Exploration Mission 2 using the Space Launch System [wikipedia.org] was originally planned to launch using the Block 1B version, which includes the Exploration Upper Stage and can carry 105 metric tons (105,000 kg) to to low-Earth orbit. Now that Congress has given NASA additional money for a second SLS mobile launcher [soylentnews.org], the agency has the ability to fly astronauts on the smaller Block 1 version of SLS [theverge.com], capable of lifting 70 metric tons to LEO:
The SLS has been in development for the last decade, and when complete, it will be NASA's main rocket for taking astronauts to the Moon and Mars. NASA has long planned to debut the SLS with two crucial test missions. The first flight, called EM-1, will be uncrewed, and it will send the smallest planned version of the rocket on a three-week long trip around the Moon. Three years later, NASA plans to launch a bigger, more powerful version of the rocket around the Moon with a two-person crew — a mission called EM-2.
But now, NASA may delay that rocket upgrade and fly the same small version of the SLS for the crewed flight instead. If that happens, NASA would need to come up with a different type of mission for the crew to do since they won't be riding on the more powerful version of the vehicle. "If EM-2 flies that way, we would have to change the mission profile because we can't do what we could do if we had the [larger SLS]," Robert Lightfoot, NASA's acting administrator, said during a Congressional hearing yesterday [youtube.com] [2h15m22s video].
[...] Meanwhile, it's also possible that the second flight of the SLS won't carry crew at all. NASA also needs to launch its upcoming mission to Jupiter's moon Europa pretty soon. Known as Europa Clipper, the mission is mandated by Congress to fly on the SLS by 2022. Lightfoot mentioned that Europa Clipper could come before the first crewed flight of the SLS. It just depends on if the Orion crew capsule, which will carry astronauts on the SLS, is ready before Europa Clipper is ready. If the Europa spacecraft comes first, then it could also fly on the small Block 1 rocket.
The trans-lunar injection (TLI) payload capacity for SLS Block 1B is 39.2 metric tons, enough to carry a ~25.9 ton crewed Orion capsule [wikipedia.org] with an 8-10 ton component of the Lunar Orbital Platform-Gateway [wikipedia.org] (LOP-G), as was the original plan for Exploration Mission 2. Block 1 cannot accomplish these two tasks at the same time. Perhaps they should launch LOP-G using Falcon Heavy instead [soylentnews.org]?