Ever heard of Apollo 11? Yeah, that was pretty much a suicide mission. Nixon had several speeches prepared to deliver to the nation covering the myriad of possibilities for failure.
Apollo 11 was not a suicide mission. It was dangerous as all space missions are, and the astronauts were heroic, but Nixon's speeches were simply a matter of planning for contingencies so that in the event that something went wrong they wouldn't be caught unprepared.
A mission where success is physically possible, and especially when estimates for a safe return exceed 50%, is not a suicide mission. Sending people to Mars without sufficient resources for them to return or subsist on the planet indefinitely -- that's a suicide mission. Give them enough fuel and the mechanisms required to return to orbit and make the trip back to Earth? That's a dangerous, but still ultimately survivable mission.