The "dead-but-alive" telecommunications satellite, Galaxy-15
<http://www.orbital.com/SatellitesSpace/Communications/PanAmSat/>, has
begun to enter the space of neighbouring craft, and their operators are
planning evasive action.
"Zombie-sat" has captured the imagination of the internet space forums
these past few weeks. It's probably the nickname that's done it.
When we sit on the sofa skipping across the smorgasbord of channels with
our remote-controls, we don't usually give much thought to the "bent
pipes" that sit 36,000km above our heads, delivering the televisual feast.
Intelsat's Galaxy-15
<http://www.intelsat.com/resources/galaxy-15/operational-status.asp>
satellite was put in geostationary orbit five years ago to re-distribute
TV services to cable companies across North America, and also to send
navigation data to aeroplanes to improve the accuracy of their GPS
receivers.
But the "bird" experienced a major hiccup at the beginning of April.
It's not known precisely what happened. One possibility is that it was
damaged by high-speed particles billowing off the Sun in a solar storm -
an ever-present danger for orbiting electronics.
The satellite is still operational: it's still "on", but Intelsat cannot
control it. Any signal it receives, it re-transmits at high power. It's
a very unusual situation.
What doesn't help is the fact that Galaxy-15, which is supposed to sit
at 133 degrees West (over the eastern Pacific), is drifting slowing
eastwards by about 0.05 degrees a day. This will take it into the path
of other satellites, and first to have an issue is AMC-11
<http://www.ses-worldskies.com/worldskies/satellites/01_amc-fleet/amc-11/index.php>,
another TV services spacecraft operated by SES World Skies
<http://www.ses-worldskies.com/worldskies/index.php>.
If SES were to do nothing, Zombie-sat would soon start picking up and
retransmitting signals sent to AMC 11. To users on the ground who depend
on AMC 11 for their daily dose of MTV, this could lead to a horrendous
mash-up.
It would be like trying to listen to two people who are shouting the
same conversation at you.
So, SES World Skies will today begin a delicate orbital dance, in which
they will allow AMC 11 to drift in tandem with Galaxy-15 while at the
same time sneaking up another satellite behind the pair.
The plan is for the SES controllers to then leapfrog many of the
services on AMC-11 across to this other satellite, known as SES-1,
thereby minimising the disruption to customers.
The manoeuvres are unprecedented, says Alan Young, the chief technology
officer with SES World Skies.
"The closest AMC-11 and Galaxy-15 will come is measured in
kilometres, and in space terms that's quite close. But the risk here
is not one of collision; we're not at all concerned about that. The
problem is that they're so close when viewed from Earth that it's
not easy to distinguish between the two satellites and seeing as
they both operate in the same frequency band, there will be
interference if we're not careful.
"We've gone to a number of measures, including moving customers on
AMC-11 on to a very large uplink antenna. This means we can very
finely discriminate between the two spacecraft so that we can direct
all of the energy into AMC-11 and as little energy as possible into
Galaxy-15. If you don't put anything into Galaxy-15, you won't get
anything out."
AMC-11 will eventually be moved back to its orbital slot to resume
normal operations once the zombie has passed through, which should be 7
June.
All satellite operators and comms companies will have to work out what
Galaxy-15 means to them. Here at the BBC, we've had to consider how some
of our international services like the BBC World News channel
<http://www.bbcworldnews.com> might be affected.
This channel is fed through Intelsat's Galaxy-13 platform. The most
recent calculations suggest everything should be fine.
Anyone sitting on their sofa in North America should be oblivious to the
space waltz that is about to take place.
There are some wider issues, however. For satellite manufacturers, there
will be keen interest in understanding exactly what happened to Galaxy-15.
Satellites have redundant, or back-up systems
<http://news.bbc.co.uk/1/hi/sci/tech/8458203.stm>; and when they have
major upsets, there are usually modes that will completely re-boot the
spacecraft automatically after a period of time.
Galaxy-15 was made by Orbital Sciences <http://www.orbital.com/>, but
Patrick Wood, the chief technical officer for EADS Astrium satellites
<http://www.astrium.eads.net/>, told me the entire industry had an
interest in finding out what went wrong:
"Part of our design review process is to check through the
architecture to ensure there isn't a single point that, were it to
fail, we'd lose complete control of the spacecraft. Clearly
Galaxy-15 has had a major event and most organisations will want to
understand what happened. From an industrial point of view, the
surprising thing is that Galaxy-15 is locked on full power. This
tends to suggest the control/tele-command side of the spacecraft has
failed and left the spacecraft in whatever mode it was in when it
was last commanded. It's a very unusual case."
And, of course, the whole episode raises once again the issue of orbital
space debris <http://news.bbc.co.uk/1/hi/sci/tech/7916582.stm>. Galaxy
15 will likely end its days in one of the two great "garbage patches" in
the sky.
These libration points
<http://en.wikipedia.org/wiki/Geostationary_orbit#Earth_orbital_libration_points>,
as they are known, are located at roughly 105 degrees West and 75
degrees East. They are gravitational "sweet-spots" where drifting
objects will naturally coalesce.
The two libration points now contain more than 150 defunct satellites
[395Kb PDF]
<http://www.secureworldfoundation.org/siteadmin/images/files/file_460.pdf>.
Satellite operators are urged to put their geostationary spacecraft in a
"graveyard orbit" once their missions are complete. This usually means
pushing the platforms even higher into the sky.
But of the 21 spacecraft which reached end of life in 2009, only 11 were
disposed of in accordance with the Inter-Agency Space Debris
Coordination Committee's (IADC) re-orbiting guidelines [99 Kb PDF]
<http://www.iadc-online.org/Documents/Docu/IADC_Mitigation_Guidelines_Rev1_Sep07.pdf>.
We may all love our satellite TV, but we're starting to build a problem
for ourselves.
Watch this space.
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