The Loose Ends Saga — Paul Levinson

Jeff Harris goes back in time to prevent the explosion of the space shuttle Challenger, but gets pulled into November 1963, and has 23 years to plan his intervention with the Challenger. He discovers that his actions in the past may result in the Soviet Union continuing in the 21st century. He strives with Laura and Karina to prevent this, and also the murder of John Lennon and the September 11 attacks, but the resilience and interconnections of history make it unlikely that they'll be able to stop all of those calamities, and the personal survival of at least one of them may be incompatible with their goals. The Saga contains Loose Ends - the novella nominated for Hugo, Nebula, and Sturgeon Awards - and its sequels Little Differences, Late Lessons, and Last Calls.