Next U.S. Mars orbiter will focus on the planet's escaping atmosphere
The spectacular images 's Curiosity rover has returned from the surface of Mars reveal an ultra-dry environment, like the Mojave Desert after a 3-billion-year drought.
Data from Curiosity and its predecessors make clear that water ran there once and the planet probably was habitable. Over the eons, something happened. The next U.S. mission to Mars will look to the red planet's pink skies for clues as to what caused the Martian climate to change so dramatically.
“Today we see a cold, dry atmosphere,” says Bruce Jakosky, principal investigator on the Mars Atmosphere and Volatile Evolution (Maven) mission. “Where did the water go? Where did the carbon dioxide from an early thick atmosphere go? What really drove the climate change that we see evidence for on Mars?”
After examining the planet's surface in spectacular detail for decades, scientists are ready to dip into the atmosphere from orbit to expand the search. Drawing on heritage from earlier spacecraft that aero-braked to achieve orbit around Mars, Maven will use its bat-like solar arrays for stability as it skims through the thin upper atmosphere from elliptical orbit and makes the occasional “deep dive” for in-situ measurements.
“There are two places that the atmosphere can go,” says Jakosky, a planetary scientist at the University of Colorado's Laboratory for Atmospheric and Space Physics (LASP) in Boulder. “It can go down into the crust, and we don't see evidence for the big reservoir of crustal minerals that would be indicative that that's where it has gone. The other place it can go is up, and be lost to space. Most of the effort over the past several decades has focused on the surface and subsurface. We're the first mission devoted solely to understanding the upper atmosphere and the role of loss to space.”
Set for launch on an Atlas V 401 in a 20-day window that opens Nov. 18, Maven is a relatively low-cost Scout-class mission. It was the onlymission to the red planet left standing after the agency bailed out of a three-year cooperative-exploration planning effort with the European Space Agency (AW&ST Feb. 20, 2012, p. 33).
Capped at $671 million—including the launch vehicle, reserves and an Electra UHF transceiver that will serve as a backup communications relay for future Mars surface missions—Maven has reachedon budget and schedule for final testing and integration with the Atlas. Jakosky and other mission managers attribute that achievement to heritage hardware and a willingness to resist “requirements creep” after NASA selected the mission in 2008.
“We gave serious consideration to a number of add-ons, but in the end we accepted none of them,” says Jakosky. “The cost, the risk, the fact that it didn't benefit our mission—really, when you come in with a focused science mission for a Mars Scout-class mission, you have to do that. You have to pick and choose what you're going to do. One of the things that I'm proud of is that the original concept we proposed is what we're flying. We resisted science creep; we resisted engineering creep.”
Also contributing to the relatively painless development was a high degree of commonality with previous interplanetary spacecraft, including the Mars Reconnaissance Orbiter (MRO), the twin Stereo solar probes, the solar-powered Juno mission to Jupiter and the twin Grail lunar-gravity mappers.
“We really wanted to take, not only the heritage of the designs from previous programs, but we also really wanted to maximize the heritage of the people and the heritage of the tools that they used, the procedures,” says Guy Beutelschies, Maven program manager at mission prime contractor. “We really wanted to take all that and deliver a high-quality product while paying a lot of attention to the cost and the schedule.”
The basic structure of the Maven spacecraft follows the same design as the MRO, which is orbiting the planet now. Essentially, it is a hexagon of composite panels with the propellant tank in the center and the rest of the components and instruments hung on the outside or on an articulated payload platform that swings out from the main body. The hydrazine tank is larger than the MRO's to accommodate the particular needs of the mission, and the arrays and high-gain antenna are fixed instead of gimbaled for the same reasons.
Similarly, the instrument suite draws on heritage hardware to minimize development time and expense. “Nothing is being done from scratch,” says Jakosky. “Instrument development for the most part went smoothly.”
The principal investigator credits the planetary science managers at NASA headquarters for helping the Maven team stay on schedule and budget by keeping a steady and adequate stream of funding flowing. Also contributing was the tight planetary launch window, which focused mission development on simplicity as much as did the cost cap.
