His poster art for the film 2001 is probably his most widely known work, but he also did many, many pieces for and in support of the NASA space program.
The piece of his I'm personally most familiar with is "Icarus Triumphant", a mural McCall did for the main branch of my local Glendale, AZ library. There doesn't seem to be a full image of the mural online, but here's a detail from the painting:
Among the comments to the news at collectSPACE.com was a note from "KSCartist" who shared some advice McCall had given in correspondence. Worth sharing, and it's advice McCall clearly followed in his own career:
To Achieve Success:
1) Evaluate your talents honestly
2) Set your goals realistically
3) Work tirelessly at your art and love every minute of the work.
4) Study the great art of the past
5) Come back from inevitable failures and diappointments with courage
6) Work relentlessly