On Tuesday, December 19, 2006, I witnessed a delightful event: the Space Shuttle Discovery and the International Space Station traveling together through the western sky, only about 1° apart.
Around 6:34 p.m., I spotted a -1 magnitude International Space Station (ISS) traveling NE above the western horizon. It quickly became apparent that there was a +1 magnitude point of light moving right along with the ISS, leading it by about one degree. It was the Space Shuttle Discovery, which had undocked from the ISS just 2h25m earlier (4:09 p.m.)!
I quickly surmised that Discovery must have fired retrorockets to put some distance between it and the ISS by lowering Discovery‘s altitude. Since Discovery was at a lower altitude, it had been orbiting faster, which is why it was leading the ISS by about a degree. As the pair approached the constellation Lyra, further evidence of Discovery‘s lower altitude occurred when it disappeared into the shadow of the Earth several degrees further west of where the ISS disappeared a few seconds later.