# What would be the (most difficult) challenge to make a “10,000 year satellite”?

Update: Bezos' recent Tweeted update on the 10,000 year clock. Also see The Verge's Construction begins on Jeff Bezos’ $42 million 10,000-year clock The discussions associated with the question Can an artificial satellite stay in orbit forever? and its answers suddenly reminded me of the 10,000 year clock project. Take a moment to look there now. After watching the VIMEO video there, here is a YouTube video. I also thought about the passive mechanical structures described in this answer — basically bi-metallic strips that open/close shutters, but their designed behavior reacting to temperature distributions inside is actually pretty complicated. An Earth-orbiting satellite that needs to remain in orbit for a long time should at perigee be high enough to avoid essentially all atmospheric drag, but not too high in order to at least reduce the gravitational perturbations from Moon, Sun, etc. It could passively absorb thermal energy from the thermal difference between Sun and space, and store it through springs and gears and latches. Then, maybe once a year it would have enough power to produce a signal - by radio, or light. 10,000 year electronics might not be impossible if they are low tech (discrete devices) and efficient, and one "beep" per year may not be very demanding. Whatever it does should be fairly limited - not impose itself on those not wanting to see it specifically. As a backup, if the active electronic "beep" fails, I suppose it could mechanically just change reflectivity, and "suddenly appear" once a year. Large surface area can be obtained in various ways — unrolling something then rolling it back up just for example. And it doesn't have to launch next month — it could be a development project (like the 10,000 year clock). Once it's up there, it wouldn't require any further management from the ground. Calculating the orbital mechanics would be a substantial project, so I'm not asking here for an orbit that can last 10,000 years. If there are calculations showing it can not exist, please post the link or calculations! But otherwise let's avoid the "Orbital Mechanical Opinions". What else besides orbital mechanics is there that would pose the most difficult challenges to making an artificial satellite stay in an orbit around the earth for 10,000 years, and "beep" or change appearance once a year? • A distant retrograde orbit around the Moon might be close. It is claimed to be stable over at least hundreds of years, and it would only rarely be eclipsed by the Earth or the Moon. However I don't know if anyone has run it out for 10,000 years. – Mark Adler May 18 '16 at 20:26 • Put it on the moon, which is already in Earth orbit. – dotancohen May 19 '16 at 15:38 • I would think that biggest/most insurmountable issue would be structutural and electronic failures due to material degradation from exposure to radiation. The radiation levels in earth orbit can be quite significant (generally much higher than interstellar space IIRC) and it can play havoc with the integrity of both structural and electronic materials, esp. in the long-term. – RBarryYoung May 19 '16 at 18:04 • Comments are not for extended discussion; this conversation has been moved to chat. – called2voyage May 19 '16 at 18:04 • Not with current technology, but:it'll be robotic, such that it can continually remake itself. Remake solar panels, renew shields, remake electronics. Remake the robots that are doing all of this. I don't think it'll be small, because it'll need spare resources (I'm guessing that remaking things can be "lossy"), and manufacturing things takes plenty of energy. – Wayne Conrad Jul 21 '17 at 18:38 ## 8 Answers # LAGEOS satellites This has, in a way, already been done, with the Laser Geodynamics Satellite (LAGEOS) satellites. LAGEOS satellites, (the second of which was launched from the shuttle on mission STS-52), have a projected orbital lifetime of over 8 million years. They are in a very stable medium Earth orbit. They are completely passive, but are illuminated by ground based lasers. And, they look like disco balls. Good info at the Wikipedia article. • WOW! I love that (those) thing(s)! I remember reading about the first one (late 1970's) - it's passive, and yet so useful due to clever design. I never knew about the Germanium reflectors for IR. Looking at this image from the page Measurement of the LAGEOS-I Spin Axis gives even more to think about. Reminds me of these siblings on the moon. – uhoh May 19 '16 at 1:08 • They might last, but will be hard to find after we lose them. – gerrit May 19 '16 at 14:26 • A beautiful thing, but it doesn't broadcast its existence, and to see it, you need to know exactly where to point your laser. – Mark Adler May 19 '16 at 15:41 • @MarkAdler Not exactly exactly. Because they have corner cube reflectors, probably the knowledge that they exist alone would be enough to find them given enough determination: a scan with an array of slightly divergent lasers could find their general