diff --git a/src/simulation.cpp b/src/simulation.cpp
index 9f152a4..eb68a68 100644
--- a/src/simulation.cpp
+++ b/src/simulation.cpp
@@ -1,4 +1,5 @@
 #include "simulation.h"
+#include <cassert>
 #include <cstdlib>
 #include <cstring>
 #include <cstdio>
@@ -129,7 +130,14 @@ static void compute_orbital_velocity_from_vis_viva(CelestialBody* body,
 
     double e = body->eccentricity;
     double a = body->semi_major_axis;
-    double speed = sqrt(G * parent->mass * (2.0 / distance - 1.0 / a));
+
+    // FIXME: there's a bug here
+    //  could be unit related, mass is in units of kg, and distance is meters
+    //  insteam of km, velocity is also m/s
+    //double speed = (double) sqrt(G * parent->mass * (2.0 / distance - 1.0 / a));
+    double v_squared = G * parent->mass * (2.0 / distance - 1.0 / a);
+    assert(v_squared >= 0);
+    double speed = (double) sqrt(v_squared);
 
     if (e > 0.01) {
         printf("  %s: eccentric orbit (e=%.2f, a=%.3e m), speed at r=%.3e m: %.3f km/s\n",
@@ -146,6 +154,7 @@ static void compute_orbital_velocity_from_vis_viva(CelestialBody* body,
 
     vel_dir = vec3_normalize(vel_dir);
     body->velocity = vec3_scale(vel_dir, speed);
+    // FIXME: this is wrong, we should be simulating in parent coord frame
     body->velocity = vec3_add(body->velocity, parent->velocity);
 }