astrojuanlu
commented
5 years ago Hi @TimothySHamilton, thanks for using poliastro and sorry for keeping you up late at night! I confirm I could reproduce both issues (which are in fact four, keep on reading) in my computer.
1. Propagator mean_motion hangs for some r, v vectors around Earth
Quick workaround: do
from poliastro.twobody.propagation import kepler
orbit2=orbit1.propagate(1.*u.h, method=kepler)This will change the default propagation method from mean_motion to kepler, which is also analytic and similar in nature.
The rest of the details are in #475.
2. Orbit propagate "flips vectors around Moon"
I put "flips" between quotes because, as you noticed, this isn't just a sign change.
Quick workaround: do
from poliastro.twobody.propagation import cowell
orbit_moon_enter.propagate(0 * u.h, method=cowell)The cowell propagator is a numerical one and does not suffer from this "flipping" issue. It just will be a bit slower.
I didn't have time to properly debug this issue yet, but I'm sure it's related to the hyperbolic branch of mean_motion, because this particular orbit has eccentricity of 1.00149526. When I have a proper analysis, I will open a separate issue.
Apart from the issue you experienced, while investigating it I also discovered #476 and #477.
TimothySHamilton
🐞 Problem Orbit propagate hangs for some r, v vectors around Earth; flips vectors around Moon
When I propagate a orbit with certain initial r & v vectors around the Earth, the calculation hangs and will not let me stop it except by closing the terminal window. It seems to happen for velocity vectors that are at low speed (maybe <1 km/s or so) and pointing generally inward towards the Earth. For example, the following causes it to hang:
On the other hand, putting similar r, v vectors around the Moon doesn't hang, but propagation turns them in different directions. For example, given this r and v,
and defining an orbit around the moon:
gives
Now propagating it by time=0:
returns different vectors for r and v:
Interestingly, they have the same magnitude as the ones put into the orbit, but they point in different directions.
A couple of plots will make it a little clearer. I am using the propagation method to do patched conics for a physics classroom exercise to simulate the Apollo moon flights. I've plotted the Earth in green and the Moon in gray (small circle). The Moon's sphere of influence is the large gray circle. The spacecraft starts out in low earth orbit and moves out in a highly elliptical path (blue), where it enters the Moon's sphere of influence. I have transformed the final r and v into the Moon's frame (the Moon is not moving--I'm putting it at a fixed point), used them to define the new orbit around the Moon, and propatated it to pericynthion (red trajectory).
The first plot shows the entire scenario, and the second plot shows a close-up of the region around the Moon. Note that it's not a simple case of flipping the sign.
Is there a hack I can do myself to fix this temporarily?
🖥 Please paste the output of following commands
conda info -aconda listpip freeze | grep astropyastropy==3.0.1 pytest-astropy==0.2.1pip freeze | grep poliastropoliastro==0.11.0python -c "import poliastro.testing; poliastro.testing.test()"🎯 Goal
💡 Possible solutions
📋 Steps to solve the problem
Addresses #<put issue number here>