Let's have a closer look at some of the dangers facing astronauts on the International Space Station (ISS).
The Danger of Radiation on the ISS
ISS astronauts are officially classed as radiation workers. A single day on board can expose them to up to one millisievert which is roughly 4 months worth of background radiation on Earth.
Space radiation comes from two sources - energetic particles from the Sun peaking at times of high solar activity and galactic rays - atomic nuclei shot across the Universe by ancient cosmic explosions. Most radiation of both types is deflected by Earth's magnetic field, which is fortunate for ISS crew as artificial shielding options are limited. Parts of the modules are lined with polyethene plastic which is more effective than sheet metal.
The risks increase the farther away from Earth. Radiation encountered by the Apollo astronauts was strong enough to have incapacitated or killed moonwalkers. Galactic cosmic rays are more hazardous still because these high energy particles cannot practically be deflected today - astronauts headed to Mars should expect every cell nuclear in their bodies to be struck during the trip.
During a spacewalk, astronauts experience a particularly strange sensation of holding onto a handheld, with nothing touching the bottom of your feet and Earth 400km away. The thought of just drifting away must enter their minds occasionally.
In reality, however, the risk of drifting away from the ISS is unrealistic - spacewalkers stay tethered at all times and are equipped with an emergency jetpack as a last resort.
Astronauts do lose equipment. In 2008 a toolbox drifted away, luckily it posed no threat. In 2006 a space suit was turned into a radio beacon and jettisoned from the ISS as an experiment. It drifted ahead of the space station for seven months after which it burnt up in the atmosphere.
Space suits are not always reliable, and more than a few spacewalks have been cancelled or cut short due to faults in the suits. EAS astronaut Luca Parmitano had his spacewalk aborted with water from the cooling system leaked into his helmet, covering his eyes and nose. Other spacewalks have been termination early die to damaged gloves, a particular weak point.
Fire behaves differently in microgravity; it stays put rather than spreading. A plasma bubble which burns hotter and hotter producing blinding smoke that can swiftly blanket a cabin. When a fire broke out on Mir in 1997 it burnt for 14 minutes and the six-man crew's fire extinguishers failed to put it out. Instead, Mirs blazing oxygen generator went on burning and spewing blobs of molten metal until finally burning out when all its fuel was exhausted.
Flames are harder to spot in microgravity, burning bluer and dimmer. The ISS has a fire detection and suppression system to monitor for smoke but as soot particles are larger in microgravity it can be falsely triggered by normal dust.
The ISS is regularly assaulted by tiny micrometeoroids and small pieces of orbital debris that are both natural and artificial. Dust from comet trails and meteoroids mingles with paint flakes, frozen water droplets or particles from solid rocket exhaust. Speed not size presents the real danger. A 1mm metal fragment can strike with the force of a bullet. Natural micrometeoroids possess higher velocities still but are comparatively rarer and less dense. Critical ISS modules are shielded to withstand objects up to 1cm in diameter as a minimum. The space station is constructed of multiple aluminium bumpers, plus Nextel and Kevlar layers which absorb the impact energy of such objects before they reach the inner shell.
Spacewalkers are also vulnerable as some parts of the spacesuits have no shielding - the helmet visor for instance. NASA has calculated that the odds of a critical penetration forming a hole larger than 4mm in the 14 layer spacesuit during a 6-hour spacewalk as one in 31,000.
Large Pieces of Debris
More than 20,000 items of orbital debris larger than 10cm can be tracked by ground-based radar. Collision avoidance manoeuvres are performed if an object comes within a few kilometres of the station and has a collision likelihood higher than one in 10000. The ISS used to average one manoeuvre annually, but the frequency is growing. Four such manoeuvres were performed between April 3011 and April 2012. In 2011 the crew were ordered aboard the two Soyuz lifeboats when some debris came 250m.
Debris comes mostly from defunct satellites, driven to explode by sunlight heating up their batteries or surplus fuel. A collision is a growing source of concern.
NASA scientists have observed that once past a critical mass, collisions will give rise to more debris in a chain reaction. This "Kessler syndrome" is becoming ever increasingly likely as debris levels in low earth orbit have increased by 50% since 2008, helped by the 2009 collision of Russians Kosmos-2251 with the US Iridium 33 satellite plus China 2007 missile test targeting its own weather system.