Recently, I found out Jeff Bell has hit a lot of the same ennui that I have, only he expressed it in an interview on The Space Show. He said many things that I agree with, and I hope he’s wrong about the things I don’t agree with. One item he brought up is that Orion, in its current form of using the European ATV module as its service module, doesn’t have enough delta-v to enter and exit Lunar orbit. He claims that this lack of delta-v is what brought about the Asteroid Rendezvous Mission. So I did a quick Wikipedia based comparison between the Apollo CSM and the Orion Capsule and Service Module:
CSM Delta-V Capability: 2,800m/sec
Orion Delta-V Capability: 1,500m/sec
Delta-V Performed on Apollo 8: 2086m/sec (from the Apollo 8 Flight Journal, and actually reflects the Lunar orbit insertion delta-v (921m/sec) and the contingency burn after the first orbit (1165m/sec))
Therefore, an Orion capsule could not recreate the Apollo 8 mission without enlarging the ATV module.