No such luck. Within just a few months after mission selection, more detailed design work on the Athena rover made it clear that it simply could not be crammed into a duplicate of the Pathfinder landing system without having its scientific capabilities crippled - both its weight and its volume were about 10 percent too great.

The Pathfinder design would have to be scaled up in size, and a great many detailed changes would have to be made in it to keep the spacecraft's total mass from exceeding its Delta 2 booster's payload limit for a Mars mission.

The estimated total cost for the two missions has now risen to over $800 million. JPL's Planetary Flight Projects Director Chris Jones told the Council that, had this been known at the time, MER almost certainly would not have been selected as the 2003 U.S. Mars mission.

That, however, is not the worst part. The reliability of the enlarged landing system - its ability to land the rover safely with adequate confidence - has given project managers conniption fits, and is still a serious problem less than 9 months before launch. The central problem can be summed up in two words: horizontal winds.

After the spectacular success on a shoestring budget of the Mars Pathfinder mission in 1997, many observers [including this journal] jumped to the conclusion that its new hard-landing system - far simpler than the intricate soft-landing systems of America's earlier Moon and Mars spacecraft - was the wave of the future.

The earlier Viking Mars landers, after their heat shields and parachutes had braked them to a descent speed of a few hundred km per hour in Mars' wispy air, had cut their chutes loose and lowered themselves the last few thousand meters using multiple throttled liquid-fueled rocket engines controlled by a computer using multiple radar beams to constantly monitor the craft's altitude and vertical and horizontal speeds.

By contrast, Pathfinder held onto its parachute until it was just 80 meters above the ground - with the chute being attached to the lander's aft cover, from which the lander had already lowered itself on a long bridle cable like a spider from its thread.

At that altitude, a simple radar altimeter ignited three powerful solid rockets fastened to the aft cover to blast away for 2 seconds, yanking the lander to something reasonably close to a dead stop - after which the lander immediately cut itself loose and simply dropped the remaining 12 meters or so to the ground, nestled in a big cocoon of 24 inflated airbags that absorbed the landing shock.

It was a nice simple system - and supposedly far tougher than soft landers, which with their footpads and landing legs are seriously vulnerable to boulders and slopes. The airbags were designed to be robust enough to survive if the spacecraft hit the surface a lot faster than the estimated nominal figure. But the system has an Achilles heel of its own that has received little publicity in the wake of Pathfinder's success.

The problem is basically building the airbags tough enough.

Pathfinder's own airbags ripped repeatedly during simulated landings on Earth; they finally ended up having to consist of four layers instead of one, weighing fully 85 kg rather than the originally planned 15 kg - and effectively making up one-quarter of the lander's total landed weight! In all likelihood, a more complex soft-landing system for Pathfinder would have weighed a good deal less.

But the greater weight of the MER lander package is not the main problem.

Pathfinder landed during Mars' pre-dawn hours, when Martian winds are at their minimum - and even then it had come down traveling sideways fast enough to bounce and roll across the surface for fully a kilometer after touchdown.

For the MER landers, due to their trajectories towards Mars, landing will instead take place mid-afternoon, when the winds are likely to be much stronger.

In various Earth based tests undertaken over the years, it appears that hitting sharp rocks at a fast horizontal clip is the main cause of airbag rips. By contrast, a soft lander cancels out all of its sideways drift by the time of landing.

Tests in 2001 showed that, given the probability of strong horizontal daytime winds, there was simply no way to make MER's airbags tough enough to be confident of surviving a landing without making them so heavy that the Delta booster couldn't carry the total spacecraft to Mars.

The result was that in 2001 the Transverse Impulse Rocket Subsystem (TIRS) was added to the landing system. This system consists of a ring of three smaller solid rockets on the aft shell of the lander, pointing out in different directions.

Sensors on the lander will provide an estimate of the likely horizontal direction and speed of its impact, and - if necessary - one or two of the horizontal rockets will be ignited at the same moment as the big main braking solid rockets to cancel out much of the lander's sideways drift.

And even if the rockets end up overcompensating and TIRS shoves the lander somewhat in the reverse direction, any resulting horizontal landing speed less than 16 meters/second will be acceptable for the $400 million Martian beach ball Mark II to bounce down on to Mars and survive.

The main danger of a fast sideways landing was thought to come from sudden wind gusts that would swing the dangling lander sideways on its cable, in turn tilting the aft cover hanging from the parachute so that its powerful braking rockets actually fired downwards at an angle and pushed the lander sideways.

So the first control sensor added to TIRS was a package of gyros and accelerometers on the aft cover to measure its tilt at the moment the braking rockets fired and ignite some of the TIRS rockets to compensate.

