Astronauts aboard the space shuttle Atlantis will have at least one extra day in orbit, after bad weather forced a delay of their return Friday.

Ground controllers at the U.S. space agency NASA canceled both landing opportunities for Friday at Kennedy Space Center in Florida.  The agency said the weather conditions, which include thunderstorms and low cloud ceilings, would not have permitted a safe landing.

NASA said there are four opportunities for landing on Saturday -- two at Kennedy Space Center (with the first opportunity at 9:16am EDT and two at Edwards Air Force Base in California.  The agency said there are several chances to land on Sunday as well.

The astronauts are returning home after a successful mission to the Hubble Space Telescope.

The shuttle's seven crew members made history Thursday when they became the first astronauts to testify before a congressional committee from space.  The crew appeared via satellite before a U.S. Senate panel considering NASA's budget.  President Barack Obama has requested nearly $18.7 billion from Congress for the 2010 space budget -- an increase of about five percent.

The astronauts lobbied the lawmakers for greater support for the space agency.

President Obama called the astronauts aboard the shuttle late Wednesday to talk to them about his nominee for the next NASA administrator, an announcement he is expected to make after the shuttle mission is over.  

He is expected to nominate retired Marine Major General Charles Bolden to the post.  Bolden is a former shuttle astronaut.

The astronauts blasted off May 11.  During their mission, the crew made five long and challenging spacewalks to repair and upgrade the 19-year-old orbital observatory.  The repairs are expected to keep Hubble operating for another five to 10 years.