Use bitlocker or diskcryptor with 30 characters (combintion of digits, lower case, upper case, space and special characters).
But I don't know whether a government agent or NASA could break it up or not.
~Rude Boy
Bitlocker is a no. If one is encrypt data on a flash drive, instead of just hiding it, he/she probably has a really good reason to do so. Bitlocker is decent, and it's main problem is the integration with Windows.
http://blog.alertsec.com/2010/01/breaking-into-bitlocker/OP I would highly recommend that you do the procedure right each time (mounting and safely demounting your flash drive).
Read:
http://www.techworld.com/news/security/bitlocker-pgp-truecrypt-encryption-weakened-by-new-attack-tool-3418189/I'm not sure if this is true or not, as you'd have to do some research yourself. Although I'd rather be safe than sorry.