Like all astronauts, the two extra crew aboard space shuttle mission STS-126 went through a rigorous selection process, fitness tests and hours of training to prepare them for their scheduled launch tonight. But there is a crucial difference: these intrepid space explorers are spiders.

Joining a human crew of seven, the orb-weaver spiders will be strapped into a special compartment aboard the shuttle Endeavour when it blasts off from Cape Canaveral in Florida.

Their destination is the International Space Station, where they will remain for the next three months, circling Earth more than 1,300 times at 17,500mph (28,200 km/h).

Their cosmic sojourn will be monitored by thousands of children on the internet as part of a programme intended to teach them the effects of micro-gravity and enthusing them about science, space and technology.

While the prospect of spending three months in orbit with two spiders lurking among the cargo would send shivers down the spines of arachnophobes - and put the imaginations of sci-fi fanatics into overdrive - the astronauts find their eight-legged crewmates strangely soothing.

“Even on shorter missions astronauts have said that they feel they are in this harsh environment of space and to see other life there with them makes them feel more comfortable,” said Carla Goulart, mission operations engineer at BioServe Space Technologies, which devised the study. “Maybe some people don't like the thought of it, but these astronauts aren't fearful of a spider. They see them as their friends.”

BioServe, a Nasa-funded space research centre at the University of Colorado, has created special habitats for the two female orbweavers.

While the astronauts set to work on expanding the space station and plumbing in a new system that will allow future crews to recycle urine as drinking water, making them less dependent on fresh supplies from Earth, the spiders will be busy on home construction of their own.

The aim is to study how they spin webs and catch food in the microgravity of space.

Their webs - the construction of which usually relies on a spider's ability to dangle and drop, but in space will require some lateral thinking by the free-floating arachnids - will also be examined to establish whether the strength of their silk is different in orbit.

Hopes are high that they will fare better than Anita and Arabella, two spidernauts that died of dehydration on board the Skylab space station in 1973 after the in-flight catering failed. This time the spiders have a personal pantry stocked with live fruit flies that have been fattened on dog food.

About 30 spiders were put through their paces, including simulated launches, to assess which of them possessed the “right stuff” that Nasa famously requires of its flight crews.

The lucky two were hand-picked for their web-spinning techniques, vigour and youthfulness.

Animals at altitude

— Fruit flies were first sent into space in 1947, aboard a V2 rocket launched by the US to explore the effects of radiation exposure at high altitudes

— Albert I, a rhesus monkey, became the first primate in space in 1948 but died of suffocation. Albert II, launched the following year, died on his return journey

— A stray mongrel from Moscow called Laika became the first dog to go into orbit in 1957 but died a few days into her mission

— France launched a stray black and white tomcat into space in 1963. Felix, the first cat in space, had electrodes fitted in his head to measure neural impulses. He returned safely

— The Orbiting Frog Otolith satellite launched by the US in 1970 carried two bullfrogs. They were kept in a water-filled centrifuge to test the effect of gravitational fields on the otolith, the inner ear's balance mechanism. They were never recovered

Sources: history.nasa.gov ; scienceray.com ; spacetoday.org

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