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Mushroom-shaped space probes

A configuration featuring a large dish for both radio communication and concentrated solar power
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What: A space probe in the shape of a narrow cylinder, with a dish at one end, whose concave side faces the cylinder.

Why: The dish can be used for radio communication with Earth, and for concentrating sunlight onto solar cells. The dish can more easily be larger than ones used on past probes. If the probe is going far out from the Sun, where such a large dish is desirable, the Sun and Earth will be in approximately the same direction anyway, so it's convenient to use the same dish for both purposes. Using concentrated solar power instead of larger solar arrays can be expected to reduce launch mass and material cost—the dish can be thin metal or aluminized plastic film. The cylinder provides the place to mount the radio and the solar cells, as well as whatever other hardware the spacecraft needs, without obstructing much of the dish. (Typical on-axis dishes (like Cassini's) require support legs for the secondary reflector, which cause side lobes, reducing gain; this avoids that.) The dish can fit conveniently within a heat shield, if the probe is planned to enter an atmosphere. If convenient, the dish can be deployed, expanded (if already partially deployed), or uncovered (if hidden inside an aeroshell) only upon arrival at the probe's destination, because its power and communication requirements are probably minimal until then.

N/A [2019-04-05]*

*I came up with this concept previously, for my ice giants mission, but I only realized today that it's worth posting as its own idea, so what I'm saying is that I'm posting it the same day I realized I should.

notexactly, Apr 05 2019

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       In most cases the orientation of the vehicle is irrelevant to vehicle operations. Only when you have an engine on one side which is actively pushing the craft, or are in an atmosphere, does it matter.
Voice, Apr 05 2019
  

       Hang on. I'm not sure of the geometry here, but if the dish is focussing parallel solar rays to a point where the solar cells are, doesn't that mean the solar cells would be in the way of transmission? But maybe there's a way round that. [+]
MaxwellBuchanan, Apr 05 2019
  

       // In most cases the orientation of the vehicle is irrelevant to vehicle operations. Only when you have an engine on one side which is actively pushing the craft, or are in an atmosphere, does it matter. //   

       As long as your spacecraft only performs orientation-insensitive experiments (like scalar magnetometry or zero-g organism growing), has an orientation-insensitive power source (like an RTG or solar arrays on all sides), isn't going close enough to the Sun that sunlight on the radiators is an issue (or has louvered radiators on all sides), and is close enough to Earth or a relay station to communicate using an omnidirectional antenna, sure. All other spacecraft (i.e. any going beyond Earth orbit, and most of those staying here) need to care about their orientations. Interplanetary spacecraft use either spin stabilization or three-axis stabilization (or both at different times) to aim their high-gain antennas at Earth.   

       // Hang on. I'm not sure of the geometry here, but if the dish is focussing parallel solar rays to a point where the solar cells are, doesn't that mean the solar cells would be in the way of transmission? //   

       Yes. There are probably a lot of potential solutions. Some I've thought of are:
* A wavelength-selective reflector to separate the beams at the focal point of the main reflector< br> * A main reflector that somehow focuses the different wavelengths to widely separated foci
* Photovoltaic cells that are transparent to radio waves
  

       Regarding the first and last of those options, have you seen the silver blankets they put over the high-gain antennas on some spacecraft? I learned the other week that those are made of plastic coated in a thin layer of germanium. The germanium reflects light, keeping the dish cool, but allows radio waves through, because it's a thin enough semiconductor layer that the longer-wavelength radio waves don't excite its electrons.
notexactly, Apr 05 2019
  

       In before any mention of Trump.
Ian Tindale, Apr 05 2019
  

       Due to the inexorable effect of inverse-square law, by the time you're at your system's asteroid belt the incident solar energy has dropped to the point where it's not viable to harvest it.   

       From there on out, you have to deploy RTGs (so crude ! ) snd similar self-contained power sources.
8th of 7, Apr 05 2019
  

       That's the point of concentrating it. Do you deny the ongoing solar-powered operation of Juno at Jupiter, or that Jupiter is further out than the asteroid belt?
notexactly, Apr 08 2019
  

       Yes. If you look at the original imaging, you can just see the extension cable in the top right corner of the frame.   

       NASA photoshop it out on the published photos, of course. They do that on all sorts of stuff.
8th of 7, Apr 08 2019
  
      
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