NASA's website says the spacewalk will last approximately seven hours. It starts at 6:30 a.m. EST. Viewers can see it on NASA TV.
McClain is a Spokane, Washington, native who was selected by NASA in 2013. According to her NASA biography, she is a West Point grad with a master's in aerospace engineering and international relations. McClain spent her pre-NASA career in the Army, reaching the rank of lieutenant colonel.
McClain is part of an expedition that launched to the ISS in December 2018. The expedition (PDF) will "test cryogenic fuel transfers, study how mixed gels form and release drugs in microgravity" and perform other scientific experiments.
McClain keeps an active Twitter feed where you can see pictures of the astronauts working out in zero gravity.
She also has an Earth plushie she likes to photograph.
Koch (who is also on Twitter), was selected by NASA in 2013. She is from Jacksonville, N.C., and has a master's in electrical engineering. She has worked at the South Pole and Greenland stations doing scientific research.
Koch just arrived at the ISS this month.