Friction In Space: The Statistical Mechanics of a Certain Missing Quora Question

I spent a good chunk of this evening around eight o'clock answer a question on Quora. Quora is a source of a host of bizarre and interesting questions. After answering the question with the most clarity I could, I thought I posted it, but it was sucked into a vacuum, never to be found through any amount of indexing or searching.

The question was, "Is there friction in space?"

I really liked the question, so much that I am devoting a blog entry to it.

The first task is to reconstruct my original answer, which goes something like:

Yes, there is friction in space because space is not empty. There is about 1 hydrogen atom per cubic centimeter of space, more or less.

When a hydrogen atom collides with an object there is an exchange of kinetic energy that obeys the rules of conservation of momentum. Part of this energy is radiated away as heat, as photons whose wavelength, frequency or color is distributed according to the energy associated with the collision. Part of this energy results in a change in direction, however slight of the two parties involved in the collision. The destiny of collision can be represented as a pair vectors with a direction and magnitude.

There is also a 'relative wind' effect. The ensemble of hydrogen atoms that the object collides with has an average velocity that can be moving along with object (running in nautical terms), against the object (upwind), crosswind, or some combination. This relative wind will affect the spectrum of the photons radiated by the travel of the object, red-shifting it when the object is running, blue-shifting it upwind or some combination for the crosswind.

When an object reenters the atmosphere the density of the gases it encounters increases rapidly and heat is dissipated as the, 'searing heat of reentry'.

If the object was initially launched from earth, it must dissipate as heat, the kinetic energy associated with putting it in orbit in the first place. Fire on the way out, fire on the way back.

Curiously only about a tenth of the energy of placing an object in orbit has to do with it achieving the height of its orbit. The other nine-tenths is expended in achieving the velocity of its orbit. This is why if you look at the exhaust plume of a rocket that it gently curves. Early in the launch it is desirable to reduce aerodynamic drag by going straight up. Later in flight the rocket has to tilt towards the horizon to achieve orbital velocity parallel to the surface of the earth at its orbital altitude.

Image Credit

There is a term, "Max Q" which translates "Maximum Dynamic Pressure" on the vehicle that you may have heard in connection with rocket launches. As a rocket ascends vertically two things are happening - the atmosphere is rapidly getting thinner, that is the density is decreasing rapidly, and the speed of the rocket with respect to that atmosphere is increasing rapidly.

There comes a point at which the product of these two quantities reaches its maximum and the structural forces on the rocket are at their maximum. This is the Max Q point. Q stands for dynamic pressure which the half the product of velocity squared times density.

The aft end of the rocket has a characteristic spectrum or heat signature and the friction between the surface of the rocket and the atmosphere can heat it considerably.

Because of staging, much of the energy associated with delivering the spacecraft to orbit is lost to the stages that got it there. The spacecraft only retains the kinetic energy associated with its own mass, and the potential energy associated with its mass and altitude. Typically only 1 to 3% of the mass of the original rocket, the 'throw weight' makes it to orbit. But there is still a lot of energy that has to be radiated as heat when it returns, thus, 'the searing heat of reentry'.

Because of aerodynamic drag, or the 'friction' of the original question, everything is destined to reenter after some period of time.

There is a rule in physics that any time a charged particle is accelerated, it is obligated to emit a photon corresponding to the energy that caused the acceleration.

This means that spacecraft flying through the ionosphere (and nearly all of them are since it extends to 1000 miles above the surface of the earth) are causing emissions of thermal radiation corresponding to the kinetic energy of the collisions or 'friction' occurring at their specific altitude and velocity.