A Russian rocket suffered a serious failure as it blasted off Wednesday carrying an unmanned cargo ship bound for the International Space Station, and pieces of the supply vehicle reportedly came crashing back to Earth.

This was the first attempt to send a spacecraft to the space station since NASA retired its space shuttles last month. The mishap was an unwelcome surprise for the space agency, which now depends on Russian rockets to carry up not just cargo but also American astronauts.

At first, everything seemed to be going fine as the Russian rocket blasted off from the former Soviet republic of Kazakhstan into a cloudless sky at 10 a.m. But 5 minutes, 50 seconds into the flight, controllers at Mission Control Moscow lost contact with the vehicle.

News reports from Russia described an explosion and pieces falling to the ground in Siberia.

“It’s a challenging process right after an anomaly, because everybody wants answers, and really what you need is time to collect the data and sort it out,” Mike Suffredini, NASA’s program manager for the space station, said in a hastily arranged press briefing for reporters.

He said the cargo ship had been loaded with nearly 3 tons of food, fuel and supplies. But NASA’s final shuttle mission last month stocked up the orbiting outpost, so there’s no danger of running out of anything critical anytime soon.

“We can go several months without a resupply vehicle, if that becomes necessary,” Suffredini said.

The rocket that failed is similar to the one Russia uses to carry up people, so the investigation could delay a launch scheduled for next month to take up two cosmonauts and an American astronaut.

The three station crew members they were supposed to replace may stay in orbit longer. Or, they may come home using a Russian spacecraft already parked at the station, and the outpost would get by with half its normal crew.

“Operating at a three-crew size is something that we are familiar with and able to do, although it would have implications on our ability to perform research,” Suffredini said.

The incident highlights how, with the shuttles gone, NASA is dependent on Russia for human spaceflight.

“I think critics of the dependence on Russia will seize on this as an example of what might happen,” said John Logsdon, a space policy expert with George Washington University in Washington, D.C. “But accidents do happen in the space business; they happen to everybody.”

Logsdon points out that Russia has had a long, excellent record of dozens of successful flights to the station for over a decade.

“But there have been a couple of recent Russian failures that are a little troubling” and raise questions about the state of quality control in the Russian space industry, he said, noting that just last week a problem with a different Russian rocket put a major communications satellite in the wrong orbit.

Robotic supply vehicles built by space agencies in Europe and Japan can take cargo up to the station, but at the moment, Russian rockets are the only way to carry up people.

Several U. S. commercial companies are developing capsules that could potentially serve as space taxis for NASA astronauts in a few years.