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How do you spot a tool bag in space?

The Magazine answers...

Track the tool bag

A tool bag which floated away from an astronaut during a space walk will be visible in the sky over Britain this week, but how do you spot it?

Losing a tool bag can be inconvenient, but when you're 212 miles above the Earth it's a whole different matter.

Last week American astronaut Heidemarie Stefanyshyn-Piper was forced to watch her tools - reportedly worth 70,000 - float off when the backpack-sized bag slipped out of her grip.

The accident happened as she was cleaning grease off her gloves while fixing a gummed-up joint on the International Space Station's solar panel. The bag went into orbit and has become a "must see" among some competitive stargazers in recent days.

You can calculate precisely where something will be in space at any given time thanks in part to Newton's first law of motion. It states that any object moving in a straight line tends to remain in such a state unless acted upon by an external force.

This also applies in space, the only difference being objects move in orbit - the bag circles the Earth due to its gravitational pull. And because space is so vast and empty, it is unlikely to be knocked off its course by anything else.

Astronaut Heidemarie Stefanyshyn-Piper
Location plotted using Newton's first law of motion
Size of space means it's unlikely to be hit by anything else - or be anything else

As scientists know information like the size of the tool bag and where it was lost, it is possible to do the orbital calculations to determine where it will be and when.

A computer model has been developed for the tool bag (see link below answer box, right). But finding its exact location in the sky depends on your location on Earth. For example, it will appear lower in the sky in the north of England and Scotland than the south.

Equipped with latitude and longitude coordinates, the model will calculate the time to see the tool bag, the altitude it will be at and the magnitude, which specifies its brightness compared with stars.

Usually the bag is below 6th magnitude, which is naked-eye visibility. This means you will need the right equipment to see it - binoculars or a telescope. Even with these it will be difficult to see, says Dr Robert Massey of the Royal Astronomical Society.

Burn up

But the typically large distances between things in space also means it's unlikely anything else will be in the same position at the same time, says Robin Scagell, of the Society for Popular Astronomy. So what you track will almost certainly be the tool kit.

"Most people have been playing too many computer games and think space is full of chunks of rock and debris, but it isn't," he says.

"There are things out there, but they are likely to be meteors the size of a grain of coffee and many miles apart. Even if they did hit the tool bag they would have little impact."

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What you will see is another matter. The bag will have no structure or shape but will look like a faint star whizzing through the sky. But it will be easily distinguishable from a plane because it won't have red and green navigation lights.

"It will be a speck of light which will not be visible to the naked eye," says Dr Massey. "I wouldn't waste too much time looking out for it. Looking at the actual space station where the bag was lost is far more interesting."

Eventually very faint traces of atmosphere will act on the tool bag to slow it down and it will come out of orbit, says Mr Scagell. But this could take years and when it does happen the bag will be burned up in the Earth's atmosphere.

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