instantiate object

Im trying to instantiate an object from my character however i have an error :Cannot implicitly convert type UnityEngine.Object' toUnityEngine.Rigidbody'. An explicit conversion exists (are you missing a cast?

using UnityEngine; using System.Collections;

public class Trajectory : MonoBehaviour 
{
    public Rigidbody projectile;
    void Update() 
    {
    if (Input.GetKeyDown("space"))  {       
            Rigidbody clone;
            **clone = Instantiate(projectile, transform.position, transform.rotation);**
            clone.velocity = transform.TransformDirection(Vector3.forward * 10);
            Debug.Log("Hello");
        }
    }
}