But as our knowledge of Mars' meteorology and horizontal wind patterns grows, concerns have also grown about steadier, longer-period horizontal winds that might be capable of blowing a lander strongly sideways at the time its braking rockets fire - without providing sufficient tilt to its aft cover to warn of trouble.

The DIMES system involves adding a lightweight camera to the lander that would take three successive photos of the surface in the 18 seconds before the braking rocket is ignited. The photos would then be immediately analyzed by the spacecraft's computer to locate surface features and calculate an estimate of the lander's sideways drift that would be added to the gyro package's tilt data to decide which TIRS rockets to fire up.

The DIMES camera system seems to work well in tests - but the decision as to whether it's worthwhile to add it to the MER spacecraft to further reduce landing risks won't be made until November or December.

And even with the whole TIRS system added, winds still remain a serious problem for the engineers building the MER landers to fully factor in.

Already, new data on possible wind speeds in different places on Mars has forced two of the top six candidates for MER landing sites to be rejected at last March's MER landing site selection workshop - both of the very scientifically interesting sites within the great Marineris Valley, which unsurprisingly channels winds to higher speeds.

Since then, another site - in the Athabasca Valley, which may be the site of recent volcanic eruptions and geothermal liquid-water outbursts less than 20 million years ago - has been rejected because Earth-based radar studies indicate that its ground may be unacceptably rough and rocky. And of the remaining three sites, studies are still going on to determine whether two of them have horizontal winds too strong for the landing risk to be acceptable.

 

Gusev Crater

Those two sites are the floor of Gusev Crater - which has a giant, apparently water-carved channel draining into it, suggesting that the crater may have been a water-filled lake during Mars' early days - and another landing ellipse located in the southern part of the Isidis Basin, whose southern rim has many smaller possible water-carved channels draining down it on to the flat basin floor, so that a lander would again have a good chance of finding ancient water-formed sediments.

The only original candidate site which scientists are certain has acceptably low winds and smooth terrain is the giant field of hematite - again, probably deposited by ancient liquid water - which the Mars Global Surveyor's IR spectrometer has mapped in the Terra Meridiani region.

It's now a near-certainty that one of the two MER rovers will land there - but, while Gusev is the other favorite judged strictly by scientific interest, it may be that both it and Isidis will ultimately also have to be rejected due to excessive wind risks.

This has led to the landing site selection committee hastily locating one more backup landing site -- a flat, scientifically rather dull site on the Elysium Plain that is a candidate only because it has low winds.

The final selection of the MERs' two landing sites won't be made until next January. And - even with all this - it is still not entirely certain that it will be considered safe to launch either of the MERs next year.

The final tests of the ultimately selected airbag design are still underway, and three candidates for the MER's big landing parachute also remain to be tested to determine which of them best combines good braking ability with the minimum tendency to be blown sideways by winds.

Officially JPL remains highly confident that the final designs for both the chute and the airbags will have been picked by the end of October - but if more unexpected problems turn up and satisfactory airbag and parachute designs still haven't been picked by next February, it will become impossible to launch the MERs to Mars in 2003.

Moreover, if additional problems rear their heads in the next few months, NASA will have to seriously consider delaying one of the MERs to a later launch window, allowing the remaining project funding to be fully focused on finishing adequate design and testing for the one remaining MER so that it can meet the 2003 deadline.

The cost-effectiveness of this is questionable, however. And in any case, NASA would like to avoid delaying either rover to a later launch if at all possible, because 2003 is an unusually favorable launch opportunity to Mars with a higher payload capacity available.

By contrast, the next opportunity in 2005 is one of the worst launch windows in the cycle. If one or both rovers do have to be delayed, the current feeling is that their landing on Mars will be set not to occur until 2008.

This doesn't necessarily require that their launch be delayed all the way into 2007, however - especially since the U.S. already plans one other Mars launch in both 2005 and 2007.

Instead, the delayed MERs may be launched in 2006, onto a path that allows them to complete one and a half orbits of the Sun before finally intercepting Mars. (The same strategy was followed with the Magellan Venus probe.) They might even be launched in 2004 and carry out three and a half solar orbits before reaching Mars, if the Mars Program's overall funding profile favors such a deployment.

Moreover, any delayed launches -- since 2003 was an unusually favorable Mars launch opportunity -- will probably have to use the bigger and considerably more expensive Atlas 3 booster (which is already planned for the 2005 U.S. Mars Reconnaissance Orbiter)." It should be emphasized, however, that this is strictly worst-case planning. MER's managers feel that they finally do have a good handle on the project's problems, that launch of both rovers in 2003 is now highly probable, and that simulations show that each one has a 90 percent or so chance of surviving landing even without the DIMES sensor added.

But this project has given JPL - and NASA - a vastly harder time than anyone dreamed possible when it was picked, and has played a major role in convincing engineers that Pathfinder's "simple" hard landing system is much less preferable to soft landing systems than had been thought.