7 ms·

Gravity assist maneuvers in space

106 points by uncertainquark 6y ago

chupa-chups 6y ago
Nice article. For even more information regarding gravity assists (and related effects like the Oberth effect) I found these articles in my comfort zone between too much detail and too superficial: https://aapt.scitation.org/doi/full/10.1119/1.2341882 https://aapt.scitation.org/doi/full/10.1119/1.2341882 - sorry, just learnt this is not an open access article https://aapt.scitation.org/doi/full/10.1119/1.5126818 https://aapt.scitation.org/doi/full/10.1119/1.5126818 (Oberth Effect)
- codeduck 6y ago
  Love the reference to Kerbal space program as orbital mechanics software :)
- TeMPOraL 6y ago
  > sorry, just learnt this is not an open access article There is no such thing as "not open access article". https://sci-hub.se/https://doi.org/10.1119/1.2341882 https://sci-hub.se/https://doi.org/10.1119/1.2341882 (With a big thank you to Alexandra Elbakyan.)
ilogik 6y ago
relevant scott manley https://www.youtube.com/watch?v=sF8roW6Cxm0 https://www.youtube.com/watch?v=sF8roW6Cxm0
- ashtonkem 6y ago
  KSP taught me more about orbital maneuvers than I expected when I bought it.
Ididntdothis 6y ago
I find it hard to get my head around gravity assists. Whatever energy you may be gaining from approaching a planet should be lost once you leave the planet. What am i missing?
- chipsa 6y ago
  Momentum exchange between the spacecraft and the planet. Spacecraft weighs next to nothing compared to the planet, so the planet's velocity change is imperceptible.
- Sharlin 6y ago
  In the reference frame comoving with the planet the probe leaves with exactly as much energy as it arrived (modulo aerobraking and such). In the reference frame of the solar system's center of mass, the planet will have transferred an imperceptible fraction of its orbital energy to or from the probe.
- sp332 6y ago
  You're moving much faster when you're leaving. So you spend less time decelerating due to gravity pulling you back. Net increase in speed.
  - simonh 6y ago
    This is the way of thinking about it that works best for me. As you fall into the gravity well of the planet from behind it, the planet is moving in the same direction as you. That means you spend more time in its gravity well catching up to it than if it was stationary. As you pass the planet you have picked up as speed boost from its pull. But as you travel away from it, because you are going faster you spend less time in its influence, so the deceleration is less than the acceleration you picked up on the approach.
    - mattr47 6y ago
      As I try to wrap my brain around this could I assume then in order for this to work, on approach to the planet you have to be going in the same direction as its orbit?
      - loeg 6y ago
        Theoretically no, but if you're going the opposite direction the approach velocity is vastly higher and thus you need a (1) very massive planet and (2) much lower periapsis to have meaningful gravitational slingshot effect. In practice, between atmospheres (the big planets being Jupiter, Saturn, and Neptune), and (for non-gaseous planets) planets not being perfectly smooth spheres, that probably decreases the tolerances below any acceptable level in this solar system.
rkagerer 6y ago
Rosetta would be in Mars’ shadow, having to rely on the limited battery supply instead of solar panels. The risk was that the batteries weren’t designed for the task. Huh? This implies a change in mission parameters or doubt whether the engineering was adequate. The link in that sentence doesn't mention either. Just that the craft entered a "sleep" mode to conserve power. I expect the power subsystem was designed specifically for the task.
- sq_ 6y ago
  Interestingly, that's apparently not quite correct. From the ESA link in the article: > Rosetta's original trajectory and engineering design did not include an eclipse, but unavoidable launch delays forced the trajectory to be replanned. Mission controllers working on Rosetta have spent months carefully planning and testing a low-power configuration which will allow the spacecraft to safely operate on batteries.
  - rkagerer 6y ago
    That explains it. I missed that bit in the link, thanks for correcting me!
    - sq_ 6y ago
      Figured it was something like that :) No way everyone can click through every link in every article they read. Glad we were jointly able to bring something interesting from the article to people's attention.
caycep 6y ago
I am just amused at all the gravity assist loop de loops portrayed in The Expanse. That seems to take just 10 minutes at a time... At any rate I suppose computers in the future can calculate these multiples of assist trajectories on the fly...
- kamilner 6y ago
  The executive producer actually wrote a post after the episode in Season 2 as a bit of an explanation/apology for how it ended up that way, which I thought was a nice touch: http://www.danielabraham.com/2017/04/04/guest-post-losing-science-drama-finding-drama-science/ http://www.danielabraham.com/2017/04/04/guest-post-losing-sc...
davidkuhta 6y ago
For fans of the "Rich Purnell Maneuver" from The Martian, check out the NASA paper: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150019662.pdf https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/201500...

