When designing vehicles for Earth orbit, the most important force to take into account
is gravity, which causes both linear acceleration and rotational torque. For example,
unless the space shuttle orbiter fires its maneuvering thrusters, gravity will inevitably
turn the orbiter so that its nose is pointing down and one wing is pointing in
the direction of travel. Building spacecraft to be durable means carrying extra
weight into orbit, which is expensive, but the alternative - building a light,
flimsy vehicle - is dangerous, because gravity could easily deform it.
Spacecraft
designers also have to take into account harmful solar radiation. The Sun
produces both electromagnetic radiation (EMR) and penetrating charged particles
(PCPs). Earth's atmosphere protects life on the surface from the harmful effects
of high-energy EMR, just as the planet's magnetic field shields surface life from
dangerous PCPs; but in low Earth orbit, where the atmosphere is thin and the
magnetic field is weak, these defenses are inadequate.
EMR
travels through substances as well as empty space in waves of energy that move
at the speed of light. Light is itself a form of EMR, which is generated by the
vibration of charged particles. Radio waves and microwaves are relatively benign
forms of low-energy EMR. On the other hand, at the high-energy end of the
electromagnetic spectrum (above the range of visible light), EMR becomes ionizing,
which means that it can alter the structure of molecules, including molecules
of human DNA. Ultraviolet (UV) light and X-rays are examples of high-energy EMR
against which astronauts and electronic equipment need to be protected.
The
high energy of PCPs is also troublesome. Along with producing their own ionizing
radiation, PCPs often crash computer systems by flipping polarities in their
magnetic memory. All computers store information in bits of binary code, with 1
corresponding to
"on" and 0 corresponding
to "off." When high-energy PCPs penetrate computers, they can change
"off" bits to "on," for example turning the eight-bit byte 10
10 10 10 into 11111111.
Laptops left on in the
space shuttle orbiter overnight typically lock up and need to be rebooted the
next morning.
Engineers
have not yet developed a way to protect spacecraft or astronauts from PCPs,
some of which are energetic enough to pass through six feet of lead shielding. The
only countermeasure currently available is to build in enough computer redundancy
so that spacecraft can operate safely. Astronauts,
meanwhile, can absorb the equivalent of two to three chest X-rays during each
day they spend in space. (‘The Bedside Baccalaureate’, edited by David Rubel)
