During Project Apollo, six highly complex manned craft landed on the Moon, took off and returned to Earth using a relatively low level of technology, with only one unsuccessful mission (an 86% success rate). Since Apollo, twenty-five simple, unmanned craft with increasingly higher levels of technology have attempted to fulfill their missions to Mars. Only seven succeeded. A mere 28% success rate. Was Apollo blessed? [David Percy]

What parallels the conspiracist sees between manned and unmanned space travel elude me. Manned space exploration is considerably more dangerous, and therefore considerably more safeguards are put in place, at astronomically greater cost.

There are other very important differences. The lunar module had two human pilots to correct any deficiency in the automatic guidance. In fact, if Neil Armstrong hadn't been at the controls of Eagle and steered toward an alternate landing site, the ship would likely have been ripped to pieces on the large jagged rocks that Armstrong discovered at the predetermined landing site.

Robot spacecraft must execute a preprogrammed set of instructions, and there are limits to how well that technology (however advanced) can deal with unexpected obstacles.

The exploration of Mars adds additional difficulties. Mars has an atmosphere, which presents problems for spacecraft landing there. A martian landing is considerably more complex because of the management of the atmosphere. Parachutes simplify the landing in one respect and complicated it in other respects. (It's another event to properly sequence.) The spacecraft must be shielded against the heat of entry, and must deal with wind gusts and other atmospheric phenomenon.

And because it takes a radio signal several minutes to travel to Mars from earth, there is little or no chance for remote control. You can't control the spacecraft in real time from earth, or receive real-time sensor information.

Wernher Von Braun said it best when he described a human being as the best possible computer you could put in a spacecraft. (He also went on to point out its advantages in being easily mass-produced with unskilled labor.)

We must also consider that NASA has changed its economics somewhat since the 1970s and 1980s. The Apollo project certainly cost billions. So did the space shuttle. The Viking landers to Mars also cost billions each. The failure of the Galileo spacecraft (which also cost billions) has hammered home to NASA just how financially devastating the failure of a high-budget spacecraft can be.

Engineers have to accept the certainty that 100% reliability is not attainable. No matter how much care is taken, there is always a slim chance of failure. But if you spend billions of dollars and years of planning and development on a mission, those are high stakes to be wagered in a single roll of the dice.

So in the 1990s NASA adopted a different planning model. Rather than sending a few astronomically priced and highly sophisticated missions to Mars, it would send a larger number of relatively inexpensive probes. That way the failure of a single mission wouldn't be so devastating. The success rate is so low because it is expected to be lower. But this way NASA gets more bang for the taxpayer's buck.

But there is a special mystique about Mars. Neither NASA nor the Soviet space program had much luck sending spacecraft to Mars. Among space scientists there is a popular joke about a big space monster that lives out near Mars and gobbles up spacecraft. The unusually high loss rate for all spacecraft sent to Mars cannot be reliably explained.

People working on manned missions to Mars say that radiation is probably the most perplexing problem. But NASA claims radiation wasn't that big a problem for Apollo missions. Which is it?

Duration of exposure is the key. Apollo missions lasted only a couple of weeks. The astronauts traversed the Van Allen belts quickly and were only outside them for a very few days. The chances of a debilitating solar event during this period was quite small. A manned mission to Mars will be months or years long, and a major solar event during that period is a virtual certainty. Apollo engineers didn't solve the problem of shielding against solar events. They just avoided the problem by playing the probabilities and hoping for the best. Mars mission planners do not have that luxury. They must come up with a reliable way of shielding against particle events.

Ironically similar problems face the crew of the International Space Station. Although the space station does not venture out of earth orbit, it "kisses" the lower regions of the Van Allen belts. And since the Alpha crews will be on station for months at a time, they actually require more shielding than Apollo crews.