Firstly, the Beagle 2 probe - whose status was rather perilous just a few months ago - has now received the official go-ahead from the ESA to be carried on its Mars Express orbiter. Second, the potential landing sites for NASA's two Mars Exploration Rovers (MER) have now been narrowed down further - but, at the same time, the final decision on their two landing sites has been delayed fully a year.

Beagle 2 - because it's a non-government project mostly funded by a consortium of universities, with some additional money from the British government - has been done on a shoestring. Its total estimated cost is only $40 million, which has led to some fears that it might be impractical to develop a Mars lander (even a small one) for that amount of money.

And, indeed, over the last few months, ESA's Space Science Committee has repeatedly expressed worries about the management of the program. For example, in December it noted that Beagle's team had told it that there might not be time to test the little lander's instruments and its robotic arm together, and in January it stated that "the total lack of a Delivery Review Board for Beagle 2 is most alarming".

However, the final decision to fly or not was made by the Committee on March 26, with the review board assigned to the project reporting to ESA and the British government: "The Board was pleased to see progress in every critical area; the risks in others were now more clearly defined, understood, and being addressed. "The schedule remains tight, but can be maintained for a January 2003 delivery, including a margin for contingency. It was recommended that the project proceed into the final construction phases. The dedication and motivation of the team was highlighted for special mention, since it was key to the project maintaining progress," the report stated.

So Beagle 2 is probably on - and, if it succeeds, will become the fourth spacecraft to report back from the surface of Mars after its touchdown on Dec. 26, 2003.

Even if it does not succeed in its primary attempt to detect complex organic molecules in the rocks and subsurface soil of Mars or traces of methane gas in the Martian atmosphere that might be produced by still-living underground Martian microbes, it will provide new and important science data on a variety of fronts.

These include:

• the most detailed study of Martian mineralogy yet. A study which could provide crucial clues as to just how much liquid water did exist on Mars' surface during its earliest days;
• to try and answer the question of how deep the organics-destroying oxidant chemicals apparently detected by Viking extend below the surface;
• conduct the most detailed analysis of the Martian atmosphere yet,
• make microscopic observations of rock crystals and soil grains at higher resolution that those to be provided by the 2003 NASA rovers;
• and attempt to carry out the first in-field age-dating of Martian rocks - which, if successful could be critical in understanding the planet's geological history.

A successful Beagle 2 mission would also be a major boost for British space exploration, which up to now has been astonishingly apathetic compared to that of other major countries. On the other hand, a failure - like the 1998 twin American Mars failures - would be a further blow to the hope that successful spacecraft can be made for costs much lower than has traditionally been the case.

Zooming In On A Landing Zone
Meanwhile, the third workshop to select landing sites for the two American 2003 Mars rovers (scheduled to land just a few weeks after Beagle) has been completed. Originally, this was supposed to be the last such workshop, selecting the rovers' two landing sites.

However, it has recently become clear that the mass margin on the 2003 MER spacecraft is a bit better than thought, allowing some additional midcourse correction fuel to be carried, which in turn allows more flexibility in the precise launch dates and directions of their boosters.

This, in turn, allows the precise design of their initial trajectories toward Mars to be decided and programmed much later than previously thought - and so the selection of the two final landing sites has now been delayed all the way to next spring - immediately before the launches - and allowing consideration of many additional observations of the remaining candidate sites both by the Mars-orbiting MGS and Odyssey spacecraft and by Earth-based radar telescopes.

Nevertheless, the list of finalists (four primary and two backup sites) was further winnowed down at this meeting - and in a surprising way.

Until now, a landing on the floor of Melas Canyon - one of the tributaries of Mars' gigantic Valles Marineris ("Mariner Valley") - had been considered one of the top two candidates, given that this site would offer a major display of layered sedimentary rock deposits. But at this meeting, Melas - and Eos Canyon, another branch of the Marineris Valley that was one of the two backup sites - were both decisively ruled out for a safety reason.

Being canyons, it now appears that they could channel and accelerate the surface winds of Mars to much higher speeds than exist on most of the planet's surface - and the MER landers are poorly equipped to deal with such winds during landing and rover operations.

The result is that three primary candidates are left:

1. the great patch of hematite (probably water-deposited in some way) discovered by MGS in the Sinus Meridiani region;
2. the floor of Gusev Crater (which has a great ancient channel, apparently water-carved, leading into it, so that the entire crater may have been water-filled in its earliest days);
3. and a site in the southern part of the great Isidis Basin, just north of the Basin's rim which is covered with ancient valley networks and possible lake beds indicating that large amounts of surface water may perhaps have drained into the basin at one time.

All the remaining sites, admittedly, have their own problems. For one thing, wind hazards at all of them may also be worse than originally thought -- as a result of which the new year available for landing site selection may also be used to examine two or three additional new landing site candidates which are less scientifically interesting than the current ones, but safer. (The current limits on the maximum permissible altitude and nothern latitude for rover landing sites may be somewhat relaxed for this search.) Gusev Crater's floor seems to be significantly covered by windblown dust, which could conceal the very rock and mineral formations the rover would be trying to study. And the Isidis site is a bit more dangerously rocky than the other two - and is also just 380 km south of the already-picked Isidis landing site for Beagle 2, which could make a second Isidis landing somewhat scientifically redundant. But they're all regarded now as more acceptable than Melas.

Also, one intriguing backup candidate site remains: Athabasca Valley, which was suddenly added to the list of candidates at the last workshop because MGS photos now indicate that it may have been the site of both surface volcanic lava eruptions and violent floods of geothermally heated surface water in Mars' very recent geological past (perhaps only a few tens of millions of years ago).

