Houston ? The international space station Wednesday marked five continuous years of people living and working aboard it.

But there wasn’t much time for celebration.

The station’s two residents spent the day cleaning air filters, upgrading exercise equipment and doing other maintenance. Astronaut William McArthur Jr. and cosmonaut Valery Tokarev, who arrived Oct. 3 for a six-month stay, also prepared for a spacewalk next week.

Former station astronaut Michael Fincke said there were handshakes, smiles and congratulatory e-mails at NASA’s Johnson Space Center, but work prevented too long a pause.

“It’s not that we’re not feeling celebratory,” he said. “The space shuttle isn’t flying right now. And we got to figure out how to finish flying the space shuttle in the next four years and to finish building the space station like we promised to.”

People first began living on the orbiting science lab on Nov. 2, 2000, after 16 countries joined to construct it.

NASA partnered with the Russian Federal Space Agency, the Canadian Space Agency, the European Space Agency and the Japan Aerospace Exploration Agency in building the station.