“The planetary launch period really drives a sense of urgency on the team,” says David F. Mitchell of, the NASA project manager on Maven. “When you can launch any day of the year, every year, there can be a delay in making decisions sometimes. When we have approximately 20 days to launch, and then you stand down for 26 months, you're breaking the program. You're breaking it financially and, in the case of Bruce's science, you're breaking it because of the different timing with the solar cycle.”
Maven will arrive at Mars on Sept. 22, 2014, assuming launch at the beginning of the planetary window. A 39-min. burn of its main thruster, followed by five orbital adjustment maneuvers, should leave the 903-kg (1,990-lb.) spacecraft in a 6,200 X 150-km (3,850 X 93-mi.) science-mapping orbit inclined 75 deg. to Mars's equator.
At that point, the Sun will be just past solar maximum, giving the science team a chance to watch how the atmosphere reacts as the Sun's influence wanes. It will also be a good time in the 11-year solar cycle for major solar storms, so the spacecraft is well-equipped to measure their impacts on the atmosphere.
“During our mission we're hoping to see a handful of major storm events hit Mars and see what the reaction is,” Jakosky says. “If we are successful and are able to continue on in an extended mission, we have enough fuel to go from just after solar maximum to solar minimum and back up to solar maximum again. We may last as long as a decade.”
Changes in solar activity will be crucial to the measurements Maven is designed to make at the interface between atmosphere and space. If the thick Martian atmosphere disappeared into space, taking most of the planet's water with it, that is where the action would have been.
“The working model that we have is one in which, early in the history of the Solar System, the more intense extreme ultraviolet light from the Sun, the more intense solar wind, was able to strip off the atmosphere,” he says. “We think this happened very early. Prior to that, there was a thick atmosphere. Mars has a strong magnetic field that we see evidence today for it having been there 4 billion years ago. That magnetic field protected the planet; it stood off the solar wind, if you will, so it didn't hit the upper atmosphere and strip it off.”
When Mars lost its magnetic field, the theory goes, it lost that protection and with it the environment that may have made it habitable.
“Although we can't measure how it happened 4 billion years ago, we can examine the same processes as they're operating today, and learn enough to allow us to extrapolate back to the early history, and to determine the integrated loss over time,” says Jakosky.
Scout-class missions are managed from the beginning by the principal investigator, and Jakosky says he was fortunate in that he and his team “came as close as you can imagine to starting with a clean sheet—What do we want to learn? And what instruments can tell us what we need?”
In the end, the team selected eight instruments to study the upper atmosphere and ionosphere of Mars, and to conduct global remote sensing as the elliptical orbit moves toward and away from apogee.
The instrument suite consists of a Particles and Fields Package built by the Space Sciences Laboratory (SSL) at the University of California, Berkeley. Within the package are the Solar Wind Electron Analyzer (SWEA) to measure solar winds and electrons in the Martian ionosphere; the Solar Wind Ion Analyzer (SWIA), to measure solar wind and ion density and velocity in the planet's magnetosheath; the Suprathermal and Thermal Ion Composition (Static) instrument, which will measure ions in the atmosphere of Mars, including moderate energy escaping ions, and the Solar Energetic Particle (SEP) instrument to measure the impact of the solar wind on the planet's upper atmosphere.
The SWEA, SWIA, Static and SEP instruments all were provided by Berkeley's SSL. Also in the Particles and Fields Package are the Langmuir Probe and Waves (LPW) instrument supplied by LASP, which includes an extreme ultraviolet sensor. The LPW instrument will measure properties of the ionosphere, wave-heating in the upper atmosphere and extreme ultraviolet inputs into the atmosphere from the Sun. And Goddard Space Flight Center rounded out the package with a magnetometer, which will measure interplanetary solar wind and magnetic fields in the ionosphere.
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.
“It's going really well right now, but we've got to get to Mars,” said Mitchell, the project manager, as technicians packed Maven for shipment from Lockheed Martin to Kennedy Space Center in July. “Anybody can look at the history of getting to Mars to see it isn't the best. So we're not complacent.”
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