A spacecraft launched from Russia last Wednesday and originally destined for Mars has yet to leave Earth orbit – and looks increasingly likely to tumble back to Earth over the next several weeks.

The craft, Phobos-Grunt (Phobos Soil), was to have traveled to Mars' moon Phobos to gather and return to Earth samples of the moon's soil and rocks. But once the craft reached Earth orbit, motors in the rocket stage that would have set Phobos-Grunt on its path to the red planet failed to ignite.

Engineers with Roscomsos, the Russian Federation Space Agency, have tried to communicate with the craft in hopes of igniting the motors before changes to the orbits of Earth and Mars close the window of opportunity over the next few days.

But according to updates on the website RussianSpaceWeb.com, all attempts have failed so far. If efforts to send the mission on its way fail, the craft – brimming with a load of toxic fuel in tanks that potentially could survive reentry – could reenter Earth's atmosphere at the end of the month.

Russia's travails serve as a fresh reminder that space exploration is hard.

"That's why it's called rocket science," says Ralph McNutt, chief scientist at the Johns Hopkins University's Applied Physics Laboratory in Laurel, Md., and the project scientist for NASA's Messenger mission, whose spacecraft currently is orbiting Mercury.

It's a point not lost on Mars-mission planners at NASA, who are preparing to launch the $2.5-billion Mars Science Laboratory Nov. 25.

Phobos-Grunt and the Mars Science Laboratory represent the most ambitious Mars-exploration missions to date for their respective space agencies.

The lab is a 1-ton rover loaded with instruments to analyze Martian rocks and soil within a vast feature dubbed Gale Crater. Although the rover isn't designed to hunt for life, it will be hunting for organic compounds that would help determine whether the planet had conditions that could have supported life.

In one sense, the world's space agencies have a success rate at the Mars-mission plate that major-league ball players would envy. Since 1960, when the then-Soviet Union launched the first mission to the red planet, which failed, 35 launches by four nations have amassed a .329 average, based on NASA's tabulation of international Mars launches.

But that average masks a wide disparity in success rates among the four.

With the apparent failure of Phobos-Grunt, Russia is 0 for 17 attempts since 1960 at a mix of Mars flybys, orbiters, and landers.

Japan, which launched a Mars orbiter in 1998, is 0 for 1. Europe, with its inaugural Mars Express/Beagle 2 orbiter-lander combo, is 0.5 for 1 at the red planet. Launched in 2003, the duo reached Mars. The orbiter has been a science success, and its mission has been extended to 2014. But the lander was declared lost after repeated attempts to contact it failed following its December 2003 descent to the surface.

NASA, meanwhile, has enjoyed 11 successful Mars missions out of 16 launched since 1964, including flybys, orbiters, and rovers.

It's easy to get used to those successes, but they are far from assured.

Designing craft for interplanetary travel means tailoring its systems to function for years in an environment far different from the conditions craft encounter in Earth orbit, where designers have far more experience, Dr. McNutt says.

Flitting between Earth and anywhere else exposes a craft and its sensitive electronics to the potentially disruptive effects of cosmic rays.

Pick your destination and you will encounter much different contrasts in temperatures the craft must endure, compared with conditions at Earth. Think the Voyager spacecraft at the far edges of the solar system versus the Messenger mission orbiting Mercury.

Indeed, the combination of the vacuum of space and temperature can be a mission-ender, he says, citing NASA's Mariner 3 mission as an example.

Launched in 1964, it was the US's first attempt at a Mars flyby. But the craft ultimately failed, felled by the combined effects of vacuum and temperature – something for which it was not tested prior to launch. Engineers conducted vacuum and temperature tests separately.

The failure led to the development of test chambers that could replicate both conditions simultaneously, McNutt says.

In the end, engineers tend to lay awake nights worrying not as much about the systems they've checked a dozen times as the so-called unknown unknowns – serendipitous combinations of system failures that can tank a mission but only become slap-the-forehead moments after they happen.

Even then, clever engineers can often devise work-arounds. McNutt cites NASA's highly successful Galileo mission to Jupiter as an example. During Galileo's cruise phase, engineers commanded the craft's high-gain antenna to unfurl. This was the antenna the craft would use to beam large volumes of data from the science instruments to salivating mission scientists back on Earth.

But the antenna failed to deploy. Speculation regarding the cause centered on the antenna's lubricants evaporating en route to Jupiter.

During the craft's seven-year cruise to Jupiter, engineers were able to compensate by reprogramming the craft's computers to compress the data and by installing more sensitive receivers at tracking stations on the ground. This allowed the team to use the orbiter' less-capable low-gain antenna both for keeping track of the spacecraft's health and for returning research data.

When it comes to the Russian space program, it clearly has had significant successes, McNutt notes. In human spaceflight, the country is providing reasonably reliable transportation for US and Russian crews to and from the International Space Station (ISS) as a bridge to US commercial crew flights to low-Earth orbit expected later this decade.

And while US planetary scientists have struggled to come up with a cost-effective mission to explore Venus, Russian missions to the second rock from the sun "have knocked the ball out of the park," he says. "Most of what we know about the surface of Venus comes from Russian data."

Which makes Russia's 0-fer at Mars a puzzle. The physics involved in getting there are the same, regardless of flag. Russia has a stable of reliable rockets. And Russian engineers are likely to be just as motivated to succeed as any other county's rocketeers.

McNutt speculates that the challenges revolve largely around money and priority. Since the end of the cold war, Russia's space program has struggled for cash, prompting its entrepreneurial foray into space tourism, for instance. US purchases of seats on Soyuz capsules to and from the space station represent another source of badly needed revenue.

Tight budgets not only reduce the number, if not the ambition, of space-exploration missions. They can also shrink investments in the engineering infrastructure needed to support the design and construction of new space probes that do receive approval, McNutt explains, recalling the Mariner 3 failure.

Phobos-Grunt reportedly was using a usually reliable upper stage for putting the craft in its trajectory to Mars. But the stage reportedly had been modified for the mission, potentially turning a less-expensive, off-the-shelf solution into an inadvertent prototype.

As for NASA's Mars Science Lab? Landing will be a thrill. The mission is using an approach for putting a rover on Mars NASA has never used before. No air bags; the rover is too big and heavy. Instead, the lander wears its descent module like an exhaust-spewing cap.

When the module descends to about 25 feet above the surface, the module will lower a bridle carrying the lander, much like a Sikorsky Skycrane helicopter lowers cargo while hovering. Once the lander's wheels touch down, the bridle's tethers are released, the descent module rockets away from the lander, and the lander is free to roam.

The approach was dictated by the rover's size and weight, explained Pete Theisinger, the mission's project manager, during a recent press briefing to preview the launch.

The lab is too big and heavy for air bags, and driving it off a descent module underneath the rover would be cumbersome and complicate.

"That would be a daunting, daunting thing to do," he said.

The various components that make up the system have been thoroughly tested, he said. But he acknowledged that the package hasn't been tested as an integrated system.

"We've done our due diligence. Mars may interfere with us, or there may be something we haven't caught" he continued. "But to the extent we've been able to think of it, we've attacked all the problems and done all the testing we can do."

Here's to a well-lubricated tether release.