
DirecTV races to decommission broken satellite before it explodes - DemiGuru
https://arstechnica.com/information-technology/2020/01/directv-races-to-decommission-broken-boeing-satellite-before-it-explodes/
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pintxo
Strange how some comments are able to get to the core of the story, while the
article somehow does not:

> Satellites in geosynchronous orbit go through an annual eclipse season when
> the Earth passes between the Sun and the satellite. Currently, the satellite
> is drawing power directly from the solar arrays and is configured to avoid
> charging the battery.

> But when the eclipse comes, they'll have to discharge the battery to keep
> the satellite powered and under control. Charging or discharging the damaged
> battery risks causing a thermal runaway and an energetic breakup.

> That's why they want to get it up into the graveyard orbit before the
> eclipse, so if it explodes, it does so in a orbit where the debris are very
> unlikely to disrupt the operation of active satellites.

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Zenbit_UX
> That's why they want to get it up into the graveyard orbit before the
> eclipse, so if it explodes, it does so in a orbit where the debris are very
> unlikely to disrupt the operation of active satellites.

A graveyard orbit sounds like it will be flying around up their indefinitely
as high speed shrapnel. Does anyone know why they would choose this instead of
destroying it on re-entry?

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tehbeard
[https://en.wikipedia.org/wiki/Graveyard_orbit](https://en.wikipedia.org/wiki/Graveyard_orbit)
puts the difference in delta V requirements for a geo-sync orbit at ~1500m/s
to de-orbit vs 11m/s to get boost up to graveyard orbit.

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dls2016
What's the physical intuition here? Is it that you want the de-orbit to hit
the atmosphere at a shallow angle instead of plunging to earth?

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penagwin
Geostationary orbit is REALLY far away from earth (and also the atmosphere)
compared to other orbits, but it's very special and so we want to keep as
little junk in it as possible. Other types of orbits are so low, that they
have to keep maintaining their orbit or they'll decay into the atmosphere.

So we're in a stable orbit with a high periapsis (closest part of our orbit to
earth). To de-orbit we need to lower the periapsis so that it touches the
atmosphere, and on each pass the satellite will slow down, further lowering
the apoapsis and periapsis. The problem is that to do this we have to burn all
that horizontal velocity that's keeping us in orbit in the first place, which
is a crapload.

An alternative is to instead change the orbit so it's no longer in
geostationary orbit and potentially interfering with live satellites, so
instead they just push the satellite further out into the "graveyard orbit".

There's 100x difference in the needed energy for the two methods, and I'm sure
you can see why most would opt for the graveyard orbit.

EDIT: To add, it's actually rather difficult to "plunge into earth". To do
this you need to burn all of your horizontal velocity (I think that's around
3.07 km/s in geostationary orbit?). Reaching the sun is actually really
difficult as well for the same reason, you have to lose all of your horizontal
velocity, and earth has a ton.

~~~
mark-r
How much of a danger is debris from an exploded satellite? Each fragment is
going to be independent and have its own orbit. Any orbit higher or lower than
the geostationary belt won't be a problem. Any orbit equal to the original
satellite's won't be a problem either. Only those pieces maintaining the same
altitude but a different direction will ever intersect the belt, and that will
be a rare occurrence.

~~~
Floegipoky
You're correct that each fragment will have its own independent orbit, based
on the delta-v that it gained from the explosion. But they won't circularize;
each of those new orbits will intersect the original orbit at the point where
it exploded. The orbital periods of the fragments will also not match the
original, meaning that the relative intersection will shift along the original
orbit every time. So an explosion in geostationary orbit would be a Really Big
Deal and could potentially threaten the whole geostationary fleet.

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r3dey3
I was really confused about eclipse season, as I thought the satellites were
in the earth's shadow every day.. But that's not the case; due to how high
they are, they usually always get sun, except for a few months in the spring
and fall where the earths axis of rotation is not pointing towards/away from
the sun. That few months is the eclipse season; and the eclipses happen
once/day for a max of 72 minutes - which is where the batteries are needed.

[http://www.intelsat.com/tools-
resources/library/satellite-10...](http://www.intelsat.com/tools-
resources/library/satellite-101/eclipse-seasons/)

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anigbrowl
Rather annoying that this article doesn't address the consequences if they
fail in their endeavors and the thing does blow up.

Also, not great timing for Boeing.

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chrismartin
Which battery chemistry?

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ThisIsTheWay
This satellite operates on a Boeing 702HP bus [1], which uses lithium-ion
battery cells sourced from Saft. [2]

[1]
[https://en.wikipedia.org/wiki/Boeing_702](https://en.wikipedia.org/wiki/Boeing_702)

[2][http://www.terradaily.com/reports/Saft_To_Provide_Lithium_Io...](http://www.terradaily.com/reports/Saft_To_Provide_Lithium_Ion_Batteries_for_Boeing_GEO_Mobile_Satellites_999.html)

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jonplackett
Boeing really can’t catch a break huh.

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jrobn
What’s the cost of just flinging it out into space towards the sun?

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lorenzhs
That's kinda not how orbits work. First, to get to the sun, you have to escape
Earth's gravity, which takes a lot of power. Once you've done so, the sun is
actually one of the _hardest_ places to go. It's easier to leave the solar
system altogether than to drop into the sun.

~~~
Dylan16807
Depends on how long you're willing to wait. In rough rounded numbers:

Earth's velocity around the sun is 30km/s.

To directly slow down enough to hit the sun, you need to remove 20km/s.

To leave the solar system you need an extra 10km/s.

But if you _almost_ leave the solar system, and wait for the very peak of your
orbit, then you'll be going so slowly that you can turn it into a pure dive
into the center of the sun with almost zero thrust. So this plan needs
slightly less thrust than escaping entirely. It will just take decades to
centuries.

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callesgg
How much is the battery cell temperature deviating?

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gabrielrdz
just call Walter White. Sure he'd be able to pull something off with Magnets..

~~~
ge96
Send Clint Eastwood and Tommy Lee Jones

