How to detect distance of a Game Object from an instantiated Game Object once

I am relatively new to Unity and right now I am having a problem with detecting distances between objects. I'm trying to make an explosion radius that will damage the player. I have the missile drop from the sky and when it hits the ground, it instantiates an explosion particle effect. So far, I've managed to figure out the distance between a game object and an instantiated one, but it repeatedly detects it so it instantly kills the player. I wan't to detect the distance of my player to the explosion once, then, if another explosion happens, it will detect it again. Here is the code for the detection:

         playerHit = GameObject.Find("Ground").GetComponent<RocketExplode>().playerHit;
         missile = GameObject.Find("Ground").GetComponent<RocketExplode>().explosionMissileCopy1;
         distance = Vector3.Distance(this.transform.position, missile.transform.position);
         if (distance <= 3 && playerHit == false)
         {
             healthloseamt = 50;
             rb.AddExplosionForce(300, gameObject.transform.position, 2);
             isHit = true;
             playerHit = true;
         }

code for instantiating the explosion:

         if (other.gameObject.CompareTag("MissilePlay1"))
         {
             Destroy(other.gameObject);
             Instantiate(explosionMissile1, other.transform.position, explosionMissile.transform.rotation);
             explosionMissileCopy1= Instantiate(explosionMissile1, other.transform.position, explosionMissile.transform.rotation);
             playerHit = false;
         }

Thanks in advance for your help!