What would have happened if Karna had been able to kill Arjuna?

What would have happened if Karna had been able to kill Arjuna? Arjuna got the same doubt and out of curiosity he asked the same question to lord Krishna. Lord krishna stated : Even if sun falls down from it’s place or the earth gets split up into 1000 fragments or the water in the ocean gets dried up or the fire gets cools down, Karna could never able to kill arjuna. Karna had two divine weapons which he safely preserved to use against arjuna. One is vasavi shakti and another one is nagapasa astra. Karna used vasavi Shakti against bhima's son ghatotkach as he seemed to be a threat against kauravas troops on the day 14th of war. When karna used nagapasa to kill arjuna, Krishna trickily lowers the chariot by putting his feet which ultimately leads the astra to take away arjuna's crown instead of arjuna’s head. Speaking frankly, it’s nearly impossible to kill arjuna by karna or by anyone as he was under the protection of lord krishna.