Out of this world: The design challenges of a mission to Mars
Mars still holds a particularly cherished place in the minds of all those concerned with space exploration. After all, we are fortunate enough to live in a time where we take all the benefits bestowed by satellite communications for granted and in which rockets heading into space barely garner a mention in the news. Given this, it takes something special to excite our interest and – in that context – Mars is it.
This is because, as our closest neighbour in the solar system and the planet that has become synonymous with alien life forms, Mars exercises a particular fascination in the human mind.
Science fiction aside, however, the reality is that Mars is more easily explored and more capable of sustaining life than any of the other planets in the Solar System. This fact has made it a prime target for exploration, a process that has been ongoing for some years now and whose latest incarnation is the MAVEN (Mars Atmospheric and Volatile EvolutioN).
MAVEN is the second mission selected for NASA's Mars Scout programme, an initiative for smaller, low-cost, competed missions led by a principal investigator. Responsive to high-priority science goals listed in the National Academy of Science's 2003 decadal survey on planetary exploration, MAVEN will obtain critical measurements of the Martian atmosphere to help understand dramatic climate change on the red planet over its history.
Long ago, Mars had a denser atmosphere that supported liquid water on the surface. At that time, the planet might have had environmental conditions to support microbial life, as the long-term presence of water is necessary to life as we know it. However, as part of dramatic climate change, most of the Martian atmosphere was lost to space long ago. Features such as dry channels and minerals that typically form in water remain to provide a record of Mars' watery past, but the thin Martian atmosphere no longer allows water to be stable at the surface.
MAVEN will provide information on how and how fast atmospheric gases are being lost to space today, and infer from those detailed studies what happened in the past. Studying how the Martian atmosphere was lost to space can reveal clues about the impact that change had on the Martian climate, geologic, and geochemical conditions over time, all of which are important in understanding whether Mars had an environment able to support life.
The first spacecraft ever to make direct measurements of the Martian atmosphere, MAVEN will carry eight science instruments that will take measurements of the upper Martian atmosphere during one Earth year, equivalent to about half of a Martian year. MAVEN will also dip to an altitude 80 miles above the planet to sample Mars' entire upper atmosphere. The spacecraft may also provide communications relay support for future landers and rovers on the Martian surface, much as Mars Odyssey and Mars Reconnaissance Orbiter have done for the Mars Exploration Rovers and Phoenix.
The craft weighs 1,784lb and will carry vital research equipment developed by partners, including NASA's Goddard Space Flight Centre and their Jet Propulsion Laboratory, the University of Colorado at Boulder Laboratory for Atmospheric and Space Physics and the University of California at Berkley Space Sciences Laboratory.
Guy Beutelschies is MAVEN project manager and the chief systems engineer at Lockheed Martin, which is the company leading the way on the project. Not only has it built the spacecraft for NASA, it will be responsible for the launch of the spacecraft and mission operations as it makes its ten month journey towards the planet.
Beutelschies says of the mission: "There are certain things that all Mars Missions share and certain design challenges they all pose. The first of these is distance. With a Mars Mission, it can take up to 20 minutes for any signal to reach Earth and another 20 minutes to respond. This means that there is a real need to put enough 'smarts' on board to deal with the time lag."
The 'smarts' in this context include software that tells the craft exactly where it is in its orbit to allow it to automate a certain number of procedures. Says Beutelschies: "Because we only communicate with the spacecraft twice a week, we needed to put some software onboard to let the spacecraft know where it was in its orbit because the orbit changes over time and we have investigations we want to trigger when it is at periapsis [the point at which an orbiting object is closest to the body it is orbiting] and is actually grazing the upper portion of Martian atmosphere."
This development of this sort of automation also serves another key purpose with a view to future, manned space exploration. Says Beutelschies: "Everybody's trying to make spacecraft more affordable. We tend to focus on the hardware, but it's easy to forget that people cost a lot of money as well. The more we can automate, the fewer people we need and the more money we can save."
The other big obstacle such a mission faces is what Beutelschies calls 'the thermal challenge'. "Put simply," he says, "it's hotter near earth and colder near Mars. That poses great difficulties in terms of coping with the effects that those wildly differing thermal conditions have on the instruments and the vehicle itself. This is further complicated by the fact that heat costs power."
Says Beutelschies: "Lockheed Martin has built every NASA Mars Orbiter so far, so we have a great deal of expertise to fall back on." One of the key areas in which this expertise is made manifest is in the highly-robust testing procedures and dynamic atmospheric simulation models the company has developed.
Summarising some of the other challenges that face the mission, Beutelschies says: "How do you make sure a spacecraft can survive in space? Facing the sun, surfaces can get hotter than any desert. In the shade, it is colder than any winter in Antarctica. The vacuum of space can wreak havoc if you don't use the right materials. Launch is even tougher. If you've ever been lucky enough to see a launch in person, you can feel the vibration rumbling in your chest from over a mile away. Now imagine what the spacecraft is experiencing as it sits on top of that 'controlled explosion'."
Naturally, a space module has to undergo a huge amount of testing. After all, as Beutelschies puts it: "Once we launch, there is no bringing it back to the shop for repairs. It's not like you can just push it through the atmosphere to see if it holds up OK." These tests are arranged in roughly the same order as the spacecraft will experience in its mission. That means that launch is addressed first. It may not be readily obvious, but the sound during launch is so intense that it can actually cause damage. To simulate this, Lockheed Martin put the spacecraft in a special test chamber with enormous speakers and crank up the sound to deafening levels.
Next comes a vibration test. Here, the spacecraft is put on a large device called a shaker table that moves a plate back and forth to provide the vibration that the vehicle will get on the rocket. After shaking it in the horizontal axes, the piston-like device on the shaker table is rotated in such a way that it moves the spacecraft up and down.
Soon after launch, the solar arrays have to be deployed. This is a difficult test to perform on Earth because of the gravity. This makes it necessary to make sure gravity is not 'helping' the arrays deploy during ground testing. To mitigate this, it is necessary to turn the spacecraft and deploy the arrays to the side so that the hinges are perpendicular to the ground. Special stands were also used to support the weight of each array while allowing them to move freely across the floor as they deploy. The team followed the same process for deployment tests on the Articulated Payload Platform boom and the Solar Wind Electron Analyzer boom.
The third major environmental test involves radio waves. Most of the regular tests use cables between the ground equipment and the spacecraft to send commands and receive telemetry. During the mission, of course, the only link to the spacecraft is through radio signals.
For this test, a special acoustics chamber is set up to block out all outside radio signals that might interfere, and then special ground antennae were used to talk to the spacecraft antennae as if MAVEN were out in space. The tests also make sure that the various portions of the spacecraft do not interfere with each other and that the radio waves from the ground do not interfere with equipment on the spacecraft.
The final environmental test is the biggest. The MAVEN spacecraft is put in a thermal vacuum chamber ("think giant thermos bottle", says Beutelschies). Once inside, the air is pumped out and the hollow walls flooded with liquid nitrogen, bringing the temperature down to -290 degrees Fahrenheit. Giant lamps in the ceiling simulate the direct heat from the sun. This test makes the spacecraft "feel" as if it's in space.
The team spend several weeks in vacuum simulating the entire mission from the heat of the sun you get being near Earth, to the cold it will experience in the shadow of Mars. It is almost as tough on the team as it is on the spacecraft because the test consoles need to be monitored around the clock for the entire test.
Says Beutelschies: "This regimen of environmental testing may sound like a lot of work, but after spending years designing and building it, we want to make sure everything work correctly. If something needs to be fixed, we want to learn about it while it here on the ground."
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