whereabouts. As Gerrit says, they'd be hard to find after we lose track of them, but not altogether impossible. – WetSavannaAnimal May 20 '16 at 10:20 • One week after you posted this answer LAGEOS is in the news: Space.com and TheWeatherChannel @OrganicMarble controls the media? :) oh, it was put on YouTube a few weeks earlier. It's quite a beauty! – uhoh May 29 '16 at 0:35 The problems would be many to transmit a radio signal for 10,000 years. However there is nothing about a 10,000 year lifetime that would violate physics. It would just be extremely difficult engineering. I would use a thermally driven Stirling engine for power, magnetic torquer for attitude control, and vacuum tubes for the electronics (which are much more radiation-resistant than semiconductors). The latter would be all analog for attitude sensing and control with a photoelectric tube and analog computing, and an analog, vacuum tube RF system. The system would not have a battery, and would only operate when in sunlight. (You might be able to come up with long-lived thermal energy storage to make an effective battery as well.) A 10,000 year orbit would not a problem. You would let it drift some. Something like a 2000 km orbit should be sufficiently high. • After all, the Moon has been in orbit for a little bit longer than 10,000 years, and as I said somewhere else (forgot where), doesn't seem to be in a great rush to get anywhere else... – a CVn May 18 '16 at 20:16 • @MarkAdler 500 years is usually considered a very long time. OP asks for components that should survive longer than human civilization has existed. I even have doubts about the stirling engine, there are moving parts, and thus friction, which would have 10000 years to add up. The magnetic torquer is dependent on field strength, so is less suitable for higher orbits. You can definitly not be in low earth orbit, those decay in less than a decade. I would like to see some math if you maintain that geosynchronous is stable enough. However I question the need for torque, broadcast undirected. – Taemyr May 19 '16 at 6:21 • I like the vacuum electronics idea very much! They wouldn't have to be in discrete glass tubes at all, and more importantly no need to be ON all the time either, so could potentially be very long lived. There is (was?) also much work done on smaller, much lower power devices compared to discrete tubes for use as logic, as well as small signal. (These days it's more high-frequency (THz / millimeter wave) or high-power stuff.) – uhoh May 20 '16 at 1:31 • Unless you have some way of minimizing thermal variations (e.g. a very large heat sink, or an orbit that stays in sunlight permanently), they will likely demagnetize the permanent magnets needed by your stirling engine generator and your torquer over that kind of time period. You'll also need to keep your satellite oriented to the Earth's magnetic field, I'd suspect, to stop variations in field direction demagnetizing them, too. – Jules May 20 '16 at 7:34 • @uhoh If the vacuum electronics should not be powered up for all time, you need something long term reliable to control ON and OFF over the 10000 years. But what should do this? No semiconductor electronics, no vacuum electronics, no relay timing circuit? A mechanic clock operating in the vacuum of space for 10000 years without lubrication or cold welding problems? – Uwe Jul 23 '17 at 14:28 To complement, not attempt to replace, the other answers, I would like to propose a difficulty I see nobody having mentioned so far, but which could potentially be very problematic over such long time scales. Micrometeorite bombardment. Even after only 15 years in low (550 km) Earth orbit, we know that the Wide Field Planetary Camera II (WFPC2) on Hubble has seen significant micrometeorite bombardment. Yes, you can build your spacecraft with whipple shields and other protective measures, but particularly against bombardment at multiples of km/s relative velocity, those don't last forever. And they increase the complexity of the spacecraft, especially if you want something like the suggested "become bigger once a year" backup to the primary radio transmission to let people know it's still there and alive. • This is a really good point - I have no idea about the flux (events per year per square meter per steradian per gram per km/sec) which I guess would be integrated over$4 \pi\$ steradians and over some combination of mass and velocity that would be potentially dangerous. Probably comes out to be 1 – uhoh May 19 '16 at 0:26
• I wonder if it would be possible to capture the energy of these bombardments to power the satellite. – Igby Largeman May 19 '16 at 6:24
• @uhoh density is ca 1 per 15x15 cm dyring 15 years on low orbits (station mir solar pannels was damaged so much) – MolbOrg May 19 '16 at 20:34
• That's orbital debris. Probably very little is natural. Just put it up at 2000 km and you won't see hardly any of that. – Mark Adler May 20 '16 at 2:30
• @MarkAdler How much less though? Over 10000 years, "very little" could add up. – called2voyage May 20 '16 at 13:56