But Athabasca has a serious problem of its own: Earth-based radar indicates that it may be much rougher on a fine scale than the other three sites, perhaps even as rough as a sharp-edged Hawaiian a'a lava field, which if true would make it extremely risky both for landing and roving.

However, spacecraft maps of the rate at which Athabasca's surface cools off at night disagree, showing that it may actually have less exposed rock than Isidis and no more than the Viking 1 and Pathfinder landing sites.

It's possible that the radar echoes reflect the fact that Athabasca's lava may have contacted large amounts of groundwater that boiled into steam, turning the solidified lava rock into pumice or some other kind of "vesicular" rock, so that the surface is actually much less rough on the whole than the radar indicates.

However, there's still enough uncertainty that this scientifically very interesting spot remains a backup to the other three candidates, and is more likely to be studied by later Mars landers, which will have more capability to land in rugged terrain than the 2003 MER rovers. Like Pathfinder, the twin 2003 landers will use a "hard landing" system that initially deploys a parachute, before a radar-triggered solid rocket ignites 200 meters above the ground to remove most of the remaining descent speed. Next a cocoon of airbags inflate that absorb the remaining landing shock. After the lander has bounced and rolled to a stop unfoldable petals open to prop the lander upright for the rovers to drive off.

Testing the Pathfinder landing system was one of the critical goals of the 1997 mission, since it was initially regarded as both much simpler and much more durable in rough terrain than the more complex "soft landing" system - using throttleable rockets, a mluti-beam radar system to measure both altitude and vertical and horizontal speeds, and landing legs - used by the Vikings, as well as by most Moon landers.

But during development, this "simple" system turned out to have problems of its own - primarily weight.

Moreover, it was much harder than expected to develop airbags tough enough to avoid ripping on sharp boulders in the frigid Martian air. As a result they ended up requiring four layers, and the final big cocoon of 24 airbags ended up weighing almost five times more than expected. The final result being that the total hard-landing system weighed much more than the more complex soft landing system would have weighed.

When the 2003 Mars rover missions were selected, one of their main selling points was that they could use copies of Pathfinder's landing system with few modifications. But once again, in practice these required extensive changes, including a larger entry heat shield and even tougher airbags.

And, despite the work done on the latter, the Jet Propulsion Laboratory's engineers are still uneasy about the MER lander's' ability to survive a landing in which its parachute is being blown sideways by high Martian winds when its solid braking rocket ignites.

Not only would this give the lander a direct shove sideways, but short wind gusts could easily cause the "backshell" to be tilted when it ignites - and the resultant rapid horizontal speeds at landing, when added to the expected vertical speed of lander impact, might prove too much even for the new airbags to endure unless they are made so tough as to become unacceptably heavy.

So in 2000 a major new addition was made to the MER landing system: "TIRS", a system using a package of gyros and accelerometers on the backshell to measure the extent and direction of its sideways tilt when the braking rockets ignite.

These will, if needed, ignite one or two of a ring of six small horizontal solid rockets spaced around the backshell to counter the resultant sideways drift in whatever direction it's occurring.

The cancellation won't be perfect, either in speed or direction - but even if the rockets accidentally ignite when the lander isn't drifting sideways at all, they'll still give it a shove of only 15 meters per second sideways, which is within the airbags' limits.

Otherwise, they'll either partially counter the sideways drift or overcompensate somewhat for it, and in either case the lander will again touch down at a horizontal speed of less than the airbags' 16 meters/sec safety limit.

This system, however, still has a problem: it can't measure the lander's horizontal drift across the surface due to steady winds that don't tilt the backshell as gusts do - and Doppler radar beams for that purpose would be expensive and complicated.

So one paper at the workshop described yet another near-certain addition: "DIMES", a little descent camera that would take three photos of the Martian surface at altitudes of 1.6 to 1.2 km, after which the rover's onboard computer (which also controls the landing system) would quickly compare their surface features, estimate the speed and direction of the lander's sideways drift at that point (only 12.5 seconds before the braking rockets fire) and combine it with the last-second data on the tilt of the lander's backshell to calculate whether and in which direction to fire the anti-drift rockets.

The rover would then store the DIMES camera's three photos for later return to Earth. Mars scientists have strongly urged inclusion of a descent camera on future Mars lander whenever possible, both to help understand the landing site's geology and to plan post-landing exploration for the rovers. But NASA had rejected the inclusion of such camera for the MERs on expense grounds.

"However, these three pictures -- taken by a duplicate of a relatively low-quality camera which each rover will use to locate the Sun for navigation -- may not be any better than the photos of the landing area taken by the high-quality telephoto "MOC" camera on the current MGS Mars orbiter." Moreover, the addition of all this new complexity to a "simple" hard landing system reflects one of the MER Project's big problems: it's been much more technically difficult than it was initially sold as being when selected in 2000, and when NASA Administrator Dan Goldin personally decided to add a second rover to it. $50 million had to be added to the project from the rest of the Mars program last year, forcing further delay of the much more sophisticated Mars "Smart Lander" and its very long-range rover from 2007 to 2009.

And even with that extra money, MER is still walking a developmental and fiscal tightrope, as repeatedly noted by both NASA's Solar System Exploration Subcommittee and its overall Space Science Advisory Committee in recent months.

The Advisory Committee went so far as to say in its March 7 report: "The MER mission is aggressively scheduled and [the Mars program's director's] presentation identified clear warning signs (cost, schedule, workforce safety) that suggest that MER is dangerously close to a path of significant risk.

"The SScAC strongly cautions against pushing risks to an unacceptable level, and path the program has been down before. SScAC believes that the success of a single rover mission is of the highest priority."

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