Detecting collisions in Solar System (openGL) - c++

Question

i'm working on a project in wich i need to develop a solar system in openGL, then i need to add a rocket and launch it into another planet detecting the collision.
This is the code where i generate the planet, the rocket and the destination:
GLfloat dirCoheteX1 = 0.0f;
GLfloat dirCoheteY1 = 0.0f;
GLfloat dirCoheteZ1 = 0.0f;
GLfloat posx = 80.0f;
GLfloat posy = 0.0f;
GLfloat posz = 0.0f;
//Planet
glPushMatrix();
//Cambiamos el angulo para que tenga una órbita diferente.
glRotatef(angulo+160,0.0f,1.0f,0.0f);
glTranslatef(70.0f,0.0f,0.0f);
glRotatef(angulo+15,0.0f,1.0f,0.0f);
glRotatef(270,1,0,0);
//Carga de texturas
glBindTexture(GL_TEXTURE_2D,texturaTerra);
GLUquadricObj* q=gluNewQuadric();
gluQuadricDrawStyle(q, GLU_FILL);
gluQuadricNormals(q, GLU_SMOOTH);
gluQuadricOrientation(q, GLU_OUTSIDE);
gluQuadricTexture(q, GL_TRUE);
gluSphere(q,10,20,20);
gluDeleteQuadric(q);
glRotatef(-270,1,0,0);
//rocket
glPushMatrix();
glTranslatef(dirCoheteX1+0.0f,dirCoheteY1+0.0f,dirCoheteZ1+0.0f);
glRotatef(anguloY,0.0f,1.0f,0.0f);
glLineWidth(2.5);
glBegin(GL_LINES);
glVertex3f(0.0, 0.0, 0.0);
glVertex3f(15, 0, 0);
glEnd();
glPopMatrix();
//Target
glPushMatrix();
//Cambiamos el angulo para que tenga una órbita diferente.
glRotatef(angulo+240,0.0f,1.0f,0.0f);
glTranslatef(posx,posy,posz);
glRotatef(angulo+15,0.0f,1.0f,0.0f);
glBindTexture(GL_TEXTURE_2D,texturaSaturno);
GLUquadricObj* q5=gluNewQuadric();
gluQuadricDrawStyle(q5, GLU_FILL);
gluQuadricNormals(q5, GLU_SMOOTH);
gluQuadricOrientation(q5, GLU_OUTSIDE);
gluQuadricTexture(q5, GL_TRUE);
gluSphere(q5,5,20,20);
gluDeleteQuadric(q5);
glPushMatrix();
The planets are generated on X axis and they rotate around the Sun on Y axis.
Right now i'm adding the rocket's X position to move it from the planet and rotating it in it's Y axis to head it to the planet, but i don't know how to detect the collision correctly.
I'm using this algorithm:
dirCoheteX1+=0.05f; //move the rocket
anguloY+=0.05f; //rotate the rocket
dx = posx- dirCoheteX1;
dy = posy- dirCoheteY1;
dz = posz- dirCoheteZ1 ;
distance = sqrt(dx*dx + dy*dy + dz*dz);
if (distance < (1.0)) {
contact = true;
}
But the collision is being detected before it actually the rocket reaches the target.