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Unless that was the mission.... to try and launch an un-trackable satellite.

nradov 9y ago
There's no possible way to build an un-trackable satellite that's large enough to make useful Earth observations. The basic limits of physics still apply. There's a difference between rocket science and magic.
- nekopa 9y ago
  Honest question: If we can make stealth planes, why not stealth sats?
  - jakeogh 9y ago
    You can try via low albedo in any required wavelength, but satellites still occult stars. Reorientation for optimal cross section is the obvious way, no idea how much that buys.
    - craftyguy 9y ago
      > but satellites still occult stars True, but it would be very difficult to 1) observe this unless you were looking for it directly and 2) determine the object's orbit. Since stars are point light sources, you can't even tell how fast it is moving since the size of the object could mislead your attempt to deduce this by timing the event.
      - jakeogh 9y ago
        You lost me on the timing. We are looking for small black rocks way past geostationary, and it's 100% at or inside that. Being a point source is what I want (since they all are). If it's in orbit, the speed bounds are already known, it's somewhere between 0 and as slow as practical, the lower the faster; faster is worse for an earth pointing platform. Once you know something is there, isn't that all you really need? Why does the speed matter for detection? An adversary can (presumably) almost immediately target that location if they are willing to admit they saw it. I don't feel like I'm saying anything non-obvious. It's way easier to hide in the atmosphere than outside of it. If Zuma made it, and it's not found by the people looking with the right tools, wow. But I doubt it. It's too hard to hide with standard assumptions. I would love to see the function for magnitude vs optical cross section, no doubt one can calculate that from first principals. It's just observable star density.
        craftyguy 9y ago
        You need to determine velocity and distance in order to determine if the thing you just saw blink out a star is in orbit around the Earth (or not). There's a lot of distance between the Earth and the star, and literally anything between your eye and the star could obscure it from your perspective. We aren't even close to detecting/tracking all of the objects in the solar system that could do that. I'm also assuming the thing is not reflective (per the comments earlier I was originally replying to). > An adversary can (presumably) almost immediately target that location if they are willing to admit they saw it You'd be late to the party. If it was in orbit around the Earth, and since you don't know its direction or distance from Earth, your impact object would have to do a lot of searching once it got into space. If you had a giant laser to destroy it and could fire instantly upon detection, it will have likely moved quite a bit in the fractions of a second that it took the light to reach it. And, again you don't know the direction it is moving.., you can't compensate.
        jakeogh 9y ago
        Ah, ya I was assuming if it blinked it is most likely in orbit around earth. Not sure how true that is, although far enough away similarly sized objects get diffracted around. I should try to math;) As for target I just meant light it up, properly diverging hp radar or laser pulse in an atmospheric window.
  - jpollock 9y ago
    Stealth planes aren't invisible. They are still visible to radar, infrared and mark-1 eyeball. The difference is that they attempt to make themselves look very small in the types of radar used to aim missiles. If the error bars on the location is large enough, you can't fire a missile and get a reliable kill. The term I keep hearing is the "kill chain", in that stealth is attempting to break one of the parts in the chain. https://en.wikipedia.org/wiki/Kill_chain https://en.wikipedia.org/wiki/Kill_chain
    - berberous 9y ago
      I thought that stealth planes had the radar cross-section of something like a marble. That was the first gen planes too, I think it might even be smaller now. I would have thought that's effectively close to invisible.
      - DuskStar 9y ago
        The problem is, radar returns decline with the 4th power of distance - so if you can detect a 1m square at 100km, you can detect a 1cm square at 10km and a 0.1mm square at 1km. Yeah, a marble is fantastic in comparison to a barn door - but that 4th power cuts both ways.
  - nradov 9y ago
    In order to observe Earth in a useful way the satellite needs large optics and/or large antennas. Changing the shape of those things or adding coatings to reduce radar reflectivity would make them ineffective as sensors. Satellites also need power. Solar panels are big and have to point at the sun so no way to hide those. RTGs are also an option, but even with those there's going to be significant waste heat which shows up on IR sensors against the cold background. And any large satellite will show up clearly on a regular optical telescope. Take a series of pictures over time and you can spot anything moving, either through direct imaging or because it occludes some other celestial object.
- rocqua 9y ago
  Based on earlier discussions, I thought it was really hard to see a satellite if you don't know the orbit. I did some calculations, and a 15m object (largest that fits in falcon 9) at 300km (lowest orbit) has an angular diameter of 0.006 degrees. That amounts to taking up about 0.000005% of the sky above the horizon. I figured the required resolution to cover the sky well enough to see that would be ridiculous, but 20 megapixels spread over the entire sky would then be enough that this satellite could occlude an entire pixel. Considering how many hobbyists there are, this does indeed seem feasible.
  - vlasev 9y ago
    The angular diameter of stars that are light-years away are really really small. Yet they are visible to the naked eye and cameras all the time. So it's not just about the angular diameter.
    - jacquesm 9y ago
      But they shine a lot of light in every direction, including directly at you, the observer. So even if they are small they compensate for that by their power output and the fact that their radiation is a near perfect sphere. A satellite trying hard to hide isn't going to be nearly as friendly as that for an observer.
      - planteen 9y ago
        How are you going to power said satellite? Those massive solar arrays are going to reflect emag across the spectrum (radar returns and solar returns).
  - nradov 9y ago
    You don't have to scan the entire sky. Based on knowing the the approximate launch trajectory, there are only a limited range of orbits where the satellite could possibly be. Satellites have very limited fuel and can only make small orbital changes.

