NASA officials were asked some years ago to catalogue the number of technical actions and successful communications necessary to fly a spacecraft safely from Earth to Mars. The number: about 10,000.

And that was for an orbiter, as opposed to a lander such as the Mars Science Laboratory, which requires an additional and very demanding descent through the thin Martian atmosphere to the surface.

No wonder the track record for successful missions to Mars is not great. About 43 flyby, orbiting and landing missions have been sent to Mars by NASA and other nations in the past 40 years, and only 12 have been fully successful. That’s a failure rate of more than two out of three.

“It’s an extremely unforgiving mission where everything has to go right,” said Scott Hubbard, the former head of NASA’s Mars space program and of the agency’s Ames Research Center in California’s Silicon Valley. “It’s basically one strike and you’re out.”

This is a sobering reality as the $2.4 billion Mars Science Laboratory and its rover Curiosity wait atop an Atlas V rocket at the Kennedy Space Center for its launch window to open on Friday.

But at least NASA has by far the best track record — with its last six missions to Mars complete successes. And as for landing on Mars and collecting usable data, only NASA has accomplished that feat.

The Russian (formerly Soviet) space program is believed to be zero for about 20 in its Mars missions, although two craft landed long enough to send back a few photos and a half-minute of data.

All missions to Mars start by launching into low-Earth orbit. Once there, a series of thruster rockets are fired — first to align the orbit path exactly with the orbit of Mars, then to propel the spacecraft toward Mars. The precision required for that second step is extreme, and the target is easy to miss.

A failure to pull off that course correction is what has apparently doomed the recent Russian effort to send its Phobos-Grunt spacecraft to a Martian moon. As of late last week, the vehicle was expected to fall back to Earth in January.

Once successfully directed toward Mars, a spacecraft is exposed to harmful radiation and temperatures unlike anything around Earth. The distance involved is also great — as much as 400 million miles, depending on the planets’ position in their orbits. Communications become especially difficult that far away; it takes 14 minutes for a radio signal to travel from Mars to Earth.

A Mars landing requires the spacecraft to decelerate from 12,000 mph to zero in about six minutes. Landing on Mars has proved especially challenging because its atmosphere is too thin to provide significant braking assistance, but, unlike on the moon, that atmosphere is changeable, which means local conditions have to be taken into account.

NASA had a bad patch in the late 1990s when several Mars missions failed, and Hubbard was one of the people brought in to fix the program. He said the biggest shortfall was found to be in systems engineering, the demanding process of making sure all parts of the mission — launch, orbit, deep-space protection and landing — work together.

It is this painstaking integration of many systems that has been Russia’s downfall, space experts believe, as well as its reliance on less sophisticated (and cheaper) electronics.