Over the last few decades, Mars has become the most explored planet in the solar system (after Earth). It has hosted dozens of visitors. Indeed, eight missions currently operate on and around the Red Planet. The oldest of these arrived in 2001—the Mars Odyssey orbiter, which is expected to remain operational until 2025. More missions are planned as NASA, and perhaps other space agencies, work toward the goal of returning Mars samples to Earth and eventually sending humans.
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Both these goals will require much more detailed maps of the ground. Rovers provide this kind of detail but can travel only a few meters a day. NASA’s Opportunity rover traveled an impressive 45 kilometers (28 miles) during its mission but took almost 15 years to do it.
So planetary scientists are looking for ways to explore the planet’s surface more rapidly. One idea is to launch a plane into the Martian atmosphere with an engine capable of keeping it aloft. Such a mission could cover vast distances but would be expensive, costing upwards of $350 million.
A cheaper approach is to piggyback on a bigger mission. So another idea is for a Mars rover to carry a quadcopter that can survey the surrounding area. But such flights will be short and of limited range.
What’s needed is a vehicle that is small enough to piggyback on another mission and yet capable of covering large distances.
Today, Adrien Bouskela, Aman Chandra and colleagues at the University of Arizona say they’ve devised just such a mission. Their idea is to launch an unpowered glider into the Martian atmosphere and to keep it aloft by using thermal updrafts—rising columns of warm air—to gain altitude. The glider will be inflatable and able to pack into a volume small enough to be carried as a secondary payload on a bigger mission to Mars.
First some background. The spacecraft currently orbiting Mars are capable of shooting images with a resolution of 30 centimeters per pixel. Planetary scientists are gradually assembling these into a relatively high-resolution map of the Martian surface.
But they need higher-resolution images to plan future lander missions. Until now, most rovers have landed on relatively flat plains, where the chances of hitting a boulder or crater are small. These areas tend to be less interesting scientifically. Instead, planetary geologists would love to visit the valleys and ridges that seem to be shaped by water. Landing safely in such places will require imagery capable of resolving small rocks and craters that could damage the craft on impact.
Landers provide this kind of detail, being capable of pictures with a resolution of 1 centimeter per pixel over the 100-meter region around them. But they are not capable of traveling the vast distances necessary to reach regions of interest. Which is why scientists have turned their attention to flying machines.
Flying on Mars is a tricky business. While gravity is low, about a third of Earth’s, the atmosphere is only a hundredth as dense. That makes it hard to generate lift.
So the new glider’s wings will have to be huge relative to its weight. The glider will carry a 5-megapixel camera, a radio communications set, an onboard computer, and a set of solar panels with batteries to store enough power during the day to keep it alive at night. Bouskela and co calculate that in addition to the inflatable wings and rudder, the glider will be 5 kilograms—about the mass of a golden eagle.
They go on to calculate that to keep this mass aloft, the wings will need to have a span of almost 6 meters (20 feet) and fly at speeds of around 100 meters per second. By comparison, golden eagles can stay aloft on Earth with a wingspan of 2 meters while flying at 15 meters per second.
The glider will deploy during the main mission’s entry into the Martian atmosphere and be jettisoned from the main payload at an altitude of about 2 kilometers. A nitrogen generator will inflate and pressurize the glider in less than 10 seconds, and its wings will harden over the next hour or so as they are cured by the Martian sunlight.
Inflatable technology like this has already been tested on Mars. Both the Mars Pathfinder lander and the Mars MER rovers relied on airbags that inflated as the craft descended through the atmosphere and then cushioned their impact with the ground. “They are also being developed for small satellites as communication antennas,” say Bouskela and co.
An important part of the mission will be the autonomous ability to exploit rising currents in the Martian atmosphere. Such systems on Earth have demonstrated the ability to stay aloft for weeks or months.
If Mars has a wind profile like Earth’s—that is, a similar change in wind speed with altitude—then a similar approach should also work on the Red Planet. Indeed, the team have created numerical simulations of these kinds of flights on Mars.
The results suggest that this kind of autonomous soaring could keep the glider aloft for long periods provided there is a large enough change in wind speed with altitude. More work will be needed to determine whether that is actually the case on Mars throughout the year and across the surface.
Nevertheless, even a few hours of flight could generate valuable data. The onboard camera would have a ground resolution of about 10 centimeters, and at a speed of 100 meters per second, the glider could cover many hundreds of kilometers. The glider would send the images to an orbiting relay station such as the Mars Reconnaissance Orbiter, which would then send them home.
Such a mission could throw light on a number of outstanding Mars mysteries. One of these is the puzzling lines that form on some Martian slopes as they warm up when summer arrives. These are thought to be created by the flow of salty water, but better observations are desperately needed.
The glider might also help sound out potential landing spots for future missions. “The Mars sailplane concept proposed here provides a whole new avenue for accelerating exploration of the Martian surface at a fraction of the cost of previous Mars airplane concepts,” say Bouskela and co.
One thing the team lacks, however, is a sense of Hollywood. The most successful missions are often those that produce iconic images. Think of Neil Armstrong standing by an American flag on the surface of the moon, or Elon Musk’s starman driving a Tesla past planet Earth. Those kinds of images play a far bigger role in the public perception of success than any data plot.
What this glider needs is a tiny wide-angle camera on its tail rudder or wing tip that will send back images of the vehicle itself in flight. The money shot would be the glider soaring over the Valles Marineris—Mars’s Grand Canyon—with a Martian sun perched over an alien horizon and, perhaps, a pale blue dot nestled nearby. Indeed, the team has created an image just like this to promote its vehicle!
Neil Armstrong’s image cost a significant portion of the US economy. So at a cost of only $36 million—chicken feed for Mars mission planners—this Mars glider image would seem pretty good value for money. And if Bouskela and co are expecting the American public to pay for this mission, the least they could do is send a picture postcard home.
Ref: https://arxiv.org/abs/1902.02083 : Attitude Control of an Inflatable Sailplane for Mars Exploration
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