Page 107 - BBC Sky at Night Beginners Guide to Astronomy - 2017 UK

WHAT TO SEE

Earth’s environment, tracking things such as global Medium Earth orbit
warming, and yet more help us communicate
around the world.
Don’t forget that the word ‘satellite’ can
include craft big enough for people to
climb into. So the International Space
Station (ISS) is a satellite, and even the ISS orbit
Space Shuttle was classed as a satellite
while it was in orbit.
The Space Shuttle was retired in 2011, but
when it was in service, it had another thing in
common with the ISS. Both would orbit quite close
to Earth for a satellite – just 340km (180 miles) from
the surface of our planet. At that range, a satellite is
Low Earth orbit
in what’s called low Earth orbit (LEO), which goes Sputnik 1,
from 160km (100 miles) to 2,000km (1,200 miles) the fi rst GPS
up. You’ll fi nd most satellites in LEO because it’s satellite satellites
orbit
comparatively cheap to put them there and you get in space
good views of Earth. At this range, satellites travel
around 8km/s (18,000mph) and zip round the planet
in about 90 minutes.
Geostationary orbit High Earth orbit
THE DEPTHS OF SPACE
The farthest away a satellite can orbit stretches
to tens of thousands of kilometres. How high you There are three distinct regions where a satellite can go into orbit around
the Earth. The ISS is close to home in Low Earth Orbit (LEO)
position one depends on what it’s going to do and
how much money you have – the farther from Earth,
the more expensive it is. After LEO is medium
Earth orbit (MEO), which stretches from 2,000km SPACE JUNK
(1,200 miles) out to just below 35,786km (22,000 If working satellites were alone process. Astronauts have even
miles). If you have a GPS unit, then it will be picking up there, all would be well. lost spanners while doing repairs.
up signals from a GPS satellite in MEO travelling But there are more spent ones Currently, nearly 9,500 bits of
at speeds of 3.9km/s (9,000mph), taking almost 12 cluttering up the space around junk larger than a tennis ball
Earth. By some estimates, there are being tracked, but there are
hours to make one orbit of Earth.
are now 25,000 objects in LEO thousands more smaller pieces
The outer edge of MEO marks the start of
alone. This includes plenty of whizzing round our planet. The
geosynchronous orbit. This is where a satellite is dead satellites, but there are problem is the speed at which
placed in order to orbit at the same speed that Earth also bolts fi red from rockets this junk travels – around 7.5km/s
rotates, so it remains over one spot on the surface. to separate one stage from (17,000mph), making it a danger
If you have a satellite dish on your house it will be another, together with paint and to spacewalking astronauts, the
metal shards chipped off in the ISS and other satellites.
pointing to one of these satellites.
Beyond that is high Earth orbit (HEO). This is
used for satellites with a highly elliptical orbit, where
their furthest distance is beyond geosynchronous
orbit. Here you’ll fi nd some communication satellites
and a few used for spying, travelling along their
orbits between 2-10km/s (4,000-22,000mph)
depending on their distance to Earth.
So what about seeing them? It’s quite amazing,
given the relatively small size of satellites, how
many of them you can see in LEO on a clear dark
night with just your eyes. The Heavens Above
website (www.heavens-above.com) will help you
locate any that are fl ying over your home. You just
enter the details of where you live and up pops a
list explaining when and where to look for any
This map shows the
satellites around, and also how bright they will
amount of space junk
appear. So, why not get out and see if you can
in orbit around Earth
identify a satellite tonight?
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