Goddard also supplied a separate Neutral Gas and Ion Mass Spectrometer (Ngims) to measure the composition and isotopes of ions and thermal neutrals in the atmosphere. LASP completed the instrument suite with an Imaging Ultraviolet Spectrograph (IUVS) for global remote sensing of the upper atmosphere and ionosphere at Mars.
“We designed a mission that would be able to tell us about the upper atmosphere, science instruments that would be able to tell us about the composition and structure, the escaping processes, the energy inputs from the Sun that drive it all, and then we put it on a spacecraft that will do two things during the mission,” says Jakosky. “It's in an elliptical orbit, so at the lower part of the orbit, at lower altitudes, it passes through the entire upper atmosphere and makes in-situ measurements. Then at the highest altitudes, we can do remote-sensing observations to extrapolate to global processes.”
That unusual approach to exploring Mars from orbit drove the spacecraft design. Because Maven does not have the imagers that Jakosky calls “data hogs,” with the exception of the IUVS, it can transmit all of its data back to Earth with two 5-hr. data dumps per week. Most of the time, the spacecraft will be set up with its reaction wheels to keep its fixed solar arrays pointed at the Sun. When it is time to transmit recorded data, controllers at the Lockheed Martin facility near Denver will slew the entire spacecraft so the high-gain antenna can link with the Deep Space Network for the call.
Five times during the nominal mission, which will last one Earth year, controllers will send the spacecraft into a “deep dive” for a look at the atmosphere closer to the planet. The two outer panels of the solar array wings are canted up to give the spacecraft more stability as it passes through the thicker atmosphere, but the spacecraft will not go deep enough to require thermal protection. Instead, it will be comparable to the “toe dips” that orbiters like the MRO made before plunging deeper into the atmosphere for aero-braking.
“Their toe dip is our deep dip,” says Jakosky. “We're still going to walk down gently so we don't screw it up.”
Controllers will be aided by periapsis time estimator software already uploaded to the MRO and baselined on Maven. It monitors the spacecraft's accelerometers and reaction wheels to determine where it is in its orbit based on actual conditions relative to the atmosphere, instead of relying on projections.
Science data will be collected at LASP, where university undergraduates sit on a console next to professional spacecraft controllers. Jakosky estimates it will take about three months to generate early results after orbit insertion and a 5.5-week calibration period. A more detailed report should be ready by the end of the nominal mission.
Still to be determined is whether it will be useful and possible to conduct any scientific measurements during the 10-month transit to Mars. “Everybody wants to, but it is not a requirement,” says Jakosky.
There should be good opportunities to observe Comet Ison shortly after launch, but there is a critical trajectory-correction maneuver three weeks into the mission, and managers do not want serendipitous observations to interfere.