Why does a satellite with a higher mass fall slower?

Question

I was doing simulations in GMAT and I could observe that if I increase the mass of the satellite, the satellite falls slower ... and if I reduce the mass of the satellite, then it falls faster (I only changed the mass, the drag area, the initial altitude, etc, everything stayed the same). What is this all about?

Answer 1

That's a great software-based experiment!

What is this about?

It's about drag and Newton's 2nd law of motion!

$$F = \frac{dp}{dt} = ma$$

but in the context of orbital mechanics.

We can re-arrange Newton's law as $a = F_D/m$ where $F_D$ is the drag force, and the drag equation is

$$F_D = \frac{1}{2} \rho v^2 C_D A$$

where $\rho$ is the density at that altitude, $v$ is the speed, $C_D$ is the drag coefficient of the spacecraft's shape (usually between 0.5 and 1.0) and $A$ is the cross-sectional area of the spacecraft viewed from the direction of motion.

Put the two together and you get

$$a = \frac{1}{m}\frac{dp}{dt} = \frac{1}{2m} \rho v^2 C_D A$$

note: If you re-run your simulation in GEO where there is essentially zero atmospheric drag because $\rho$ out there is extremely close to zero, you'll see that you can drop the mass even further without detecting much loss of altitude. But with very very low mass, if you have photon pressure turned on in your program then sunlight could start to do strange things to your orbit, because you've effectively built a solar sail.

Other effects like gravitational perturbations from the Sun and the Moon will always be present, but those accelerations are independent of mass of the spacecraft because it cancels out (i.e. drop a feather and a hammer on the moon and they accelerate the same).

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If this were a linear problem, you'd stop here and say that the deceleration was inversely proportional to the mass.

But in orbit things work counterintuitively. While you'd think that you lose linear momentum $dp/dt$ you actually speed up as you lose altitude and drop deeper into Earth's gravitational well.

You can read further at: https://www.grc.nasa.gov/WWW/K-12/rocket/newton2r.html

Answer 2

From Force=Mass*Acceleration for a given starting drag force (from the constant drag area, altitude and velocity) increasing the mass will reduce the acceleration (deceleration in this case) slowing the satellite and causing the perihelion to lower.

So being heavier does not directly make the satellite fall slower, but does change how quickly atmospheric drag reduces the orbit.

Answer 3

This is not always the case. Suppose the satellite with the higher mass has, say, the form of a sphere, and the lighter one that of a long massive spear with a sharp point gradually extending to the small diameter of the spear, which has a higher mass at the front than at the back. On entering the atmosphere, the spherical satellite will experience more friction than the spear-shaped satellite.
The spear-shaped satellite will, due to the friction forces, adjust its orientation, so it is heading to earth vertically, thereby experiencing less drag force. Therefore it will arrive on earth before the lighter satellite.
Of course, it also depends on the ratio of both masses, but if the mass of the spear-shaped is not too much lower than the spherical one, it will surely arrive on earth faster. Which means falling faster.