How can a satellite’s speed decrease without its angular momentum changing?

Answer 1

What WERE you thinking of ?! :

If it's in a circular orbit around a spherically symmetric mass distribution, it CAN'T decrease its speed while at the same time keping its original angular momentum!

In fact, the satellite's speed ' v ' in a circular orbit is proportional to 1 /sqrt(R), while its angular momentum (= m v R) is proportional to sqrt(R).

Thus (angular momentum) x (orbital speed) = constant!

So the satellite's speed decreases as R increases and therefore as the angular momentum increases.

If you change from an originally circular orbit to a non-circular (that is, elliptical) orbit without changing the angular momentum, the satellite's speed will be alternatively smaller than, or larger than, what it was in the originally circular orbit. That speed will be least at ap-planet (the greatest distance from the planet), and greatest at peri-planet (the closest it gets to the planet).

"And that's the way it is."

Live long and prosper.

Answer 2

The only way to decrease the satellite's speed is to either

1) increase the radius
2) increase its mass

Since increasing its mass doesn't make realistic sense, the only sensible thing to do is increase the radius... that is, increase the distance the satellite is from the thing that it's orbiting.

Answer 3

Change the radius of its orbit.