Zombie Satalites
Introduction
A zombie satellite, as the name implies is a dead satellite which has unexpectedly come back to life and starts talking to its command point again after an extended period of time inactive. The satellite is usually in a state of expected deactivation having come to the end of its workable life. So a little background first.
I'm an amateur radio operator, here in the UK, I've always had little interest in ground based communication,, but fascinated with off world communication, moon bouncing, listening to the international space station, that kind of thing. I first heard of zombie satellites when I was on a radio forum about tracking satellites, and some excitable character who had just established communication with a long lost satellite, and had made a post on the subject. I was hooked, before I knew it I was designing and building vast aluminum antennas to listen to the depths of space and scanning amateur radio astronomy websites to see what kind of equipment I could get or fabricate. From that point it wasn't amateur radio, it was amateur radio astronomy.
So back to zombie satellites. Try and imagine you are a NASA command controller, you have just finished a 5 year mission and your satellites are now defunct bit of equipment hurling into outer space has all been written off by NASA months ago. Just then something extraordinary starts to happen, somehow the satellite starts communicating again? You start receiving data from it, the question as a NASA controller is at this point what do you do? So this is another interesting aspect of Z-Sats (zombie satellites). This all depends on if the satellite is commercial or explorative. One other important distinction to make, a satellite which has ceased transmitting and is no longer under power is considered space debris, and believe me there is a lot!
The above image isn't of Saturn, its actually earth with all the current space debris mapped as singular dots. So lets now dive a little deeper into what makes a Z-Sat.
Ignore the satellite
When that thing first fires up and starts communicating after years of nothingness, the first question to ask is what purpose did the sat serve. Nine times out of ten it was a commercial sat, either weather or communications, in both cases it has served out its useful purpose to the company who owns the sat and as far as they are concerned it is floating space junk which occasionally is calling out and saying hi. But that doesn't mean we should ignore them, simply cast them out as nothing but extraordinary, in most cases Z-Sat's whose purpose was commercial are tracked still by amateur radio astronomers as a kind of version of what radio hams call DXing which is in short us demonstrating ones technical ability to receive very weak signals. These types fo Z-Sat's are useful in that they can be fun to try and locate and identify, as well as a good indication as to how good your equipment is.
Collect the data
The other purpose a sat may have is that of exploration. These are usually owned and run by the main space agencies although some private companies dabble in exploratory satellites. The data these SATs send back is usually very useful and interesting, they could be placed in orbits around planets of bodies in space that are being studied, so when a dead sat comes back to life and starts transmitting data, the agency it communicates with jump at the chance of some extra data. Take for example Voyager2 (no one seems to talk about voyager1?), the deep space sat sent from earth in 1977 to study the outer planets. Now this probe was suppose to have died a long time ago as it passed Pluto and outwards into the cosmos, however on the 29th of October 2020 to NASA's total surprise they re-established contact with Voyager2, and immediately started to figure out that it was transmitting some interesting data about what life is like beyond our solar system.
Power supply
This is a very key point with Z-Sat's. In space, there are generally two types of power supply, solar and nuclear.
Solar
Solar energy is somewhat abundant in space, with no clouds in the way and certainly nothing to stop those juicy photons hitting your array of photovoltaic cells, solar energy is the perfect solution for sat's, that is until a pesky planet gets in the way and plunges your satellite into total, powerless, darkness. This is workable with an orbital sat but becomes a little less workable for anything in a non orbital trajectory through our solar system as often a planet could get in the way for some time. Orbital sat's have batteries which are charged as a percentage of what is produced by the cells , usually 50% and the other 50 goes into powering the satellite, that way when the lights go out the sat can continue to work on the dark side of what ever planet it is orbiting. There are some situations where even if there is an abundance of sun light, solar isn't the right solution, New Horizon is a perfect example of this. Launched to study Pluto and its surrounding solar winds New Horizon although regularly near sun light and in orbit, is regularly plunged into darkness hence a Nuclear Battery is used. When a solar powered Z-Sat comes back to life, there is usually a predictable pattern of communication based on when the sat is in sun light or not, and often it failed because at some point towards the tail end of its mission it was exposed to a prolonged period of darkness draining it's internal batteries. These types of Z-Sat's are called strobes or sleepers. (not sure why strobes but hey) Quick pro's and con's
- Solar energy is abundant in space (close to a star anyway)
- Solar panels are quite efficient and can be made to specific size to suit the situation, including specific power needs.
