In fact, three of them would focus largely or entirely on studying Mars' atmosphere - and four of the missions would never even touch Mars' surface. This most definitely does not mean, however, that they're scientifically inferior - only that the Scout program is now much different from what it used to be.

In describing them, we will begin with the three atmosphere-oriented missions - two of which are Mars orbiters to study the planet's atmosphere and weather from above. One - MACO (Mars Atmospheric Constellation Observatory), is proposed by Robert Kursinski of the University of Arizona.

It would use the time-honored technique of radio occultation to make vertical profiles of Mars' atmosphere. But instead of the spacecraft transmitting the radio beam to Earth, MACO would consist of four little satellites that would continuously transmit radio beams back and forth between each other, monitoring their strength, frequency and phase as the beams periodically sliced through a cross-section of Mars' atmosphere.

Despite the simplicity of MACO's observation system, it could in many ways do an excellent job at monitoring global weather changes on Mars - especially since the four minisatellites, once they had been aerobraked into low circular orbits around Mars with different inclinations, would have their interlinked radio beams occulted over 200 times per day in positions scattered all around Mars.

Moreover, such occultation cross-sections can provide much better vertical resolution of the many properties of a planet's atmosphere than even the best radiometer-type sensors on a single satellite can do with resolutions as little as 100 meters possible. The GPS network of navigation satellites is currently being employed using this same technique to provide data on Earth's weather.

The MACO satellites would profile temperature, pressure, and humidity, while providing significant data on wind speeds and even some measurement of the trace levels of deuterium present in Mars' water vapor - which we already know is somewhat more concentrated than on Earth, confirming that part of Mars' initial atmosphere has been swept away from the planet by the solar wind.

The MACO birds would also each carry a lightweight IR sensor to measure atmospheric dust - which is one of the biggest influences on the temperature of Mars' thin air - and use a microwave receiver to measure water vapor.

The Mars Environmental Observer - proposed by Michael Janssen of the Jet Propulsion Laboratory - is a more conventional Martian meteorological satellite, but a very capable one.

It would be a modified version of the Stardust spacecraft - which, to set up its flyby of the comet Wild 2, has to carry out one deep-space maneuver so big that Stardust's fuel tanks are quite adequate to brake a copy of the craft into Mars orbit. MEO would then aerobrake into a 400-km orbit around Mars, and set about its atmospheric observations, using four different instruments.

Two of them would be radiometers measuring very short wavelength microwaves, which are now starting to be used for space studies in a major way.

One, a French instrument with the unlikely name of BOSCO, would measure waves with a wavelength of a few millimeters, while the American, "MIMAS"-- based on the American "MIRO" microwave spectrometer which will fly on Europe's Rosetta comet probe in 2003 - would measure shorter, sub-millimeter waves.

There's some feeling, though, that the two are redundant - and the French are seriously considering deciding instead to fly BOSCO on their 2007 test flight of their big Mars sample return orbiter.

Either or both of them, peering at the Martian horizon, would provide a very sensitive profile of air temperature and humidity throughout the thickness of the Martian atmosphere - and they could also measure winds at various altitudes down to only two or three meters per second, a capability that the PMIRR instrument doesn't have.

Another copy of the miniaturized PMIRR from the 2005 U.S. Mars orbiter would, however, also be carried, and by comparing its data to the microwave data, the amount of dust at different altitudes could be very sensitively measured.

Finally, MEO would carry a miniature version of the "ATMOS" instrument flown on the Space Shuttle: an extremely sensitive IR Fourier spectrometer capable of taking such detailed spectra that it could measure a huge variety of different trace gases in Mars' air down to only a few parts per billion - and, for some gases, a few parts per trillion!

Not only would this provide vast new knowledge of Mars' atmospheric chemistry - and allow more isotopic measurements to study the atmosphere's history - but it could detect tiny near-surface concentrations of such gases as methane and nitrous oxide, which, if found, would provide strong evidence that Mars still has a small subsurface population of living bacteria to manufacture them.

Thus MEO - despite being an atmospheric orbiter - is one of the few Scout proposals that could look for actual evidence of Martian life.

Another Scout proposal - "Pascal", from Robert Haberle of the Ames Research Center - would be a network of 24 tiny hard landers scattered all over the Martian surface as a network of miniature weather stations. (Haberle has proposed Pascal before as a Discovery mission, although it wasn't picked as a finalist.)

These rugged little cylindrical capsules, weighing only 2.1 kg each, would be dropped by simple parachute - being padded enough to survive impacts of 140 km/hour - and their sensors would be evenly distributed over their surfaces so it wouldn't matter which way they ended up lying.

They would be powered by tiny nuclear batteries, allowing them to measure air temperature, pressure, humidity and dust content for fully 10 Martian years (18 Earth years) - thus enabling a portrait of long-term variations in the yearly cycle of Mars' weather, such as the occasional global dust storms, to be formed.

Despite their tiny size and low communications bit rate, they would also each carry a tiny color CCD camera, which would take 10 pictures of Mars' surface on the way down, and then transmit those pictures very slowly over the following months.

Haberle is also considering adding another tiny camera to each of them that could send back a single photo per month. They would be carried to Mars on a copy of the Mars Climate Orbiter/Mars Odyssey spacecraft , which would eject them in various directions a few weeks before reaching Mars to arrange their landings scattered over the planet - and the carrier bus would then brake itself into a low Mars orbit to daily receive their data transmissions and relay them back to Earth. To save money, however, the bus would carry no instruments of its own.