Long-duration space travel

The Mars500 experiment aimed at testing human resilience in the isolation of long space journeys has concluded after almost 18 months of solitude for the group of six men.

Topic of the moment – long-duration space travel
NASA

But as well as the test of psychological endurance, any long-term space mission would raise some tricky physical challenges too.

The hazards of spaceflight
Astronauts on a long-term mission will find that there are several threats to their health and wellbeing created by the hazardous environment of space.

On top of the psychological stress of being confined in a small space with one or more other people for an extended period of time, there are actual and potential physical effects on the body too, which come from the absence of gravity and the presence of radiation.

These hurdles will all have to be overcome if spacefarers journeying to Mars or beyond are to remain fit and healthy when they get there.

Gravity
Experiments in Earth orbit have shown the deleterious effects that weightlessness has on the human body.

Living in zero or negligible gravity for extended periods has a range of health consequences: from a puffy face and dizziness to muscle wastage and bone decalcification.

The more serious effects can be mitigated with a programme of exercise, but it’s possible that they could be eliminated entirely by using an artificial source of gravity.

There are several ways this could be achieved – but none of them is easy.

Rotation
Inhabitants of a rotating spaceship would feel a force pushing them away from the centre of rotation.

A doughnut-shaped habitation section could then be used to provide an equivalent to gravity, with the ‘floor’ being the inside wall of the outer radius.

This is not without challenges of its own, however.

A source of energy is required to get the spacecraft rotating to begin with. If it is connected to nonrotating sections, such as a propulsion system, then friction will slow the rotation without further energy input.

Additionally, the Coriolis Effect will apply to any objects that move, giving them an apparent motion opposite to the direction of the habitat’s spin. This can be minimised by limiting the rotation of the spacecraft to around two revolutions per minute or less.

However, since the force experienced is proportional to the radius of rotation and to the square of the rotational speed, this would require a very large spacecraft – to rotate at 1 rpm it would have to have a radius of 900 m.

As well as taking a lot of resources to build, this would present engineering challenges in making the structure strong enough at that size.

Acceleration
In formulating his General Theory of Relativity, Einstein realised that gravity and acceleration are equivalent.

Theoretically, if a spacecraft could maintain an acceleration of 1 g for the first half of a journey and the same deceleration for the second half, this would provide an equivalent to full Earth gravity for the entire trip.

Even smaller accelerations are enough to negate the worst health effects of weightlessness.

Higher accelerations are likely to be impractical as they would require a huge amount of fuel to be carried on board – unless it can somehow be obtained from interplanetary space, such as in Robert Bussard’s ramjet design.

But a lower-thrust electric propulsion system – which uses electromagnetic fields to expel electrons as a reaction mass, rather than chemical combustion to heat and expel fuel – could provide low levels of acceleration for prolonged periods.

Radiation
Unprotected by the Earth’s atmosphere and magnetic field, astronauts are at greater risk from the radiation emitted by the Sun and by distant stars and galaxies.

Around 90% of cosmic rays are protons, with the rest being made up mainly of alpha particles, the nuclei of heavy elements, electrons, and small numbers of antimatter particles.

Long periods of exposure to radiation can trigger cases of cancer, while even short-duration exposure to extremely high levels, such as that generated by solar flares, can cause potentially fatal radiation poisoning.

High-energy particles also risk damaging a spacecraft’s electronic components.

In Earth orbit, such as in the International Space Station, astronauts are protected from periods of increased solar activity with thick walls that stop radiation in its tracks. The US National Ocean and Atmospheric Administration also maintains a Space Weather Prediction Center to monitor and predict radiation in the immediate vicinity of Earth.

For longer-range missions where spacefarers will be outside of the Earth’s magnetosphere for extended periods of time, physicists have suggested using a plasma shield, confined by a magnetic field, to reduce the energy of any incoming particles.

With plenty of danger to keep things interesting, long-distance astronauts might not suffer too much from boredom and loneliness at all.



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