The biggest challenges are going to be what people have already mentioned - the funding (don't skirt over that comment, you did ask for the challenges), an energy source for 10,000 years, and how to make a flash or radio pulse based on parts that can last that length of time.

Electronics, by the way, is a particular problem. I've heard people boast low power computers can last for centuries, when we know the electrolytic capacitors will fail after a few decades. Amazing how easy some people assume this sort of thing is, when really it's very difficult. That's probably a question for the Electronics stack exchange.

Putting it in a safe orbit for 10,000 years is probably the easiest bit. (Especially a low orbit around Jupiter, for example. Not many things would have the energy to disturb it there. I admit Jupiter is out of scope though, as Earth orbit was the requirement.)

Edit: Meaningful testing is an interesting thought experiment. How would we test something to be sure it would last 10,000 years? (Without actually making the test period that long.) I'm not even sure transistors will last that long, due to solid diffusion at the junctions. (Probably why Mark Adler opted to use vacuum tubes.)

• Funding is a challenge for sure - I didn't mean to skirt, I just responded to: "...maintaining funding for operations for 10,000 years". For this question right now, it should be around earth, so it can do something every year that we can somehow notice, in analogy to the 10,000 year clock. It's very cool even around Jupiter, but it would be much harder to have it do something that we'd notice here on earth. Of course in a few hundred years we (some of us) may be there, but I'd like to think it does something here, within our lifetime - speaks to the funding issue. – uhoh May 18 '16 at 17:01
• Okay, yes you are right no long term funding would be required but possibly a huge effort to design, test and build it. (High quality testing in a single human lifetime might even be the hardest part of all... and funding the testing over several Government cycles may be the single most ambitious engineering achievement ever.) – Andy May 18 '16 at 17:11
• Are you sure bimetallic strips would last 10,000 years? They can separate if not well made... even if joined with rivets the soft metal can fatigue around those points... – Andy May 18 '16 at 17:25
• @Andy, if you make your bimetallic strip out of two alloys of steel, you can keep the stress below the fatigue limit and it should last nearly forever. Of course, now you've got the problem of finding two steel alloys that have sufficiently different coefficients of thermal expansion... – Mark May 18 '16 at 20:36
• You bring up a good point about capacitors; another problem at these long timescales is that if you have any sort of complex circuit (to send a radio signal), you are going to get electromigration. As far as I can recall, we have no idea how to solve that issue. – 0xDBFB7 May 19 '16 at 21:34

I think it should be possible to put a big, dumb sphere into such an orbit around Earth that it (partially) eclipses the Sun (seen from the Earth's surface) exactly once a year.

This idea is exploiting the fact that the Earth-Sun system is already (per definition) the best time-keeping device for measuring exactly one year. Does that count as an answer? The orbit would lie in the ecliptic and its period would be one year, which would put it at around 2'151'500km...

Edit: ...which is actually outside the Earth's sphere of influence (thanks for pointing that out @hiergiltdiestfu). I guess the idea could still work by putting it in a synchronous orbit around the sun, but that's not what the question asked.

Edit 2: What I was going for is a design that doesn't require any electronics or mechanics whatsoever, so there's nothing that can break. To detect the relatively small object passing in front of the Sun (or Moon for that matter), you would need specialized instruments (like a telescope), but that's true for receiving radio signals from the satellite as well.

