Would backing up my private key be enough? Lets say I put my wallet's private key on a rocket and fire it towards mars.  In 50 years time I travel to mars for a cheap vacation and find my private key.  Would I then have full access to my wallet, including all the transactions over the past 50 years and the current balance (from when I landed on mars in person) etc?

In other words, to fully backup my wallet and be able to restore it with full control, do I only need to backup my private key?
That's exactly how private key works. All you need to have is a private key to access your wallet. And this is the reason it is not a good idea to fire your private key to mars, if someone get there before you, they will steal all your coins.