While Mars Global Surveyor (MGS) is still struggling to reach its final mapping orbit, the launches of the two follow-on missions, Mars Climate Orbiter (MCO) and Mars Polar Lander (MPL), are rapidly approaching. The overall theme of both missions is "Volatiles and Climate History"; the role and distribution of water is a common thread among the investigations. Again we have an orbiter (again using Mars aerobraking to reach its final orbit) and a lander. The basic mission statistics are as follows:
| MCO | MPL | |
|---|---|---|
| Nominal Launch Date: | 10 December, 1998 | 3 January, 1999 |
| Arrival Date: | 23 September, 1999 | 3 December, 1999 |
| Launch Mass: | 643 kg | 615 kg |
| Primary Mission Duration: | 2 y mapping, + 3 y relay | 90 d |
MCO arrives first, and follows a procedure very much like MGS, with an initial rocket-powered capture maneuver, followed by a 2-month aerobraking into a 400 km circular orbit. Two instruments are on board: the Pressure Modulator Infrared Radiometer (PMIRR), and the Mars Color Imager (MARCI). PMIRR is designed to observe the atmosphere in one visible and 8 infrared bands between 6µ and 50µ, to measure vertical profiles of atmospheric temperature, dust, and water vapor, with 5 km vertical resolution over the entire planet. It will also map clouds, and allow a quantitative determination of the surface radiation balance: bookkeeping energy in and energy out as a function of wavelength. The MARCI consists of two small CCD cameras with similar focal planes but different optics to yield wide and medium fields of view (FOV). The Wide Angle (WA) camera views fully 140° in 5 visual and two ultraviolet channels, with a spatial resolution of about 7 km/pixel. The Medium Angle (MA) camera has a 6° FOV and a 40 m/pixel resolution; it will map the entire planet (except the rotational poles, due to the slight inclination of the orbit) in 10 spectral channels from 425 - 1000 nm. As on MGS, the nearly-polar mapping orbit will slowly precess, synchronous with the Sun, to maintain a constant illumination angle, crossing the equator at about 4 PM local time. In two Earth years (one Mars year), the entire surface will have been surveyed. Before beginning in earnest on its own science objectives, MCO will serve as a radio-relay station in support of MPL. The old Viking orbiters served the same function for the Viking landers, back in 1976. Later, after its 2-year mapping objective is complete, MCO will be available for up to three additional years to support future missions as a command and data relay station.
MPL will deploy two "New Millennium Microprobes" from the discarded Earth-Mars cruise stage, which will impact independently about 200 km from the main MPL lander, and penetrate about 2 m into the subsurface to search for water ice. As on the Moon, the presence of water is critical for the economics of future exploration, since it is the raw material for H2/O2 rocket fuel. Unlike the Moon (until Lunar Prospector's discoveries early this year), the mere presence of water is fairly well assured on Mars; but the amount and distribution are yet to be determined in detail. The microprobes will be targeted at the layered terrain near the edge of the south polar cap. Each has a detachable penetrator that connects to a small surface station by a flexible cable. Besides looking for subsurface water, they will measure temperature and soil mechanical and thermal properties, all of which will be relayed to Earth via the surface transceiver and the MCO orbiter.
The main lander, like Pathfinder, will use a protective heatshield and a parachute to get rid of most of the energy it must shed before landing, but the final descent will use small rocket thrusters rather than the Pathfinder airbags. The landing target area is near the rim of the south polar cap at about 80° S latitude. A particularly interesting innovation is the Mars Descent Imager (MARDI), which will return pictures of the surface during the descent. The lander will be also be equipped with a surface stereo imaging camera, a microphone, a LIghtning Detection And Ranging experiment (LIDAR), and a trench-digging robotic arm to examine the subsurface soil conditions. It will also have the capability to monitor local weather, and to conduct analyses of gases and volatiles in soil samples retrieved by the robotic arm. The surface package is so complex, in fact, that a full description of its capabilities will have to await some future issue.
Meanwhile, MGS continues with the revised program that was developed in the wake of the realization last October that it has a dangerously weakened solar panel attachment. If you recall, the drag forces developed during the original aerobraking schedule threatened to rip off the panel, so a gentler program was devised, with the consequence that it will not reach the final mapping orbit until next January. Last November gentle aerobraking resumed, but (in accordance with the revised plan) it was already too late to reach the planned 2 PM orbit. This "Phase 1" braking continued until March 1998, when the period was 11.6 h. Since last March the spacecraft has been in what is termed a "science phasing orbit", meaning no braking is taking place as the orbital mechanics wait for the right moment to resume, in order to reach the goal of an orbit with the desired illumination angle. (Readers of this column may recall that when the low orbit is reached, the plane will begin to precess synchronously with the Sun, maintaining the correct angle despite Mar's orbital motion. But opportunities to reach the right angle occur only about once per year.)
Meanwhile, the scientists have busied themselves collecting all the data they can from the higher orbit; which (of course) turns out to be quite a lot. As most of you will have seen, spectacular images are being returned every week (not to mention the heartless destruction of that piece of fabled lore, "The Face on Mars"). In any event, aerobraking ("Phase 2") resumes in mid-September, and will continue for the rest of the year. After the final orbit is reached, the real mapping will begin, currently expected in March, 1999.
Just as I write it has been announced that Donna Shirley, who has managed JPL's Mars Exploration Program Office since it was founded four years ago, in the dark days after the 1993 Mars Orbiter disaster, is retiring. In a very real sense the current reborn Mars program is a monument to her efforts. In over 30 years at JPL she has made major contributions to numerous Lab programs, including the early design of the Cassini Saturn mission, the space station, and automated planetary rovers. She is a recipient of numerous awards, including the NASA Outstanding Leadership Medal. Most recently she has published a popular book, "Managing Martians", about her experiences with the Mars Pathfinder project, its little rover, and its people.
Donna and I became friends years ago when our kids were both tots at the JPL day care center, where we served together on the board of directors. She has been an inspiration ever since, a seemingly endless source of constructive energy and genial sanity. All who care about space exploration as a human enterprise will wish her well.