After all, a soft landing is a good deal more complex than a "simple" braking burn to go into orbit around another planet -- and that's not even taking into account the uncertain ruggedness of the landing site terrain itself.

JPL insists that it is taking the advice of the NASA investigative board that looked into the MCO accident, and that -- largely through greatly augmenting the size of the spacecraft's operating team, and particularly its navigation team -- any risk of repeating that appalling incident is over.

But even so, in addition to the standard unknowns of any landing attempt, there are at least three specific problems that have been mentioned over the past week as perhaps endangering MPL. The question is how worrisome are they actually?

(1) On Nov. 7, Keith Cowing's "NASA Watch" Website quoted JPL inside sources as saying that an even more horrific mistake had been made during the assembly of the Lander: the heaters had accidentally been left off the small pyrotechnic charges designed to separate the lander from the "cruise stage" that had supported and guided it during the long trip to Mars, and which was supposed to be separated 5 minutes before the Lander entered the Martian atmosphere!

As a result, these charges would be so cold that they might very well fail to detonate -- so that the Lander, still attached to its cruise stage, would crash uncontrollably onto Mars. The report stated that a JPL "tiger team" had concluded that the decision might well be made to leave the cruise stage attached until entry into the Martian atmosphere had actually begun, in the hope that the heat of air friction would warm the pyrotechnics to the point that they would then fire when commanded -- which, if true, would have been a tremendously risky maneuver.

However, when the MCO Board report was released on Nov. 10, JPL stated that this report was actually just a garbled version of another possible problem that the Board did uncover.

Indeed, Mars Surveyor Program spokeswoman Mary Hardin personally assured SpaceDaily.com on Nov. 15 that this is indeed the case, and that no known problem exists with the pyrotechnic charges.

(2) But what are the real problems that the MCO Investigation Board has turned up?

It involves the main "descent engines" that the lander will use to carry out the final 1800 meters of its final descent to the surface of Mars' after cutting itself free from its parachute, since even a huge chute cannot brake the craft below about 300 km/hour in the thin Martian air. These 12 thrusters burn hydrazine propellant, which is ignited when it contacts a bed of chemical catalyst that causes the hydrazine to break down explosively into ammonia and water vapor.

But the Board raised the possibility that, if the catalyst was chilled below 0 deg C during the long trip to Mars, it might be chemically sluggish in igniting the hydrazine, making the thrusters unreliable.

"...The cold catalyst bed-induced ignition delays, and the resulting irregular pulses on startup, could seriously impact MPL dynamics and potentially the stability of the vehicle during the terminal descent operations, possibly leading to a non-upright touchdown" -- known to the average person as "crash and burn".

Moreover, the MCO Board also noted that if the lander's fuel lines were comparably cold, the hydrazine might freeze solid in them before it even reached the engines.

Having decided that JPL had underestimated these dangers, the MCO Board recommended that the electric heaters for the fuel lines should be turned on earlier than had been planned to ensure that they were properly warm -- and it also recommended that JPL should consider firing the descent thrusters in a series of extremely short bursts during the first few seconds of engine startup, to ensure that the catalyst beds were warm enough to work properly afterwards.

JPL agreed to turn on the fuel line heaters several hours before the lander arrived at Mars, which would raise the temperature of the engines themselves to 8 deg C at the time they were started.

It also stated that its tests showed that the engines' catalyst beds would work properly at temperatures as low as -20 deg C, making those short startup bursts unnecessary -- although it added: "More ground-based test firings are scheduled to better characterize engine performance at various temperatures." At any rate, the odds look good that these particular problems have been dealt with.

(3) However, the Board also expressed another worry. Every previous soft landing that the U.S. or the Soviet Union has ever carried out on the Moon or Mars has involved the use of throttleable rocket engines whose thrust can be controlled over a wide range -- allowing the craft to control its descent speed in response to the data coming in from its radar -- and each engine has also been separately throttleable, allowing the craft to tilt itself in order to cancel out any horizontal drift that the radar detects.

