NASA的新火星着陆方式.doc

NASA Bets Big Rover On Novel Landing Scheme

Aug 1, 2011
By Frank Morring, Jr.
Washington
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NASA will try a completely new—and scary—technique to land the largest rover yet on the surface of Mars next year, in a spot that scientists hope will tell them whether the planet can, or ever could, support life.
2012年虽然是电影中的地球毁灭之日，不过NASA还是打算继续实施自己的火星探索计划。NASA计划在2012年发射一个大型漫游车到火星表面，用来进一步探究火星上是否曾经存在生命，是否可以供人类居住。这个漫游车名为“火星科学实验室”，从名字上就可以看出它功能多、个头大。“火星科学实验室”重850千克，有6个轮子，大小和常见的家用轿车差不多。所以，如果要用常见的伞降方式把它着陆到火星表面，就要携带一套巨大的降落伞。这对地球上的航空伞降和近地轨道飞船伞降返回来说，不是什么大问题，毕竟不需要把这套东西送离地面太远，消耗的飞机或者火箭燃料有限。但如果是火星登陆，为运送降落伞系统而消耗的燃料就有些难以承受了。因此，NASA打算另辟蹊径。
Crews from the Jet Propulsion Laboratory (JPL) and Kennedy Space Center are in the final stages of preparing the $2.5 billion Mars Science Laboratory (MSL) for an Atlas V launch to the red planet as early as Nov. 25. If all goes as planned, the probe will reach Mars next August and begin its harrowing 6-min. descent through the atmosphere.
After an entry that includes a temporary shift in the capsule’s center of gravity to gain lift before parachute deployment for a precision approach, the lander will go into a rocket-powered hover and lower the rover to the surface on nylon cords. Engineers at JPL adopted the “Sky Crane” landing technique because, at 850 kg (1,870 lb.), the six-wheel rover, dubbed Curiosity, is more than twice as heavy as the twin Mars Exploration rovers (MERs) combined. That makes car-sized Curiosity too large for the parachute/airbag combo used so successfully by the Pathfinder mission’s Sojourner rover and the two MERs, Spirit and Opportunity.
着陆舱进入火星大气层以后，要先调整重心位置来获得更大的升力，然后释放出一部“飞行吊车”，漫游车就用多根尼龙绳吊在下面。“飞行吊车”启动火箭发动机降低下降速度并控制姿态，最后打开降落伞实施精确着陆。JPL的工程师们
指出，MSL重850千克，大约是前两台火星漫游车重量之和的两倍还多。由于个头太大，已经不适用降落伞+气囊的降落方式了。
“It has to be a propulsive entry, and then it becomes a question of where do you put the propulsion stages,” says Peter Theisinger, MSL project manager at JPL. “Do you put it over the rover or under the rover? If you put it under the rover, you have the problem of how you deploy the rover. If you put it over the rover, then you think to yourself, ‘OK, what’s going to contact the surface?’ It’s the wheels, and the mobility system. They are already built to contact the surface. The only question is how hard you’re going to hit it.”
“必须采用动力再入的方式，所以，把推力系统放在什么地方就是个大问题了。”MSL项目经理彼得·泰辛格说，“把它放在漫游车底下吗？那漫游车怎么落地呢？放在漫游车上面的话，你自己想想会发生什么吧。什么东西会接触到火星表面？当然是轮子和行走系统。他们就是用来接触地面的，问题是我们打算让他们在落地的时候承受多大冲击力。”
Aside from the novel design for entry and landing, NASA, JPL and the MSL science team have spent a lot of time and brainpower figuring out exactly how the big rover should touch down. The mission’s objective is to move beyond the life-seeking “follow-the-water” approach of earlier landers. It is clear that water exists on Mars. The MSL will try to find out if it actually makes the planet habitable, and the choice of a landing site was critical.
除了再入和着陆方式上的创新以外，NASA和JPL海耗费了大量精力，研究大型漫游车触地时的精确状况。MSL的任务是继续跟随前几任着陆器所发现的水的痕迹，寻找火星上的生命。目前已经能够确认火星上存在谁。而MSL的任务是确认火星是否真的适合人类生存，因此为MSL选定合适的着陆点是非常关键的。
NASA says the rover will touch down next to a 5-km-tall mountain in the middle of Gale Crater, an equatorial feature that was considered too tricky for a landing by one of the less-accurate MERs. The Gale Crater site offers “a diverse number of environments over a long period of time; possibly tens to hundreds of millions of years,” according to Dawn Sumner of the University of California/Davis, who was co-chair of the site-selection working group.
