an immense amount of fuel, only a little
less than that needed to start the spacecraft from its launch pad. A spacecraft
would have to carry this fuel for braking throughout its whole journey, and also
a corresponding amount of additional fuel to launch it in the first place,
because the fuel for braking would obviously have to be carried into space. The
braking thrusters would also add extra weight.
Such a spacecraft would need roughly three or four times as much fuel as one
that does classic atmosphere braking, making it much more expensive. The
technical difficulties of how to transport such an amount of fuel up into space,
and the huge mass to be manoeuvred around while the spacecraft carries out its
mission, add to the problem. Compared with this, heat-proof tiles are simple and
cheap.
Torge Zengen
Undeloh, Germany
Steel smuggler
Question: As a result of a broken leg, I now have a piece of stainless steel
that runs down the centre of my femur. It is about 30 centimetres long and 1.5
centimetres in diameter. The bone and rod are held together by four screws, each
about 8 centimetres long.
The steel plate has never caused airport security alarms to go off although a
small bunch of keys that I carry has. Why?
Answer: A stainless steel femur rod does not set off airport metal detectors
because it is made of nonmagnetic stainless steel.
Stainless steel is made in two forms: austenitic and ferritic. Austenitic
stainless steel is nonmagnetic. It is the common form used in cooking pots and
surgical hardware. Ferritic stainless steel is magnetic. It is used in some
knives and guns and for various industrial purposes. Nonstainless steel (carbon
steel) is magnetic. It is used in most knives and guns.
Metal detectors do not detect metals as such, they detect the distortion of a
magnetic field produced by materials of high permeability. Magnetic materials
have high permeabilities which greatly distort the field and so are easy to
detect. Nonmagnetic materials have low permeabilities and as a result have
little effect on the field and are difficult, if not impossible, to detect. The
small bunch of keys detected at the airport is probably on a magnetic steel ring
because most keys are made of nonmagnetic metals, such as brass or
aluminium.
I find that I can avoid setting off airport metal detectors by limiting the
number of bank and credit cards I carry (they have magnetic strips), and by
touching all my metal things with a strong magnet and not carrying those it
attracts.
Ross Firestone
Winnetka, Illinois
Suckers for punishment
Question: What do leeches do when they are not feeding on hosts and what do
they live on in an environment where there are few mammals? How long is their
life cycle and how long can they survive without feeding?
Answer: Leeches vary greatly in their biology. They range from little ones a
couple of millimetres long, living only in water, to big jobs many centimetres
long, which can emerge from the water to seek mammalian hosts in rainforests.
Leeches vary also in their prey and what they do when they find it.
Many are bloodsuckers, often of fish or amphibians. If you look at the
underside of a healthy tadpole, you can see the heart pumping red, white, red,
white as the muscle contracts and relaxes. I have seen a tadpole heart pumping
white, whiter, white, whiter after an encounter with an aquatic leech. Some
leeches live on invertebrates鈥攃ertain South African leeches, for instance,
feed on aquatic snails and actually kill them, liquefying and sucking up their
flesh. Other species eat worms or insect larvae.
Some species of leech larvae remain on the mother as a brood for some time
after hatching. The life cycle varies, but some species from temperate zones
have an annual cycle.
Many leeches鈥� biology has not yet been studied, but they are an unexpectedly
interesting group. They also have a certain charm, when seen from the comfort
and security of an armchair behind a glass barrier. For one thing, their
underwater 鈥渨alking鈥� motion is one of the most beautiful movements in
nature.
Jon Richfield
Dennesig, South Africa
Answer: Not all leeches are bloodsuckers鈥攎any are predators, feeding on
invertebrates. If they are not feeding or hunting, leeches just hang around
between meals.
Their multipurpose suckers are also used for locomotion and hanging on. Fish
leeches have the largest suckers, enabling them to hang on to their
fast-swimming hosts between meals.
Parasitic leeches increase their chance of a meal by not feeding on one
species alone, although their range tends to be restricted to fish, amphibians
or mammals. The medicinal leech, Hirudo medicinalis (the one favoured
by the physicians of yore) can suck out up to five times its own weight of blood
in a single meal. This can support it for a whole year. If things get really
tough, leeches metabolise their own bodies, shrinking to a fraction of their
original size during periods of famine.
Like earthworms, leeches are hermaphrodites. Breeding is triggered by the
warming of water in spring and some of the segments near the middle of the body
form a saddle-like clitellum, which secretes a cocoon in which the eggs develop.
Hirudo medicinalis deposits its cocoon a little above the water line.
Some species attach the cocoons to stones underwater and others carry the
cocoons with them until the young hatch. There is little information on their
life span in the wild鈥攑erhaps leeches are difficult to track. Medicinal
leeches in captivity can live for many years, but nobody in my local hospital
knows precisely how long.
My vote for the leech with the most unpleasant lifestyle must be
Theromyzon tessulatum. This species enters the nostrils of water birds and
sucks blood from the nasal cavities. Apparently this can cause death in young
birds, although I would rather not speculate as to precisely how.
Richard Scrase
Bath, Somerset
This week鈥檚 questions
The big sleep: Why do you feel tired in the morning after a night鈥檚 sleep of
perhaps 10 hours, but after only a few hours鈥� sleep you feel wide awake?
N. Pollard
Bristol