# Astrophysics Question

Hello. I have, on occasion, seen footage of satellites being launched into space from the space shuttle. Almost always these are spinning. I assume the spinning is in order to facilitate stability however I am wondering how long these will continue to spin. In zero gravity will they continue to spin at the same RPM forever? Will they slow down on their own in time? (I assume that micro-dust particles might have an effect.) Thanks.

First, satellites orbiting Earth are not in zero gravity, otherwise there would be nothing holding them in orbit. They are in microgravity, where the lack of acceleration mimics the effects of true zero gravity. All of this has no impact on the rotation.

The only thing that could reduce the angular momentum of the satellite is a torque. On Earth, such torque is provided by friction, against the surface or against the atmosphere. In LEO, there is very little to provide friction. What is there is interplanetary dust and gas, various manmade debris particles, and maybe a little bit of radiation pressure. But the sum total of all of these effects on a spinning satellite weighing in at a ton or five is going to be very small indeed. So, they won’t spin at the same RPM forever, but it’s going to be a very very long time before there is any noticeable slowdown.

In normal terms, what is set spinning n space continues to spin indefinitely – the effects of atmospheric drag, friction, etc., that slow spinning things ‘normally’ (i.e., on Earth with gravity and atmosphere effects) will apply.

In more practical terms, objects in LEO (low earth orbit, ‘just outside the atmosphere’) are actually in the “exosphere”, the ultrathin outermost layer of atmosphere, and will slow and reduce spin, eventually losing altitude and burning up in the atmosphere (the largest objects, like Skylab, falling to Earth).

Things farther out will in the very long term suffer from tidal drag, gravitational pull (mostly Earth, Moon, and Sun) etc., and the spin will slow, very gradually, measured in multiples of millennia.

That’s a very generalized answer, without any mathematical precision, addressing effects. Essentially the answer is No, in general, but with Yes being true under two circumstances – low orbit or greatly extended time span for the question.

There are two reasons to spin a satellite. One is spin stabilization during launch vehicle operation, which reduces the amount of off-trajectory deviation to a specified coning angle and reduce the offset mass effect of slosh of liquid propellant or slag. The other is to induce a “barbeque roll” that evenly distributes solar radiation across the body of the satellite to prevent it from becoming excessively or unequally heated, and provide long term gyroscopic stability.

For spin stabilization, this is typically done with solid motors that remain attached after action time. Because solids will continue to provide a small amount of residual thrust after the primary burning “action time” due to burning up any remaining slivers of propellant (tail-off) and outgassing of the elastomeric liner from heat being emitted by molten aluminum residue from the propellant (slag), it is often desired to hang onto the motor after action time in order to prevent it from running into the satellite or space vehicle. The preferred solution from a strictly controls point of view is to thrust terminate the motor; that is, either fire retro motors that push it back away from the satellite after separation, or blow open the forward end so that it can’t provide net thrust. However, this can produce undesirable debris and residue that might damage the space vehicle or interfere with its sensors, and so the motor is often retained until the thrust is low enough that it won’t run into the space vehicle. When this is done, it is necessary to keep the vehicle from being pushed off course by the residual thrust. The low and uneven level of thrust makes normal thrust vectoring control (TVC) from the nozzle (either by a gimballed nozzle or a fixed nozzle with liquid injection TVC) unusable for control, and thus requires either an external attitude control system (ACS) or restricting how much the thrust can push the vehicle off course by constantly reorienting the off-axis thrust (spinning). Much of this is due to the unequal distribution of slag mass, so spinning also helps to keep that material pinned to the side and smeared somewhat evenly around the base. This type of maneuver is generally done about the minimum axis of rotational inertia, and once the satellite is fully deployed, some kind of retro-roll motor or a flywheel is used to slow or stop the roll motion as it will not be stable long term and may interfere with the other functions of the vehicle like observation or communication.

The barbeque roll maneuver, on the other hand, is typically done about the maximum axis of rotational inertia (either naturally or the space vehicle will deploy arms or tethered weights). The roll rate for this is slower and is intended to provide some degree of gyroscopic stability against any tumbling motions, and to maintain the satellite in thermal equilibrium. For instance, on the Apollo capsules, a slow (nearly imperceivable) roll was intermittently done in order to keep the lightly insulated capsule from overheating in the constant sunlight.

Once you start an object spinning in vacuum of space where there is essentially no drag to slow it down, it will keep spinning practically forever until you induce torque (via a reaction wheel or roll control thrusters). Even a slight amount of unintended and uncontrolled roll can be a significant control problem.

Just a pedantic point of note, but this isn’t actually an astrophysics question. Astrophysics is the study of the natural physical behavior of stars and other celestial bodies like planets, galaxies, et cetera. This query is more of a space vehicle dynamics and orbital mechanics question.

Damn, Stranger, that’s impressive. You in the biz or something?