How to simulate the trajectory of movement along an ellipse under gravity?

Question

Physics

There are two bodies, one has a huge mass, the other has a single one. There is a code:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class force : MonoBehaviour {

public Vector2 force1;
public Rigidbody2D myRigidbody;
public Rigidbody2D myRigidbody2;
public float f12;
public GameObject myObject1;
public GameObject myObject2;
public int nObject;

public float inputX;
public float inputY;
public float inputX2;
public float inputY2;

public float y;

// Use this for initialization
void Start () {

  if (nObject == 1) {
myObject1 = GameObject.Find ("Sphere1");
myObject2 = GameObject.Find ("Sphere2");
myRigidbody = myObject1.GetComponent<Rigidbody2D> ();
myRigidbody2 = myObject2.GetComponent<Rigidbody2D> ();
 }

 if (nObject == 2) {
myObject1 = GameObject.Find ("Sphere2");
myObject2 = GameObject.Find ("Sphere1");
myRigidbody= myObject1.GetComponent<Rigidbody2D> ();
myRigidbody2= myObject2.GetComponent<Rigidbody2D> ();
}
}

// Update is called once per frame
void Update () {

}

void FixedUpdate()
{

 if (nObject == 1) {
Vector2 ve = new Vector2 ();
ve.y = y;
ve.x = 0;
myRigidbody.AddForce (ve, ForceMode2D.Impulse);
}
Vector2 input = new Vector2 ();
input = myObject1.transform.position;
inputX = input.x;
inputY = input.y;

input = myObject2.transform.position;
inputX2 = 0;
inputY2 = 0;

float r12 = Mathf.Sqrt (Mathf.Pow (inputX2 - inputX, 2) + Mathf.Pow (inputY2 - inputY, 2));
float cosq12 = (inputX2 - inputX) / (r12);
float sinq12 = (inputY2 - inputY) / (r12);
float G = 7 * Mathf.Pow(10,-1);
f12 = (G * myRigidbody.mass *  myRigidbody2.mass) * (Mathf.Pow (r12, -2));
force1 = new Vector2 ();
force1.x = f12 * cosq12;
force1.y = f12 * sinq12;
myRigidbody.AddForce(force1,ForceMode2D.Force);

}
}

If you do not assign ForceMode2D.Impulse to a small body, then it will simply be attracted to a larger one (which, in theory, should be). But I need to get the rotation around the larger ellipse. So, if I apply momentary force, the ball starts moving along an unknown trajectory. That is, first it rotates in one direction, then in the other, then it is attracted, and then flies away.
I apologize for this code right away) I think there are two options: a problem in physics or a problem in the methods that I use. But I googled and read the documentation :(