The Zarya control module went up Nov. 20.
Unity is the first piece delivered by space shuttle.
After returning from their 1969 trip to the moon, the Apollo 11 crewmen underwent a 21-day quarantine. From left to right are astronauts Neil Armstrong, Edwin Aldrin Jr. and Michael Collins greeting their wives. Quarantines can help a society protect itself from dangerous infections. The word comes from the French, for the 40 days of isolation once faced by new arrivals who may have been infected. Quarantining an entire planet against potentially harmful extraterrestrial diseases becomes more difficult.