But MPL uses a new "pulse-mode" engine system. Instead of three smoothly throttleable engines, it carries three clusters of four thrusters each whose thrusts are rigidly fixed at 27 kg per thruster -- and it controls its descent rate and its tilt by rapidly flicking the 12 separate thrusters on and off for as little as a small fraction of a second in order to control the craft's overall thrust level.

The Board noted: "This type of powered descent has always been considered to be very difficult and stressing for a planetary exploration soft landing" because of the vibrations it produces -- which is why it has never been used before.

"The concern has been that the feedline hydraulics and water hammer effects could be very complex and interactive. This issue could be further aggravated by fuel slosh, uneven feeding of propellant from the two tanks, and possible center of gravity mismatch on the vehicle... Under extreme worst-case conditions for feedline interactions, it is possible that some thrusters could produce near-zero thrust and some could produce nearly twice the expected thrust when commanded to operate."

JPL had concluded that enough was known by now about these possible problems that computer guidance software could be written that would deal reliably with them -- and since it is much cheaper and easier to develop a fixed-thrust rocket engine than it is to develop and manufacture a new throttleable engine, JPL decided to go with the pulse-mode landing technique this time.

The Board noted: "It was stated many times by the MPL project team during the reviews with the Board, that a vast number of simulations, analyses and rigorous tests were all carefully conducted during the development program to account for all these factors during the propulsive landing maneuver. However, because of the extreme complexity of this landing maneuver, the EDL [Entry-Descent-Landing] team should carefully re-verify that all the above described effects have been accounted for in the terminal maneuver strategies and control laws and the associated software for EDL operations."

Given JPL's blunders in navigating the Mars Climate Orbiter -- and the similar problems the Board uncovered in the management of the MPL, due largely to an inadequate number of personnel and a poorly designed control organization -- this author is not greatly confident that this has been adequately done.

Unfortunately, by this time there isn't much time left to do it thoroughly, or to change the landing software in response. It is therefore reasonable to assume that because of this fundamental problem, there is a distinct element of a risky gamble in the MPL landing, even apart from the unknown terrain features of the exact landing point.

This area of Mars was thought to be one of the smoothest on the planet -- but recent photos by the Mars Global Surveyor's high-resolution telescopic camera have shown that it is somewhat rougher than expected.

Even if MPL does fail, though, at least we'll know why -- right? Wrong. For the first time ever, a spacecraft will have no radio contact of any sort with Earth during its landing sequence -- even the rudimentary kinds of signals that Mars Pathfinder sent immediately on "bounce down" were able to indicate that its landing had been successful.

However, as the landing site is so near Mars' south pole, the Lander's low-gain antenna isn't properly aligned to allow even simple low-power signals to be sent to Earth -- let alone any engineering telemetry on the functioning of the craft's systems.

After separating from its cruise stage, the craft won't reestablish radio contact with Earth until fully 20 minutes after the landing (set for noon Pacific time), at which time it is scheduled to finish pointing its small dish antenna at Earth to allow direct contact.

Later, it will start using its low-gain UHF antenna to communicate larger amounts of data to the Mars Global Surveyor, which will in turn relay it to Earth -- but, for several reasons, MGS can't be used for that purpose until several days after the landing.

If MPL's landing fails, we'll never know why -- whether it's a design flaw in the craft, or simply an unavoidable landing on bad terrain. It will be a replay of the loss of the Mars Observer spacecraft during a brief planned period of radio silence, which forced its accident review board to come up with several possible failure causes and try to guess which was most likely.

Again, this is not reassuring. It's too late to do anything about this with MPL; but for this author it is hoped that by the time of the next landing mission in 2002, NASA will have modified the lander so that it can send engineering data to one of the Mars orbiting spacecraft that are already scheduled to regularly receive data from it after the landing.

In any case, given the number of unknowns in MPL's landing, there will be even more reasons than usual for us to hold our breaths until that confirmatory signal comes through -- or not.