# What pushes a spacecraft during a flyby?

xkcd today posted the following graphic:

Of these, the only one that I really don't understand is "What pushes a spacecraft during a flyby?" I don't understand why this is an issue. How energy is passed during a flyby is well known, however, this question implies there is a mystery. Is there a mystery surrounding some phantom force during flybys, or is this just a reference to how flybys seem mysterious?

• There is a wiki, explainxkcd.com/wiki/index.php/1547. It's usually my first stop when there is something on xkcd that I don't understand. – Taemyr Jul 7 '15 at 8:27
• xkcd must not be cited without hover text. – phresnel Jul 7 '15 at 9:04

Whilst the main factors involved in transferring energy to space craft during a flyby are well known, I believe this is a reference to several observed anomalies that have occurred to various space probes, covering everything from the early Pioneer probes to the much more recent Rosetta probe.

The researchers looked at five deep-space probes — Galileo to Jupiter, the NEAR mission to the asteroid Eros, the Rosetta probe to a comet, Cassini to Saturn, and the MESSENGER craft to Mercury. Each spacecraft flew past the our planet to either gain or lose orbital energy in their quests to reach their eventual targets. (Galileo made two flybys.)

In five of the six flybys, the scientists have confirmed anomalies.

• That's kind of spooky. I like to think we know most everything about how stuff moves in space, but we don't! – Thane Brimhall Jul 6 '15 at 18:13
• @ThaneBrimhall Yeah. Been trying to remember where I first heard it. I thought it might have been in this documentary (worth a watch if you get the chance), but that covers the Voyagers and the presenter, Dallas Campbell, confirmed it wasn't in that. I think it must have been covered at least once by New Scientist – James Thorpe Jul 6 '15 at 18:17
• These effects are at about the $10^{-6}$ level. There are many ways to be slightly off at that level, so any excitement about "new" physics should be tempered. We have seen this sort of thing come and go many times (remember faster-than-light neutrinos?). So now to increase the excitement, I will henceforth name this "Dark Gravity"! Now watch everyone jump on it. – Mark Adler Jul 6 '15 at 18:35
• @MarkAdler quite a catchy name, please start a plot theory about that! – o0'. Jul 7 '15 at 10:07
• For reference, Here's the New Scientist article I must have read some time ago... – James Thorpe Jul 7 '15 at 12:09

Michael Minovitch worked out in the early 1960s that a spacecraft which flies past a planet orbiting the Sun will gain or lose speed. It gains speed if it flies behind the planet and it loses speed if it flies in front. Gravity is responsible, combined with the planet's momentum as it moves around the Sun. The planet loses momentum, slows down, as the spacecraft speeds up. The planet gains momentum, speeds up, as the spacecraft slows down. Thus nature balances its books. It's not really complicated, though folks have done their best to make it that way.

• Welcome David! I hope you can contribute here with your deep historical knowledge of space flight. Note that this question is not about swingbys in general, but rather the observation of slight differences in the velocities attained from several actual swingbys, as compared to what was expected using the best navigation and gravity field information. – Mark Adler Jul 17 '15 at 0:14

I think your assumption is correct, and this is just a reference to how flybys seem mysterious.

Example of a gravity assist around Jupiter, wherein velocity is added. (Courtesy NASA JPL)

To anyone without a reasonable knowledge of orbital mechanics, upon hearing about spacecraft accelerating during a gravity assist, will presume that it was 'pushed' by some force.

We do of course know that it is in fact the gravitational pull of a planet that imparts the force. However to the layperson, a push comes close enough to describe the phenomenon.

• The problem is, the author of XKCD isn't really a layperson. Still... – PearsonArtPhoto Jul 6 '15 at 15:45
• @PearsonArtPhoto agreed, but these questions seem to be modelled around the mind of a layperson. – Vedant Chandra Jul 6 '15 at 16:04
• Not really. A third of the questions require some really specific stuff, and only a small portion are even in the realm of a layperson. – PearsonArtPhoto Jul 6 '15 at 16:47
• typically Heat from the RTG pushes the probe, try laying that on your average layman. If gravity was the answer to that question, it would be in the chart. – njzk2 Jul 6 '15 at 17:03
• The actual question in the comic is "What pushes spacecraft slightly during flybys?". If this was just a reference to how gravity assists work, the word "slightly" does not seem to make sense. Additionally, the other questions are not asked at the level of a person who does not understand gravity assists. This is much more likely a reference to the flyby anomaly, where several spacecraft experienced unexplained speed increases during Earth flybys. – neelsg Jul 7 '15 at 8:03