NASA说，漫游车将会降落在盖勒火山坑中部一座5000米高山的旁边。此处位于火星赤道附近，NASA曾经发射的较低精度MER曾经认为此处地形过于复杂，不易降落。根据选址工作组主席，加利福尼亚大学的多恩·萨姆纳若说，盖勒火山的位置“提供了长期以来多种不同环境的样本；可能有数亿年之久。”
“That will give us a history of some of the ancient environments on Mars, how it has changed, and help us evaluate the habitability of the planet,” Sumner says, adding that the site should be “incredibly beautiful” and reminiscent of the U.S. desert southwest.
Following its launch on an Atlas V 541, with four solid-fuel strap-on boosters and a Centaur upper stage, the route to Gale Crater will be set with the first trajectory correction maneuver (TCM). Depending on when in the Nov. 25-Dec. 18 launch window the mission actually lifts off from Cape Canaveral AFS, Fla., the spacecraft will use some or all of its eight hydrazine thrusters to begin homing in on the landing site at the same time that controllers bias its trajectory away from the aimpoint to avoid accidental contamination of the planet if they lose control later in the mission.
The gross trajectory set up at that first TCM 15 days after launch will be gradually refined as the cruise stage approaches Mars. However, the time of flight to the planet established then will bring the spacecraft into the atmosphere at a point calculated to land the rover within an ellipse 20 km long by 25 km wide. That target probably will wind up “significantly smaller” once the trajectory is better understood during cruise, according to Theisinger.
The transit to Mars will take about nine months, with additional TCMs and health-monitoring contacts through NASA’s Deep Space Network. As the spacecraft approaches the planet, it jettisons the disk-shaped cruise stage to expose the descent capsule, consisting of a backshell and heat shield. It also jettisons ballast—tungsten cylinders—to move the center of gravity so it can gain lift as it descends into the atmosphere.
窗体底端
The spacecraft will be moving at almost 6 km/sec. when it impacts the thin Martian atmosphere, so the entry, descent and landing sequence is autonomous. The navigation software uploaded during cruise fires the thrusters to rotate the spacecraft and direct its lift so that it literally flies toward the landing zone, accommodating variations in atmospheric drag as it goes. The 4.5-meter-dia. (14.8-ft.) phenolic impregnated carbon ablator (PICA) heat shield, the largest ever used for an atmospheric entry, protects the rover as it descends.
After deploying more ballast to restore the center of gravity and dropping the heat shield, the computers deploy a supersonic parachute at an altitude of roughly 7 km. That slows the vehicle to 100 meters/sec. at an altitude of 1.8 km, where the descent stage fires up its eight Viking-heritage descent rockets inside the composite backshell, and then drops out to hover above the surface.
At this point the mission will draw heavily on the same 1-meter-resolution imagery from the Mars Reconnaissance Orbiter (MRO) that helped scientists pick the landing site. The MRO images will be matched with imagery collected from an altitude of 3.7 km by a camera on the bottom of the rover.
“With the MRO images . . . we actually can see the terrain,” Theisinger says. “We now are in this position where we can actually see rocks that will break us, as opposed to previous missions where we had to guess rock distributions or model rock distributions.”
The landing system will position the rover over a safe touchdown spot and lower it on a set of three nylon cables and a power-and-data umbilical to a distance of 7.5 meters, then throttle down its rockets to descend until the rover contacts the surface. Even though the system has never been tried, project managers are confident it will work.
“You build a very good radar and a very good throttleable propulsion system,” Theisinger says. “So you land at a few tenths of a meter per second, and you’re good.”
The confidence is based on heritage and testing, he says. In addition to the Viking heritage in the 3.1-kn-thrust descent engines, supersonic parachutes were used on Viking, Mars Pathfinder and the MERs.
“At the beginning there was a lot of skepticism,” Theisinger says. “They’ve done probably 50 reviews of [entry, descent and landing] in various manifestations and pieces of it. The viability of the sky crane and the verification and validation program for the sky crane [gave] a very early review on the project to make sure that [despite] everyone’s initial skepticism . . . that there was in fact good engineering behind it.”
At the same time, the mission’s science team has whittled down a list of potential landing sites from more than 50, to four, then two and finally to Gale Crater. The care was necessary because of the subtlety of the habitability question.
“When we use the term ‘habitability,’ this is the next step forward,” says John Grotzinger, MSL project scientist at JPL. “To have a habitable environment, you’ve got to have water; you’ve got to have a source of energy for a microorganism to utilize in order to do its metabolism, and you’ve got to have a source of carbon—for life as we know it on Earth. So we start with carbon as the basic building block of all life, and now we’re not just looking for water. . . . We need a more sophisticated understanding of how microorganisms work.”
