Now that we are able to detect when a UFO is destroyed, it would be more realistic to make them explode, and I think you're ready to develop this by yourself.
Create one explosion whenever a spaceship is destroyed by using the file explosion.png that you will find in the Resources folder.
You will need to go back a few pages to achieve this challenge because in this case, we're going to create a CCParticleSystemModeRadius particle.
First of all, add explosion.png to the project and then add the following lines inside the method detectCollisions, after [_ufosToRemove addObject:ufo];:
CCParticleExplosion *explosion = [CCParticleExplosion node];
explosion.texture = [CCTexture textureWithFile:@"explosion.png"];
explosion.emitterMode = CCParticleSystemModeRadius;
explosion.startSize = 100.0;
explosion.startRadius = 20.0;
explosion.endSize = 1.0;
explosion.endRadius = 100.0;
explosion.totalParticles...