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Helios
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Would a spaceship that is half reflective ( the stern ) and half flat-black ( the bow) be propelled through space just by the ambient radiation from stars and the CMBR?
The black side will heat up and radiate.Helios said:Would a spaceship that is half reflective ( the stern ) and half flat-black ( the bow) be propelled through space just by the ambient radiation from stars and the CMBR?
Solar sails work fine with particles coming from one direction. But the OP seems to ask about approximately isotropic radiation from all sides. It seems to me, that converting such ambient radiation continuously into a directed movement of a ship would reduce entropy, and thus violate the laws of thermodynamics.Ryan_m_b said:Technically yes, you're essentially describing a solar sail.
A.T. said:The black side will heat up and radiate.
Very little, and not in the direction you think! Details follow.Helios said:Would a spaceship that is half reflective ( the stern ) and half flat-black ( the bow) be propelled through space just by the ambient radiation from stars and the CMBR?
Heat up? It will cool down. In fact, the black side will radiate a lot until it cools down to T0=2.72548 K, the CMBR temperature. There will be no net thrust if the black side cools to the CMBR temperature. Ignoring thrust from starlight, the only way to get thrust is if the black side has a temperature above that of the CMBR.A.T. said:The black side will heat up and radiate.
Sorry, bad choice of words. I meant that you cannot prevent it from radiating, by cooling it down to 0K.D H said:Heat up? It will cool down.
There are two problems with your line of thinking.Nugatory said:Right. Of course if we were willing to supply power to an on-board refrigeration system, we could transfer heat to the shiny side to maintain the thrust.
I assume that is just the initial thrust. As soon it starts moving, the incoming radiation stops being isotropic due to Doppler effect and aberration.D H said:The net thrust will be [itex]\frac P c[/itex], in the direction of the reflective side.
P/c is an incredibly small value. Even at Mr. Fusion levels (1.21 gigawatts), P/c amounts to a whopping 4 Newtons. Presumably the spacecraft won't consume power at that rate. There are no garbage cans with which one could replenish Mr. Fusion in those voids in space. We need to be miserly with our power consumption to cross that void.A.T. said:I assume that is just the initial thrust.
That exists, but it's even smaller than the minuscule acceleration described above. The Doppler shift is a rather small effect for a vehicle moving much, much slower than the speed of light. For smallish velocities (v<<c), this will result in a very, very small drag term that grows linearly with velocity.As soon it starts moving, the incoming radiation stops being isotropic due to Doppler effect and aberration.
A.T. said:Solar sails work fine with particles coming from one direction. But the OP seems to ask about approximately isotropic radiation from all sides. It seems to me, that converting such ambient radiation continuously into a directed movement of a ship would reduce entropy, and thus violate the laws of thermodynamics.
That's not applicable here because radiation pressure most certainly is a real effect. Just because the rotation of the Crooke's radiometer does not result from radiation pressure does not mean that radiation pressure doesn't exist. It most certainly does. It is the basis for solar sails, and this concept has been successfully demonstrated with the IKAROS mission.256bits said:This is the wiki on Crookes radiometer, which may be of interest.
http://en.wikipedia.org/wiki/Crookes_radiometer
A spaceship drive by light, also known as a light sail or photon sail, is a propulsion system that uses the momentum of photons from a light source to propel a spacecraft forward. This technology is based on the principles of radiation pressure, where photons exert a force on an object when they reflect or absorb off of its surface.
A spaceship drive by light works by using a large reflective sail to capture and reflect photons from a light source, such as a laser or the sun. As the photons reflect off the sail, they transfer their momentum to the spacecraft, propelling it forward. The sail can be adjusted to change the direction and speed of the spacecraft.
One of the main advantages of a spaceship drive by light is its potential for high speeds. Unlike traditional rocket propulsion, which requires fuel and can only go so fast, a light sail can continue to accelerate as long as it has a light source. Additionally, this technology is more efficient and environmentally friendly, as it does not produce any exhaust or emissions.
While the concept of a spaceship drive by light is promising, there are still some challenges and limitations that need to be addressed. One major challenge is the development of a powerful and reliable light source that can provide enough photons to propel a spacecraft. Another limitation is the need for a very large and lightweight sail, which can be difficult to manufacture and deploy in space.
Yes, there have been several successful demonstrations of light sail technology in space, including the Japanese IKAROS and the Planetary Society's LightSail 2. These missions have shown the potential of light sail propulsion and have paved the way for future space exploration using this technology.