Why Space Matters: GEO Satellite operations, Part 6–Eclipse

All that you touch, all that you see…

Yep, you guessed it!  We’re going to talk about another issue common to geostationary (GEO) satellites:  the eclipse.  This issue is almost opposite from the problem discussed in Part 5 of the GEO lesson series.  Instead of being overpowered by the sun’s energy, the satellite can’t function because of a lack of it.

Here’s how that works.

Twice a year, the Earth gets in the way of the GEO satellite’s view of the sun.  Those two periods occur around the vernal and autumnal equinoxes.  The eclipse period starts in very small time increments at first, gradually increasing over a period of days.  Eventually the eclipse peaks for nearly 70 minutes, then starts decreasing over the next few days.  Each eclipse period during each equinox lasts a total of 45 days.  For some satellites, though, these periods can be too long.

Forever’s gonna start tonight…

You see, the satellite’s systems are powered by electricity.  There’s no extension cord long enough to plug the satellite in, but the satellite must be powered somehow.  The common engineering answer has been to use the sun’s energy, absorbed and changed to electricity through a satellite’s solar panels.  The power is then saved in the satellite’s batteries.  The batteries then power the rest of the satellite, sub-systems, payload, etc.  But what happens when there is no sun as a source of energy for the satellite solar panels to convert?  Well there’s nothing I can say other than it’s a total eclipse of the part (of the satellite).

Again, you may be thinking, “No biggie.  I have a cell phone in my pocket that has more capability than a satellite, and it can last for days.  Surely satellite weighing tons can last longer on batteries than my tiny phone.  Right?  Right?”  The answer to this is:  It’s complicated.

You would think the designers of these very expensive satellites would put enough batteries on board a satellite to compensate for an eclipse.  They try, but here’s a question for you:  Have you ever lifted a car battery?  Now pretend the satellite has same car battery weighing at the low end of 15lbs—and multiply that weight by 103.  That’s about what the Nickel Hydrogen batteries of a modern GEO satellite weigh.

…Everything under the sun is in tune…

What this heavy fact means is there are only so many batteries that can be put on a satellite before it becomes too heavy, and therefore, too expensive to launch.  So batteries are part of a price/weight/required power/risk management balancing act.  For most satellites, the eventual design is just fine.  When eclipses come around, the satellites can rely on the battery to eventually get them through the full 70-minute eclipse.  Operations can continue without a hiccup.

But what can be done if an operational satellite’s batteries start going south?  Or if some other part of the power sub-system starts degrading or just stops working?  Suddenly the capacity to power a satellite is greatly diminished.  Suddenly the satellite may not have enough power to not even turn itself on again (at least until the sun charges them up again—but totally depleting a battery is sometimes not a good thing either).  There are also certain preventative maintenance functions a satellite operator uses to help with satellite battery life.  But all of this is what makes the battery balancing act tricky.  Now, here’s another question for you:  How long do batteries last in your household?  Especially the rechargeable ones?

…I don’t know what to do, I’m always in the dark…

There’s no Walmart satellite servicing garage in space just now.  Satellite servicing and battery change capability aren’t available to satellites (yet—see this interesting tidbit).  The batteries that are designed and eventually loaded onto a satellite’s bus must be robust, long-lasting, and reliable.  If any of those criteria are violated, the satellite just becomes a very expensive man-made star.  Since satellites are so pricey to get into orbit to begin with, quite a few owners expect the satellites to be designed to last a long time–some as long as 15 years.  So guess what also needs to last that long?  Would it amaze you to know the batteries do last this long on some satellites?  They do, but eclipses still cause power issues.  At least they are predictable.

Of course, this sort of problem isn’t only the province of the GEO satellite.  The other satellites also have some power issues to deal with, including eclipse.  But this problem is very obviously seen on GEO satellites, especially since a lot of the public relies on communications and weather from those sources.

There is another problem GEO satellites do face, but that’s a lesson for another day.

HOLD ON!!!  I JUST ADDED THIS:  It seems appropriate for my readers, and I like DIY things.  Those of you from a different age might recognize Patrick Norton from ZDTV.  He’s been for the longest time a podcast host on Revision3.  The particular episode (below) posted on 27 Jan 2014 is all about making your own batteries to charge a cell phone–without lemons.