Why the bad science Cecil (Planetary movement)

[Threadkiller]

Cecil,

In today’s column you mention:

…Discouraged? Don’t give up so fast. One idea out there is to change the orbit of a good-sized asteroid or comet — about 100 kilometers in diameter — so that it swings by one of the inner planets and then back out to, say, Jupiter. As the cosmic tow truck passes our target planet, it uses its gravitational attraction to tug the planet in the desired direction, then hurtles back to slingshot around Jupiter, picking up more energy in the process. Repeat a few thousand times with a few score asteroids and there you go: Mars in your backyard…[bolding mine]

In an earlier mailbag column (I would post the URL but the SD is suffering from a dead link)–I can however get a discussion on said column we learn that the ‘gravity-assist maneuver’ does not impart greater energy only a change of direction.

So is this wrong or was you? Or is it me–my kids are getting closer to being teenagers so I am getting used to being wrong.

[ftg]

What? Gravity assist is called that precisely because it assists in increasing the speed of the spacecraft. This is not an obscure fact.

The spacecraft is bouncing off the planet’s gravitational field. Instead of a reflection it’s refraction. Depending on orientation, one can add or lose speed. (With the planet then losing or adding speed, respectively.) Guess which one is usually chosen?

It’s not rocket science.

[Una_Persson]

I don’t have time to explain in detail myself, but here is one reference Cecil used which might help explain the process.

Korycansky, D.G. et al. “Astronomical Engineering: A Strategy for Modifying Planetary Orbits” Astrophysics and Space Science 275 (2001): 349–366.

[ZenBeam]

If you are on the planet being used for the gravitational shift, then in that frame of reference, the maneuver only changes the direction, not the energy. From the frame of reference of the Sun, the maneuver does change the energy of the vehicle or asteroid. In the column, Cecil is talking about changing orbits of planets, so a Sun centered frame of reference is the most relevant one. So it’s accurate to say that the asteroids or comets pick up energy from Jupiter.

[Stranger_On_A_Train]

@Threadkiller

In an earlier mailbag column (I would post the URL but the SD is suffering from a dead link)–I can however get a discussion on said column we learn that the ‘gravity-assist maneuver’ does not impart greater energy only a change of direction.

I’m not certain where you get this, but although the overall (kinetic + potential) energy of the planet-spacecraft system is conserved (unless additional propulsive thrust is applied by/on the spacecraft) the energy of the individual players almost certainly changes. In the typical swing-by maneuver the spacecraft picks up more kinetic energy as well as the change in momentum (direction and speed), and the planet loses the equivalent amount, the net effect of which is that the planet falls into an (immeasurably lower) orbit. A clear example of this is the Pioneer 10 and 11 and Voyager probes which have attained sufficient energy to achieve escape orbit, thereby taking their little bit of energy from the respective planets and moons that they performed swing-by maneuvers upon.

In general, any gravitational interaction between two bodies results in energy transfer via force interaction; that is, each body exerts a force upon the other, and the resulting change in energy as measured from some inertial reference frame is proportional to the square of that differences in speed. It is possible to have momentum change without a change in energy, but that would be a very specific case of exact symmetry, and would be virtually impossible with moving bodies.

Stranger