There's no possible way to build an un-trackable satellite that's large enough to make useful Earth observations. The basic limits of physics still apply. There's a difference between rocket science and magic.

nekopa 9y ago
Honest question: If we can make stealth planes, why not stealth sats?
- jakeogh 9y ago
  You can try via low albedo in any required wavelength, but satellites still occult stars. Reorientation for optimal cross section is the obvious way, no idea how much that buys.
  - craftyguy 9y ago
    > but satellites still occult stars True, but it would be very difficult to 1) observe this unless you were looking for it directly and 2) determine the object's orbit. Since stars are point light sources, you can't even tell how fast it is moving since the size of the object could mislead your attempt to deduce this by timing the event.
    - jakeogh 9y ago
      You lost me on the timing. We are looking for small black rocks way past geostationary, and it's 100% at or inside that. Being a point source is what I want (since they all are). If it's in orbit, the speed bounds are already known, it's somewhere between 0 and as slow as practical, the lower the faster; faster is worse for an earth pointing platform. Once you know something is there, isn't that all you really need? Why does the speed matter for detection? An adversary can (presumably) almost immediately target that location if they are willing to admit they saw it. I don't feel like I'm saying anything non-obvious. It's way easier to hide in the atmosphere than outside of it. If Zuma made it, and it's not found by the people looking with the right tools, wow. But I doubt it. It's too hard to hide with standard assumptions. I would love to see the function for magnitude vs optical cross section, no doubt one can calculate that from first principals. It's just observable star density.
      - craftyguy 9y ago
        You need to determine velocity and distance in order to determine if the thing you just saw blink out a star is in orbit around the Earth (or not). There's a lot of distance between the Earth and the star, and literally anything between your eye and the star could obscure it from your perspective. We aren't even close to detecting/tracking all of the objects in the solar system that could do that. I'm also assuming the thing is not reflective (per the comments earlier I was originally replying to). > An adversary can (presumably) almost immediately target that location if they are willing to admit they saw it You'd be late to the party. If it was in orbit around the Earth, and since you don't know its direction or distance from Earth, your impact object would have to do a lot of searching once it got into space. If you had a giant laser to destroy it and could fire instantly upon detection, it will have likely moved quite a bit in the fractions of a second that it took the light to reach it. And, again you don't know the direction it is moving.., you can't compensate.
        jakeogh 9y ago
        Ah, ya I was assuming if it blinked it is most likely in orbit around earth. Not sure how true that is, although far enough away similarly sized objects get diffracted around. I should try to math;) As for target I just meant light it up, properly diverging hp radar or laser pulse in an atmospheric window.
- jpollock 9y ago
  Stealth planes aren't invisible. They are still visible to radar, infrared and mark-1 eyeball. The difference is that they attempt to make themselves look very small in the types of radar used to aim missiles. If the error bars on the location is large enough, you can't fire a missile and get a reliable kill. The term I keep hearing is the "kill chain", in that stealth is attempting to break one of the parts in the chain. https://en.wikipedia.org/wiki/Kill_chain https://en.wikipedia.org/wiki/Kill_chain
  - berberous 9y ago
    I thought that stealth planes had the radar cross-section of something like a marble. That was the first gen planes too, I think it might even be smaller now. I would have thought that's effectively close to invisible.
    - DuskStar 9y ago
      The problem is, radar returns decline with the 4th power of distance - so if you can detect a 1m square at 100km, you can detect a 1cm square at 10km and a 0.1mm square at 1km. Yeah, a marble is fantastic in comparison to a barn door - but that 4th power cuts both ways.
- nradov 9y ago
  In order to observe Earth in a useful way the satellite needs large optics and/or large antennas. Changing the shape of those things or adding coatings to reduce radar reflectivity would make them ineffective as sensors. Satellites also need power. Solar panels are big and have to point at the sun so no way to hide those. RTGs are also an option, but even with those there's going to be significant waste heat which shows up on IR sensors against the cold background. And any large satellite will show up clearly on a regular optical telescope. Take a series of pictures over time and you can spot anything moving, either through direct imaging or because it occludes some other celestial object.
rocqua 9y ago
Based on earlier discussions, I thought it was really hard to see a satellite if you don't know the orbit. I did some calculations, and a 15m object (largest that fits in falcon 9) at 300km (lowest orbit) has an angular diameter of 0.006 degrees. That amounts to taking up about 0.000005% of the sky above the horizon. I figured the required resolution to cover the sky well enough to see that would be ridiculous, but 20 megapixels spread over the entire sky would then be enough that this satellite could occlude an entire pixel. Considering how many hobbyists there are, this does indeed seem feasible.
- vlasev 9y ago
  The angular diameter of stars that are light-years away are really really small. Yet they are visible to the naked eye and cameras all the time. So it's not just about the angular diameter.
  - jacquesm 9y ago
    But they shine a lot of light in every direction, including directly at you, the observer. So even if they are small they compensate for that by their power output and the fact that their radiation is a near perfect sphere. A satellite trying hard to hide isn't going to be nearly as friendly as that for an observer.
    - planteen 9y ago
      How are you going to power said satellite? Those massive solar arrays are going to reflect emag across the spectrum (radar returns and solar returns).
- nradov 9y ago
  You don't have to scan the entire sky. Based on knowing the the approximate launch trajectory, there are only a limited range of orbits where the satellite could possibly be. Satellites have very limited fuel and can only make small orbital changes.