Sergei Kushnarov (Сергей Кушнаров) is a Russian Cosmonaut. His career appeared to be dedicated to proving the existence of wormholes. He was celebrating his birthday on an international space station with two other Russian astronauts while awaiting an expected greeting from Phineas Flynn and Ferb Fletcher.
However, when Phineas and Ferb were set to send out the greeting, an RC helicopter with a Mindy Mimmick doll taped to it put together by Candace in an attempt to get Linda to see their contraption crash-landed into the microphone receiver. The recording of Candace saying, "Turn around", which were broadcasted to the astronauts brought to their attention that an asteroid was on a collision course with their satellite.
The evasive action they took sent them into a wormhole. Nothing is known about them after that, other than that "somewhere beyond Jupiter, and beyond time" an aged Sergei on what we could assume was his death bed received his birthday greeting from an alien Ferb fetus ("Mommy Can You Hear Me?").