15

Space probes like Voyager 1, 2, New Horizons, etc, traveled beyond those gas giants, how did they cope up with their extreme gravity?

How was the trajectory of these probes unhindered by the immense gravity of those giant gas bodies? How was the trajectory decided?

Is it because of the small size of the space probes?

Rodia
  • 135
  • 6
Paran
  • 965
  • 1
  • 9
  • 14
  • 3
    A trajectory that is not influenced by the immense gravity of those giant gas bodies is impossible. Very careful planning and a lot of numerical simulations allow to fly a trajectory that is acclerated by a swing by maneuver. Bo it does not depend on the size of the space probes. Probes of gigantic size would modify the orbit of the gas giants itself, but they are impossible to build and launch. Earth is a small pebble when compared with those gas giants. A probe of 10 % the Earth's mass is still too small. – Uwe Jun 11 '18 at 11:58
  • 2
    Math; the part that makes rocket science actually difficult. What they didn't know was if the circuitry would survive the immense radiation, and if it'd make it through the asteroid belt unscathed. – Mazura Jun 11 '18 at 23:13

2 Answers2

50

The trajectory was not only "unhindered" - it was enhanced!

Knowing mass of the planet you can calculate very precisely how the trajectory of a probe flying by will be affected. You modify the trajectory on arrival in such a way, that the departure trajectory will be exactly as desired. And due to some rather unintuitive physics caveats, you can make it so that the speed of the probe (relative to the Sun) at departure can be much higher than on arrival. This is called "gravity assist" or "Slingshot maneuver" and allows for some quite huge fuel savings. Voyagers performed good few gravity assists on their mission, and they are leaving the solar system faster than any other probe.

SF.
  • 54,970
  • 12
  • 174
  • 343
  • 5
    Unintuitive? When a slow-flying tennis ball hits a running locomotive, it bumps with much more speed. (Analogy by Randall Munroe). – kubanczyk Jun 11 '18 at 13:28
  • 11
    @kubanczyk: That's a very simplified analogy, which tends to be confusing because bumping against the front of the car is quite dissimilar of the gravitational swing behind the planet. – SF. Jun 11 '18 at 13:56
  • Having in mind all the basic physic laws, shouldn't it also alter the path, orbit and the speed of the planet? If so, by how much Voyagers have changed the speed of other planets by 'stealing' speed from them? – kukis Jun 11 '18 at 18:51
  • 2
    @kukis: Absolutely yes. Throwing tennis balls here on Earth also affects the path, orbit and speed of the planet, and the effect is about the same size. Planets are big. You can certainly do the math to work out how much energy was "stolen" by doing the integral of force applied over distance; that's the work, which has units of energy. The work that Jupiter did on Voyager results in a very, very small retrograde acceleration of Jupiter, and hence a very, very small lowering of its orbit. – Eric Lippert Jun 11 '18 at 19:13
  • 3
    @kukis: Law of preservation of momentum. Mass of planet * change of speed of planet = mass of spacecraft * change of speed of spacecraft. Gravity assist benefit is capped at half of planet's orbital speed, Jupiter's 13 km/s so at best 6.5km/s (likely much less). (721.9 kg * 6500m/s) / 1.9e27kg = 2.47e-21 m/s. That's 0.78 angstrom / millennium, – SF. Jun 11 '18 at 19:18
  • 3
    @SF: http://www.planetary.org/blogs/guest-blogs/2013/20130926-gravity-assist.html: "during the Voyager encounters with Jupiter in 1979, Jupiter slowed down by roughly 10 to the -24th power kilometers per second", which is 1e-21m/s, so your approximation is definitely in the right ballpark! – Eric Lippert Jun 11 '18 at 19:25
  • @SF. - Gravity assist delta v is capped at circular orbital velocity just above the top of the planet's atmosphere, which in the case of Jupiter amounts to a $\Delta v$ of about 41.8 km/s - i.e., more than three times Jupiter's orbital speed about the Sun. – David Hammen Jun 11 '18 at 19:54
  • @DavidHammen: Let's imagine a small neutron star orbiting on far outskirts around a giant star. The neutron star's orbital velocity is 1m/s, it takes millions of years to complete one revolution around the giant star. Meanwhile, you can orbit just above its surface at 1% c. How the heck would a probe coming from any direction gain 1%c through gravity assist against it? – SF. Jun 11 '18 at 21:10
  • 1
    @kubanczyk, I'm guessing that what SF meant by unintuitive is that all things being equal, when a planet applies an attractive force to the probe on the way in to speed it up and then uses the same attractive force on the way out to slow it down, why would the probe's velocity change at all? – Tracy Cramer Jun 11 '18 at 23:21
  • 1
    @TracyCramer: The probe's speed (scalar; velocity is vector and changes direction) relative to the planet remains unchanged, but speed relative to the star changes. Imagine you could just swing 180 degrees around the planet. (unrealistic, but hey, example.) You arrive slowly, head-on. Planet moves at $v_{pl}$; Your speed relative to the star is $v_{s0}$, your speed relative to the planet is $v_{p0} = v_{pl} + v_{s0}$ You swing around and leave at same $v_{p1} = v_{pl} + v_{s0}$ in opposite direction, but now relative to the star you're moving at $v_{p1}+v_{pl} = 2 v_{pl} + v_{s0}$! – SF. Jun 11 '18 at 23:37
  • @SF., so is what I said what you meant by "unintuitive physics caveats" or was it something else? – Tracy Cramer Jun 11 '18 at 23:52
  • @TracyCramer: Yes, it is; also the arrival and departure trajectory that result in optimal assist are non-obvious. Add to this departure towards a specific target, and possible inclination change and it gets quite tricky! – SF. Jun 12 '18 at 00:02
25

They did not !

This is the trajectory of Voyager 1 at Jupiter.

enter image description here credits wikipedia

mattdm
  • 103
  • 2
Antzi
  • 12,640
  • 2
  • 46
  • 75
  • if the trajectory was pre planned to provide gravity assist then the trajectory remains unhindered – Paran Jun 11 '18 at 10:32
  • 16
    @qwerty if by unhindered you mean “followed the intended path” yes. If by unhindered you mean that trajectory was unchanged then you are wrong. See the image: the deflection is significant – Antzi Jun 11 '18 at 10:40
  • 2
    @Antzi To be fair, the deflection was only significant because we shot the probe at the planet. If we just launched the probe "between the planets" the deflection would be much less pronounced (but I bet it would still tug over time!) – corsiKa Jun 11 '18 at 15:34
  • 7
    Note that this hyperbolic orbit is from the perspective of Jupiter, and not from the perspective of the sun. From the perspective of Jupiter, as you can see, Voyager's orbit is perfectly symmetrical. From the perspective of the Sun though, Voyager is being accelerated at a huge rate in the prograde Jupiter orbit direction at the close approach, and so there is a large acceleration from the perspective of the Sun. – Eric Lippert Jun 11 '18 at 19:27
  • 1
    @EricLippert yes, but it still looks like broken ellipses. I should add the graph too – Antzi Jun 11 '18 at 23:40
  • 1
    @Antzi - They're broken hyperbola (better: hyperbolic segments), not broken ellipses. The graph is fine. – David Hammen Jun 12 '18 at 06:44