# Adding Disk to Parametric Plot Animation

I have a simple animation of a spacecraft being gravitationally deflected by Mars and wanted to be able to represent Mars as a red disk, but am having trouble doing this. Here is the code (where the last animate is my attempt at adding a red disk to the animation):

m[2] = 6.4185*10^23;
r[2] = 3.3899*10^6;
G = 6.672*10^-11;
tmax2 = 2.5*86400
xr = 5.74143*10^8
yr = 4.64259*10^7
vxr = -3275.65
vyr = -227.858

soln = NDSolve[{
x''[t] == -((G  m[2] x[t])/(x[t]^2 + y[t]^2)^(3/2)),
y''[t] == -((G  m[2] y[t])/(x[t]^2 + y[t]^2)^(3/2)),

x[0] == xr, y[0] == yr, x'[0] == vxr, y'[0] == vyr}, {x[t],
y[t]}, {t, 0, tmax2} , MaxSteps -> 1000000,
Method -> "StiffnessSwitching"]

Show[ParametricPlot[Evaluate[{x[t], y[t]} /. soln], {t, 0, tmax2},
AxesLabel -> {x, y}, PlotStyle -> Automatic, PlotRange -> Full,
ImageSize -> Large], Graphics[{Red, Disk[{0, 0}, r[2]]}]]

Animate[ParametricPlot[{{x[t], y[t]}} /. soln /. t -> a, {t,
Max[0, a - 10000], a}, AxesLabel -> {x, y}, Axes -> False,
ImageSize -> Large], {a, 0, tmax2}, AnimationRate -> 1000]

Animate[Graphics[{Red,
Disk[{0, 0}, r[2]]}], [ParametricPlot[{{x[t], y[t]}} /. soln /.
t -> a, {t, Max[0, a - 10000], a}, AxesLabel -> {x, y},
Axes -> False, ImageSize -> Large], {a, 0, tmax2},
AnimationRate -> 1000]]


As can be seen below the disk shows up nicely in the "still" version, but I get a syntax error when trying to put the disk in the animated version. Any help would very much be appreciated, thanks very much.

• Look up Epilog and Prolog. Jun 9, 2015 at 12:33
• Also take a look at using Show to combine Graphics objects. You already used it for the still version but not for the animated one. But for a simple Disk object using Epilog is probably the best. Jun 9, 2015 at 13:01
• I tried many combinations of trying to get Show working with Animate and Graphics but could never get it to work for reasons unknown to me. I've had a look at Epilog and Prolog but there don't seem to be any examples using Animate and so I'm unfortunately still getting syntax errors no matter what I try. Syntax seems to be my problem here, as the tools are staring me right in the face (Show and Epilog), which is extremely frustrating Jun 9, 2015 at 13:08
• Some more tinkering got it to work, thanks very much guys. Jun 9, 2015 at 13:18
• "got it to work" - then, can you answer your own question please? Jun 9, 2015 at 13:52

Animate[ParametricPlot[{{x[t], y[t]}} /. soln /. t -> a, {t,