The principal objective of the MSL will be to learn as much as possible about how these three factors—water, energy and carbon—have interacted over time. Most of that work will involve detailed study of the mineralogy of hydrated rock and soil at the landing site—mainly clays and sulfated salts—although there is a slim chance the rover may stumble across organic molecules that have been preserved in the rock record. One of the attractions of the Gale Crater site, aside from the layers of sedimentary rock rising from an alluvial fan in the landing zone, is a water-cut canyon that the rover should be able to climb.
“After the deposition of all these layers, we had a time when you had water flowing down the mountain, and that cut a canyon a lot like the Grand Canyon,” says Sumner. “That gives us the opportunity to read the environments through time and those changes, but that canyon-cutting event also represents an environment that could have been habitable. You have the flowing water; you have erosion and you have deposition of those sediments at the mouth of the canyon.”
To move around the terrain, JPL has built a 900-kg rover that stands 7 ft. tall and has a 7-ft. reach with its sophisticated robotic arm. For stability, the six-wheeled rocker-bogie suspension system carries a body measuring 10 ft. long by 7 ft. wide. For endurance, its wheels are turned by brushless electric motors; and for power on the planet’s dusty surface, it eschews the solar arrays of past rovers for a plutonium-238 fueled radioisotope thermoelectric generator that produces 110 watts of electricity and warms sensitive components inside the rover body.
“This rover was built to go 20 km and last two Earth years,” Grotzinger says. “One of the things that we intend to do on this mission is rove. Spirit and Opportunity were intended to drive around in a relatively small area, and it turned out that they became spectacularly successful in roving. So armed with that confidence, we built a rover that we think will last, hopefully, two years, and if it scales the same way that the mission did for Spirit and Opportunity, which was supposed to have been three months, we could be at this for an awfully long time.”
MSL carries a sophisticated set of tools and an internal chemistry lab, with a suite of instruments that would be the envy of most university geology departments back on Earth. More than half of the rover’s science payload is taken up with the Sample Analysis At Mars (SAM) suite of three instruments—a quadrupole mass spectrometer, gas chromatograph and tunable laser spectrometers.
Taken together, the three instruments are designed to analyze atmospheric and solid-material samples to inventory carbon compounds and lighter elements and their isotopes near the planet’s surface. The SAM’s sample-handling hardware can heat soils and rocks collected by the rover’s robotic arm to 1000C in a gaseous helium stream, reusing the 59 quartz sampling cups over and over by baking them clean between tests.
Also working with samples collected by the arm’s drill and other specialized tools will be the Chemistry and Mineralogy instrument, which will use X-ray diffraction to identify and measure minerals in soil and rock samples delivered by the arm. Mounted outside on the rover’s mast, the Chemistry & Camera instrument will fire laser pulses at rocks as far away as 7 meters, and use a telescope to funnel light from the resulting plasma to a set of three spectrographs inside the rover body via fiber-optic cable for analysis of minerals and microstructures in the rock (AW&ST Oct. 4, 2010, p. 24). That will help scientists decide if they want the rover to move in for some direct sampling, says Grotzinger.
Also available for analyzing the composition of rocks and soils is the Alpha Particle X-Ray Spectrometer, a direct-contact probe that uses a curium source to generate X-rays and alpha particles. When they interact with the in-situ sample as the probe is held against it, X-rays are produced for analysis. Earlier versions were carried on the two MERs and Pathfinder’s Sojourner rover.
Working in association with the contact spectrometer will be the Mars Hand Lens Imager, a close-up camera that will enable scientists on Earth to look at features smaller than the diameter of a human hair as they decide where to place the instrument. Both will be mounted at the end of the robotic arm, in a structure called the turret that can rotate 360 deg. Also in the turret are three tools for collecting and preparing samples of soil and rock for analysis inside the rover body.
The elaborate instrumentation is necessary because the science team wants to know how water interacted with the rest of the Martian environment, and because the search for carbon is vastly more difficult than looking for evidence of water.
“Only a tiny subset of those [possible habitable] environments might ever preserve organic carbon, if it had been there to begin with,” says Grotzinger. “So we talk not only about habitability, but we also talk about preservation. Everybody has to understand how hard this is on Earth. On a planet that is teeming with life, it almost never gets preserved in the rock record.”
While the MSL instruments will do more than follow the water, water remains a key element in the search for habitable zones past and present. To search for subsurface water, most likely either as ice or bound in mineral crystals, MSL will use the Dynamic Albedo of Neutrons instrument. Funded by the Russian space agency, it sends a neutron beam into the ground to detect water content as low as 0.1%. It should be able to resolve layers of water down to 2 meters below the surface.
Facing in the opposite direction will be the Radiation Assessment Detector, which will measure high-energy radiation from space and from the interaction of gamma rays and energetic particles with surface materials. The data will help scientists gauge the radiation threat to any Martian microbes and to future human explorers, and to learn more about how the radiation environment has shaped the composition of the planet’s surface.
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