Anticipated deep budget cuts for U.S. unmanned space programs are prompting protests from scientists on both sides of the Atlantic and pushing Europe to cooperate with Russia on future Mars exploration.

In his budget proposal next week, President Barack Obama is expected to announce major funding reductions for NASA's future robotic missions, leaving the agency's ambitious plans for Mars exploration in limbo. Among the biggest casualties, say scientists and industry officials, will be a pair of joint U.S-European rovers that...