- They weigh considerably less making them cheaper to launch.
The one and only disadvantage of course is they are useless if there is no sunlight where the probe or satellite is going its not going to last very long at all!
So what does this mean? In short it means that if the sat comes back to life, its likely to be in communication only while it has direct sun light, making these satellites of particular intriguing / challenge for any radio astronomy satalite hunter.
Nuclear
Nuclear powered satellites rely on something called an RTG or Radioisotope Thermoelectric Generator. (wiki) I want to do a separate article on these things because they are actually used on earth, and there usage in space has been controversial at times. In short you get a bunch of radioactive stuff, something not very useful in a conventional fission reactor, shove it in a tube and surround it by thermo-couplings which convert the heat of its decay into electricity. Its not much (another reason they are only used for space craft) about 80Watts and 30 Volts. Enough for a space craft. The advantage of using them in space is as follows
- They last a long time. Nuclear material can have a very long half life, so it stands to reason will produce a pretty steady output of heat for a long time.
- They are safe in space. Yes if this thing was to blow up on the launch pad or in our atmosphere it could cause a problem as it does contain radioactive material, for this same reason these things are generally only used for deep space missions and never for earth orbiting satellites, because they are more than likely to re-enter earths atmosphere at some point or another ,called retrograde.
- They do not rely on the sun, so can provide constant energy output regardless of light levels.
So what are the dis-advantages
- They are heavy (lots of shielding and cooling vents) so this adds to the overall cost of launching them. (more weight more fuel required to escape earths gravity)
- Because at the Centre of these things is a tube containing lots of elements which are bombarding everything around them with neutrons, kind of like putting sand in a washing machine, over time component failure due to neutron damage is to be expected.
- They pose a health risk while being constructed, launched and while they are close to our planet in orbit.
- We need to consider the long game, are we ever likely to encounter one of our RTG's when we send people to planets such as mars (Where a few were actually used)
- Efficiency isn't very good. It ranged from 25% to 30% of total heat generated is converted into electricity. Not the best.
So what does this mean? Probes (its rare you will find an earth orbiting satellite with one of these things in it!) will function constantly with an RTG in it, up until the point one of two things happens
- The heat output falls below the threshold required to generate sufficient power. Probe shuts down, communication stops. boo!
- The core damages the thermocouples through neutron bombardment. Probe shuts down, communication stops. boo!
Both of these scenarios end in the same result, however one of them is recoverable. Should the conditions of the probe change, for example it gets access to some direct sun light, this could push the the heat generated just above the threshold and the probe could come back to life. Woohoo - Zombie satellite is born.
To Summarise!
Zombie satellites are a cool thing, really they are, hunting for them is the radio astronomical equivalent of metal detecting, only the beach is the size of France, the metal is the size of a pin and there are only about fifteen or so. I will leave you with some examples while you digest just how rare they are. Enjoy
Transit 5B-5
One of the oldest out there, It was launched in 1965 as part of the Transit system, one of the first GPS systems. Transit 5B-5 is actually one of only a few Nuclear powered satellites in a stable polar EARTH orbit. (that's right its up there floating around) more alarmingly its still communicating but operators are unable to control it,due to it being out of fuel for maneuvering.
LES 1
This one is quite cool. Remember how I said not many Z-Sat's are in earth orbit and even fewer of them solar flavored ones come back to life? Well this one is a great example. LES 1 was the Lincoln Experiment satellite, launched in 1965 to study UHF communication. It never achieved correct orbit, and was considered lost. In 2012 though it began to start broadcasting again, no one really knows why, but the discoverer (a Cornish radio ham) thinks
It is possible that, after 47 years, the batteries failed in a manner that allows them to carry charge directly through to the transmitter on 237 MHz, allowing the satellite to resume transmissions when it is in sunlight.
LES 5
These sats seem to be indestructible or something, in 2020 yet another long defunct Lincoln experiment satellite came back online and started broadcasting. Again solar panels so only operating during sunlight hours (for the sat not for you!),
Here is a video of LES-1 on Youtube Interesting enough the software being used is the same software I use, with my SDR radio. (I might do a post about how to listen to satellites at a later date.)