Contracts-Excusable Non-Performance

Objective Impossibility
No person on earth could perform due to unforeseen and unforeseeable factors arising after formation. (Subjectively and objectively impossible; subjectively and objectively unforeseeable.)

Impracticability
1. Unexpected contingency subsequent to K.
2. risk not allocated by K or custom of marketplace
3. consequence of contingency=cost of performance grossly (objectively) commercially unreasonable.

UCC Version of Impracticability
Where there is casualty to identified goods without fault (including negligence) of either party, contract is avoided entirely (both parties discharged) OR, at election of buyer, where loss is partial, buyer can accept goods "as is" with due allowance from the contract price.
Where casualty is not to goods, but to mode of delivery or payment, commercially acceptable substitute, where available, must be tendered and accepted.

Frustration of purpose
Subsequent, unforeseen and unforeseeable circumstance renders value of other party's performance totally or nearly totally destroyed.