The American National Aeronautics and Space Administration (Nasa) is attempting to regain control of the Near spacecraft, currently more than 200 million miles from Earth, by sending a series of command signals once every three hours.
"We're still trying to communicate with it," said Helen Worth, spokeswoman for the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. "We're still optimistic. We just have to find an avenue of communication."
The Near probe was launched in 1996 and has travelled about 1.5 billion miles on its indirect route to the Eros asteroid, which is about 240 million miles from Earth.
Nasa lost contact early yesterday morning after attempting to fire the spacecraft's rocket engines as part of the first stage in the sequence of rendezvous manoeuvres.
Dr Worth said that the next 48 hours will be critical if the spacecraft is to make its rendezvous with Eros next month as planned, although a failure to regain contact in that time will not result in the mission being abandoned.
One possibility is that the space probe had gone into a "safe" mode, which it is designed to do if it receives conflicting commands or detects something that should not have happened.
"We think the spacecraft has shut itself down. At the end of 20 minutes everything seemed well but 20 minutes after that we got some communications that something had happened," Dr Worth said. "If we don't make contact within a couple of weeks we can still make a rendezvous later," she said.
Near was scheduled to orbit the Eros asteroid for 12 months, mapping its magnetic and gravitational fields and analysing its mineral composition. Scientists had hoped to bring it within feet of its surface, even perhaps to attempt a soft landing.
Nasa now has a difficult job trying to regain contact, although it has had the experience of recovering the errant Soho satellite earlier this year, which was lost for several months before scientists regained control.
Near is one of the first Nasa robot craft built under a programme designed to emphasises less expensive and more effective ways of exploring the universe.