After over three hours of making sure the hatch on the Soyuz spacecraft was properly sealed, astronaut Scott Kelly began his journey back to Earth at 7:02 p.m. Tuesday evening. Kelly and his colleagues’ separation from the International Space Station in the Soyuz craft marks the end of NASA Expedition 46 and the beginning of Expedition 47.

Meanwhile, at Northwestern University, plastic astronaut dolls and rocket ships were scattered around the Pancoe Auditorium, where Tang and cosmic brownies were served to celebrate the homecoming of Scott Kelly and his microbiota.

Hosting the celebration were Northwestern Professors Fred Turek and Martha Vitaterna, sporting NASA gear. In keeping with the space theme, videos about how to do stuff in space — like going to the bathroom, brushing your teeth and making a sandwich — were playing on the projector.

Vitaterna and Turek are leading of one of the 10 research teams selected by NASA to study Scott and his identical twin Mark. The Northwestern team is studying the microbiota of the gastrointestinal tract.

“Microbiota refers to the communities of microorganisms, mostly bacteria, that live in and on the human body,” according to Vitaterna.

She and Turek are looking for changes between Mark and Scott’s microbiota to determine the effects of long-term space travel on these microorganisms. They hope to discover if these changes will make astronauts sick or more susceptible to health issues.

NASA has never been able to do research quite like what they will do with the Kelly twins. Because they are identical, their genetic makeup is also identical. This will give NASA powerful data showing the effects of a long-term space expedition on the human body. Data like this will be immensely helpful in future missions that will require astronauts to spend extended periods of time in space, like a journey to Mars.

The team is waiting until they receive all of their samples to begin their research because they are also looking into the effects on the body after Scott’s return to Earth.

Earlier Tuesday, before handing the command of the International Space Station over to his colleague Tim Copra, astronaut Scott Kelly took a minute to thank everyone who has been part of Expedition 46.

“The one thing I need to recognize is all the people that have been on board with Mikhail [Kornienko] and I since we have been up here,” because they have all played key roles in their mission, he said.

Tim Copra, the new commander of Expedition 47, expressed his gratitude to Kelly during their farewell. “Thank you for your leadership, you’ve been such a great role model to us in every aspect. So we’re very, very thankful. Thank you.”

Tuesday evening marked the end of astronaut Scott Kelly’s 340-day mission on the International Space Station. Kelly landed in Kazakhstan about 11 p.m. Tuesday along with Kornienko and Sergey Volkov, who piloted the Soyuz capsule.

Contributing: Associated Press