• According to en.wikipedia.org/wiki/Sphere_of_influence_(astrodynamics) this orbit would be outside the Sphere Of Influence for Earth. – hiergiltdiestfu May 18 '16 at 20:00
• Interesting idea! Eclipsing the sun (or the moon for that matter) would require it to be really big, but that's just what solar sail concepts to leave the solar system have to address also. I suppose it could transit the sun or moon, and occasionally become large enough to make a distinctive shape in telescopes - like those photos of the ISS transiting the sun and moon. – uhoh May 19 '16 at 0:43
• @uhoh "occasionally become large enough", yeah, but see my edit... – mb21 May 19 '16 at 8:10
• Yep, I see what you mean I think. If you need both a telescope and an accurate prediction to see it, it would be more interesting to a small group of entheusiasts, but inaccessible to most people, but maybe eclipsing the sun or moon is far too intrusive. I have a feeling that just drifting across the night sky similar to other bright satellites but more slowly, maybe changing brightness or color through passive means might be a good ballance. – uhoh May 19 '16 at 9:37

The biggest problem I suspect is not the orbital mechanics, but rather the power. We don't have any method for generating power that would last that long, solar panels will lose power eventually. One could presumably come up with such a system, but it wouldn't be easy.

Radiation is another significant problem. I suspect anything involving electronics would be worn away with time given such conditions.

As for the orbit, I would put it in a sun synchronous orbit with 7 orbits per day, at 5172 km high. Or even lower would be fine, I think anything above, say, 1300 km or so would work for 10,000 years.

• I've hinted at mechanical conversion of solar heating directly into mechanical energy - for example winding a spring like a mechanical self-winding watch. It could spin a generator once a year if electricity is needed. A passive LC circuit could make a radio "ping" without semiconductors at all, if that's necessary. – uhoh May 18 '16 at 15:20
• What about an RTG using a long half-life substance? – Jerard Puckett May 18 '16 at 15:27
• That would be a very low power system. Batteries we have today wouldn't work. The spring idea, well, it might work, I'm not sure. Hmmm... – PearsonArtPhoto May 18 '16 at 15:32
• @JerardPuckett if it could be used to make energy that could be stored in ways that can last 10,000 years - use its heat to wind a spring? Without a working fluid you'd need a motor that can run for 10,000 years continuously, or an RTG that makes a lot of power all the time and used occasionally - which would be nastily radioactive. – uhoh May 18 '16 at 16:22
• Springs stretch/break/become brittle. A spring wouldn't last for 10,000 years. – Stack Tracer May 21 '16 at 20:31

Suppose we were make the satellite largely out of a radioactive isotope with a suitable half-life. For instance molybdenum 93 which decays to the stable isotope niobium 93 with a half-life of 4000 years. Apart from easy detection with X-ray and maybe gamma-ray telescopes, it would remain relatively hot and so should be easily detectable in the IR. Making half the surface shiny and the other half black would add some time variation to the IR signature as well.

We could also use the decay as a source of timing. As the molybdenum decays, the power produced will fall, and so the satellite would cool. That cooling could bend a bimetallic strip and open a shutter which would allow sunlight (concentrated by a lens, perhaps) to set off a chemically powered flare or explosive.

With a flat firm solar sail can increase decrease the surface area facing the sun to gain speed. Angle and correct geometry in shape to keep orbit and position. Moving parts would also be a problem. Still working on the shape where it rotates with the sun with no moving parts.

Can a satellite work like a radiometer?

• No, a 10,000 year orbital lifetime is no problem with a high-enough orbit. So long as you don't make it look like a solar sail. – Mark Adler May 8 '18 at 5:13
• A solar sail on a lower orbit will increase drag. What orientation of the sail should gain speed without increasing drag? – Uwe May 8 '18 at 10:12
• it might look like a box kite for all I know – Muze May 8 '18 at 12:39
• A mechanism that turns the sail twice per orbit will be a failure point and very difficult to make reliable enough. In current satellites, the mechanical parts are often the first to fail (e.g. Voyager's scan platform) – Hobbes May 9 '18 at 9:40
• @Hobbes then there must be a nontraditional or not used before solid unchanging shape where a sail can just rotate in position while orbiting like a clock using solar vains? – Muze May 9 '18 at 16:09