Why can’t photons, e.g. from a laser, be used as the thrust medium? Is it just that there is too much wasted heat in any known such setup?
In a standard rocket, the “thrust medium” must have mass, which photons are notable for lacking.
Photons have momentum, so they can produce thrust. That’s how solar sails work.
The momentum of a photon is its energy divided by the speed of light, so it would take a very powerful laser to produce even measurable thrust. Nevertheless, the concept of Photonic laser thrust has been considered.
No one cares about mass in the rocket equation - they only care about momentum.
Ok, I’m being somewhat glib but consider a rocket sitting in space
m[sub]0[/sub] = 0
Now throw a photon out the back with energy E
m[sub]1[/sub]=m[sub]ship[/sub]v[sub]ship[/sub]-E/c
By conservation of momentum m[sub]0[/sub]=m[sub]1[/sub]
so m[sub]ship[/sub]v[sub]ship[/sub]=E/c
Photons do have momentum, even if they don’t have mass; so you could (in theory) use them for propulsion. This is the basis for the so-called ‘photon drive’. Unfortunately the thrust you can get from a photon drive is fantastically small; three hundred megawatts (the output of a small power-station) will get you one newton of thrust.
A modest jet engine in Earth’s atmosphere can produce 27 000 newtons, so one newton-per-power-station-output is ridiculously feeble.
see
http://www.armaghplanet.com/blog/whatever-happened-to-photon-rockets.html
As this is General questions I need to correct this, while it is based on Newtonian physics and is a common understanding this is incorrect under current generally accepted understandings of physics. SkyLab was intentionally placed in an orbit which would decay, and orbits to not necessarily decay.
The Earth as an example is actually traveling in the straightest path possible, at a constant velocity through spacetime, but the Sun curves that path which results in an orbit. For the orbit to grow or to decay there would need to be a transfer of angular momentum which is conserved.
In some cases like the Earth Moon system the tidal forces actually increase the orbital distance over time as it also slows down the Earths rotation. In the case of Mars and Phobos the orbit is decaying as angular momentum is transferred to Mars making it spin faster.
The “Acceleration” perceived happening by an orbiting body is a “fictional force” or an apparent force that is due to a frame of reference, like “centrifugal force”
The Space Station or the moon is not being accelerated, it is following the geodesic, or shortest straight path through spacetime. It is actually you sitting on your chair that is being accelerated away from your shortest path through spacetime and that is what you feel as your weight. This actually means that your clock runs slightly slower, and it well tested.
It is harder to understand especially with how our Physics education is kind of stuck in the mid 1800’s so I don’t expect people to know this generally but as I have to follow the ideals behind this site I wanted to clarify.
Evidently I am no rocket scientist, and we can all be glad for that. Thanks for the enlightenment.
To answer the OP; to hit the sun you basically need to nullify the orbital velocity or you will just end up missing it.
Ceres: 17.9 km/s = 40,041 mph
Earth: 29.8 km/s = 66,660 mph
Venus: 35.0 km/s = 78,292 mph
Pluto: 4.7 km/s = 10,513 mph
Note that without using planets for gravity assist that it takes a lot more speed to hit the sun from Venus than from Pluto. Without gravity assist it is actually more energy efficient to send a spacecraft to the outer solar system first BTW.
For Ceres surface the escape velocity is 0.51 km/s and the sum of the two deltas is 17.9 km/s + 0.51 km/s = 18.41 km/s or ~ 41,181 mph.
So to hit the Sun from the surface of Ceres a projectile would need to be going 41,181 mph. As the average bullet is only going ~1,700 mph there is a lot of speed to make up to actually send something into the Sun.
(note all of the above math is back of envelope in quality, do not use for space travel)
We can debate (again) whether gravity is a force or a warping of space-time. In general, however, Newtonian physics is a very accurate approximation of Einstein physics, unless the speeds involved approach an appreciable fraction of c. Energy losses in an orbiting pair system, typically due to tidal effects, are a third-order detail in what is essentially a close-to-perpetual system. After all, the lensing effect of the sun’s gravity on light rays grazing its surface is about 1.75 seconds of an arc - not a lot at all.
Newtonian is useful, but not descriptive of fundamental properties. The issue was with the claim of a retrograde orbit being the norm is why I had to bring this up.
I agree that Newtonian physics is a very useful approximation, but it is not descriptive of the current best theories on the actual causative properties.
This is an important distinction here to show the limitations of Newtonian physics, which would assume that because there is “acceleration” that there is some type of energy being transferred to “accelerate” an orbiting body into a circular path. While almost is good enough for horseshoes and hand grenades, and interplanetary travel it does obfuscate the reality that there is no “work” being done in an orbit. The fact that there is no “acceleration” purely due to the fact that something is orbiting (ignoring tidal forces etc…) is very important. When purely considering the orbit one can assume the conservation of momentum is the primary driver in those interactions.
This is exactly the proper point to bring up the implications of GR, when improper assumptions are made when conflating the useful approximations of Newtonian physics with a causative nature.
It is exactly like centrifugal force, which is very useful if you are designing a Tilt-a-Whirl, but incorrect to use past that ‘pseudo’ force context.
To clarify, Skylab’s orbit decayed due to atmospheric drag, not due to any tidal or relativistic effects. These would be much too small to affect a satellite in earth orbit on human timescales.
Also, I wouldn’t say it was intentionally placed in an orbit that would decay. It was placed in a low earth orbit for practical reasons, mainly so that crew-carrying rockets could reach it easily. It was known that the orbit would decay due to atmospheric drag, but pre-launch predictions of how long it would take were not particularly accurate, mainly due to unpredicted factors such as solar activity which affect the density of the upper atmosphere. Late in Skylab’s life there were plans to use the Space Shuttle to boost its orbit to prevent reentry, but the Shuttle was not finished in time to save Skylab.
Ages ago when Bad Astronomer used to post here he said (at least I think it was him, may have been someone else) that the sun is the hardest thing to hit in the universe from the earth.
By that he meant the most energy expensive thing to get to. In theory once you have escape velocity from the earth (or solar system) you can coast the rest of the way to your destination even if it is 10 billion light years away.
To reach the sun will cost more energy than getting anywhere else in the solar system. Certainly doable though (in fact we plan to send our first probe to the sun in 2018).
Yes I have to balance between post length and readability, low earth orbit does have issues with drag, but pretty much anything placed in geosynchronous orbit will never decay within practical time lines.
There is no tidal interaction to remove momentum from the orbiting body and transfer it to the Earths rotation, lowering the orbit until frictional effects come into play.
Skylab was intentionally put in low Earth orbit mostly due to the costs and scientific needs. As low Earth orbits will decay, it did come down but it was intentionally put into that orbit.
Relevant Schlock Mercenary strip … they really don’t want that gravitational drive to fail.
Larry Niven wrote a story where the narrator ranted about journalists who described a payload “spiraling into the sun” after it was sufficiently accelerated so as to cancel out Earth’s orbital velocity. The title of the story was, appropriately, “Spirals.” I found a copy online by Googling, but I suspect it was a pirated version so I won’t link to it, but I enjoyed it.
Heinlein also wrote a book (can’t remember the title) where a character was testing the abilities of another character to pilot a spaceship, so he mentioned “spiraling into the sun”. The second character correctly responded that a spiral is not a possible orbit.
That was a line in “Starman Jones” where Sam is quizzing Max to see if he could really pass as a spacehand.
Yes! Starman Jones. Thank you. God, it’s been decades since I read that book, but that line is one of the few things I remember about it. Also the way that they had to input numbers into the navigation computer directly in binary, by clicking two buttons, one held in each hand.