Nice article. For even more information regarding gravity assists (and related effects like the Oberth effect) I found these articles in my comfort zone between too much detail and too superficial: https://aapt.scitation.org/doi/full/10.1119/1.2341882 https://aapt.scitation.org/doi/full/10.1119/1.2341882 - sorry, just learnt this is not an open access article https://aapt.scitation.org/doi/full/10.1119/1.5126818 https://aapt.scitation.org/doi/full/10.1119/1.5126818 (Oberth Effect)

codeduck 6y ago
Love the reference to Kerbal space program as orbital mechanics software :)
TeMPOraL 6y ago
> sorry, just learnt this is not an open access article There is no such thing as "not open access article". https://sci-hub.se/https://doi.org/10.1119/1.2341882 https://sci-hub.se/https://doi.org/10.1119/1.2341882 (With a big thank you to Alexandra Elbakyan.)

ilogik 6y ago

relevant scott manley https://www.youtube.com/watch?v=sF8roW6Cxm0 https://www.youtube.com/watch?v=sF8roW6Cxm0

ashtonkem 6y ago
KSP taught me more about orbital maneuvers than I expected when I bought it.

Ididntdothis 6y ago

I find it hard to get my head around gravity assists. Whatever energy you may be gaining from approaching a planet should be lost once you leave the planet. What am i missing?

chipsa 6y ago
Momentum exchange between the spacecraft and the planet. Spacecraft weighs next to nothing compared to the planet, so the planet's velocity change is imperceptible.
Sharlin 6y ago
In the reference frame comoving with the planet the probe leaves with exactly as much energy as it arrived (modulo aerobraking and such). In the reference frame of the solar system's center of mass, the planet will have transferred an imperceptible fraction of its orbital energy to or from the probe.
sp332 6y ago
You're moving much faster when you're leaving. So you spend less time decelerating due to gravity pulling you back. Net increase in speed.
- simonh 6y ago
  This is the way of thinking about it that works best for me. As you fall into the gravity well of the planet from behind it, the planet is moving in the same direction as you. That means you spend more time in its gravity well catching up to it than if it was stationary. As you pass the planet you have picked up as speed boost from its pull. But as you travel away from it, because you are going faster you spend less time in its influence, so the deceleration is less than the acceleration you picked up on the approach.
  - mattr47 6y ago
    As I try to wrap my brain around this could I assume then in order for this to work, on approach to the planet you have to be going in the same direction as its orbit?
    - loeg 6y ago
      Theoretically no, but if you're going the opposite direction the approach velocity is vastly higher and thus you need a (1) very massive planet and (2) much lower periapsis to have meaningful gravitational slingshot effect. In practice, between atmospheres (the big planets being Jupiter, Saturn, and Neptune), and (for non-gaseous planets) planets not being perfectly smooth spheres, that probably decreases the tolerances below any acceptable level in this solar system.

rkagerer 6y ago

Rosetta would be in Mars’ shadow, having to rely on the limited battery supply instead of solar panels. The risk was that the batteries weren’t designed for the task. Huh? This implies a change in mission parameters or doubt whether the engineering was adequate. The link in that sentence doesn't mention either. Just that the craft entered a "sleep" mode to conserve power. I expect the power subsystem was designed specifically for the task.

sq_ 6y ago
Interestingly, that's apparently not quite correct. From the ESA link in the article: > Rosetta's original trajectory and engineering design did not include an eclipse, but unavoidable launch delays forced the trajectory to be replanned. Mission controllers working on Rosetta have spent months carefully planning and testing a low-power configuration which will allow the spacecraft to safely operate on batteries.
- rkagerer 6y ago
  That explains it. I missed that bit in the link, thanks for correcting me!
  - sq_ 6y ago
    Figured it was something like that :) No way everyone can click through every link in every article they read. Glad we were jointly able to bring something interesting from the article to people's attention.

caycep 6y ago

I am just amused at all the gravity assist loop de loops portrayed in The Expanse. That seems to take just 10 minutes at a time... At any rate I suppose computers in the future can calculate these multiples of assist trajectories on the fly...

kamilner 6y ago
The executive producer actually wrote a post after the episode in Season 2 as a bit of an explanation/apology for how it ended up that way, which I thought was a nice touch: http://www.danielabraham.com/2017/04/04/guest-post-losing-science-drama-finding-drama-science/ http://www.danielabraham.com/2017/04/04/guest-post-losing-sc...

davidkuhta 6y ago

For fans of the "Rich Purnell Maneuver" from The Martian, check out the NASA paper: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150019662.pdf https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/201500...