Frequent visitors to Vintage Space are doubtless aware that I am fascinated with the problem of landing from space. Faced with this unknown, the US and Soviet Union developed very different methods, parachute-controlled descent and splashdown and Earth-landing via parachutes, retrorockets, and pilot ejection respectively. (Pictured, the view from Viking 1, the first successful robotic landing on Mars. 1976.)
Part of what interests me in studying landings is the lack of attention paid to this critical mission phase in favour of the more exciting launches. But there is one area were landings are not only a major focus but a vital aspect of a mission: robotic planetary exploration. Without a successful landing, there could be no robotic mission.
Like manned return from space, planetary landings have developed and become increasingly sophisticated over time. The more scientists and engineers know about a planet, the better chance they have of successfully touching down on its surface. After all, each body in the solar system has different characteristics and presents difference challenged to the entry, descent, and landing (EDL) stage.
Robotic planetary exploration began in tandem with early manned spaceflight. The central US/Soviet race to the moon eclipsed the race to secure a planetary first. In the US, the planetary effort has and continues to be linked to the Jet Propulsion Laboratory, one of the research centre that was absorbed into NASA after its 1958 inception.
NASA had the same enthusiasm for its unmanned program as it did for its manned program. In developing robotic exploration goals in the late 1950s, JPL and NASA decided on a program with grand goals. They would develop new launch vehicles and spacecraft capable of sending probes to planets late in the 1960s. A manned landing on other worlds would be accomplished by 1980.
The challenges facing a planetary landing are many. First is the problem of how to get there. Since all the planets orbit relative to one another, there are better and worse times to launch a spacecraft to, say, Mars. An efficient launch uses the orbit of the Earth as additional propellant for the spacecraft, which is launched into an orbital pattern such that it will intercept Mars months later. (Pictured: a schematic representation of the journey from the Earth to Mars. This is the path taken by the Mars Reconnaissance Orbiter which entered into Martian orbit in 2006. The positions of both planet at launch and arrival are clearly marked.)
If the spacecraft is going to land from orbit around the target planet, it must first slow itself to get into orbit. This requires a perfectly timed retrofire burn, slowing the spacecraft enough that it becomes a temporary artificial satellite.
To land, the spacecraft has to slow down further from orbit to begin its fall into the atmosphere. At this point, each planet’s unique characteristics come to bear on the landing method. Differences in atmospheric density, gravity, and surface features all play a part in determining the best landing method. The type of payload landing on the planet also affects the choice of landing method. The payload has to survive the landing if the mission is going to be any kind of success.
Two planets in our solar system have been targets for robotic exploration since the dawn of the space age: Mars and Venus. Both planets have strikingly different surface features and conditions and have seen very different landing methods.
The brightest point our night sky, Venus is the closest planet to Earth. At its closest it is 23.7 million miles away and 162 million miles at its furthest. Venus shares some similarities with Earth; its similar size and mass have led to its nickname of “Earth’s Twin”. That, however, is where the similarities end.
The planet has a carbon dioxide-rich atmosphere 93 times thicker than the Earth’s – a spacecraft on its surface has to withstand 92 times the pressure it experienced on Earth. This carbon dioxide-rich atmosphere has the effect of heating Venus’ surface with a runaway greenhouse effect. Temperatures on the surface are known to have reached as high as 461 degrees Celsius (roughly 861 Fahrenheit).
These planetary features provide certain benefits and detractors when landing on Venus’ surface. The principle challenges are the heat and atmospheric pressure the lander has to sustain. But the atmosphere that threatens to crush a landed spacecraft actually makes its descent easier. A thicker atmosphere provides significant drag throughout the descent stage. A parachute is also an extremely effecting slowing mechanism in such an atmosphere.
The Soviet Union achieved the first successful landing on Venus. The Venera program developed in the 1960s sent a series of landers to the planet.
The first three, Veneras 4, 5, and 6, were crushed by the atmospheric pressure before landing. Veneras 7 and 8 landed successfully in 1970, sending surface temperature and atmospheric composition measurements to Earth. Venera 9 returned the first images from the surface of the planet. It functioned for a little under an hour before rising temperatures in its core halted communication with Earth. (Pictured, a landing test of Venera 9.)
In the early 1980s, the Venera program gave way to the Vega program that continued Venusian exploration as a side effect of the Soviet focus on the comet Halley. En route to the comet, Vega dropped balloons into Venus’ atmosphere to measure planetary winds.
For their part, the US has spent its Venusian energies in orbit, measuring the structure and composition of the atmosphere and thick cloud layers. Mars is where NASA has achieved many successful landings, as well as some spectacular failures.
At the risk of stating the obvious, Mars is a strikingly different planet than Venus. It is much smaller than Earth; the surface of the planet is as large as Earth’s continents put together. Because of its small size, the pull of gravity on the surface is less. One effect of weaker gravitation pull is that the Martian atmosphere has all but escaped the planet. The atmospheric pressure on Mars is about 1 percent of that felt on Earth. Mars’ scant atmosphere is comprised largely of carbon dioxide with minimal amounts of nitrogen and argon.
Its thin atmosphere affects descending spacecraft; there is very little drag produced as the spacecraft moves through the atmosphere. Parachutes, too, are only somewhat effective. A thinner atmosphere means there is less air pressure moving through the fabric to inflate the chute.
An additional challenge in any planetary landing is the communications delay, in this case the time it takes for signals to travel between Earth and Mars. A signal takes roughly 20 minutes to travel one way, which makes real-time adjustments to a landing sequence impossible – the time from atmospheric entry to landing on Mars is about six minutes.
The completed EDL sequence must therefore be preprogrammed into the spacecraft’s onboard computer. It must be able to sense its altitude, orientation, lateral movement, as well as speed for the landing to work.
The Soviet Union actually reached the surface of Mars before the Americans. The Mars 2 lander entered the Martian atmosphere on November 27, 1971. Its angle of descent was too steep, and it crashed into the surface. Its sister lander, Mars 3, met a similar end.
NASA’s Mars program got a boost in 1968 when President Johnson called for the country to achieve a Martian landing to mark its bicentennial in 1976.
The US’s first Mars lander, Viking (pictured), was born in the Apollo era; its design is reminiscent of the Apollo Lunar Module. The lander is boxy on top, covered with scientific equipment, with four splindly legs to provide clearance of surrounding rocks. It was also born in a time when NASA could design and build spacecraft free from budgetary constraints.
It is worth mentioning here that at this point in its life, NASA had never successfully landed a manned spacecraft on land, something the Soviets had accomplished and used exclusively in their manned program. The two 1,323 pound Viking landers used a ballistic EDL method relying on parachutes and retrorockets.
During its trip to Mars, the lander was stored inside a protective casing made of a two-piece heat-shielding ‘sphere-cone bioshield’. Once Viking was in orbit around the planet, the lander in its conical bioshield casing was jettisoned, beginning its entry into the Martian atmosphere.
After atmospheric entry, the lander was on its way down, and only a perfect sequence of event could ensure its survival. First, the upper and lower bioshield covers were jettisoned, revealing a tripartite descent lander: the physical lander with its leg stowed was sandwiched between a cover housing a parachute and an aeroshell and ablative heatshield base. As the lander began its descent, the ablative heat shield slowed the craft as it plunged through the atmosphere.
Around 4 miles above the surface, the lander’s 52-foot parachutes deployed, stabilizing and slowing the spacecraft to about 200 feet per second. The aeroshell and heatshield were then jettisoned, exposing the lander from the bottom and allowing it to unfold its legs. A little under a mile above the surface, the lander separated from the parachute and its retrorockets ignited, slowing the lander to about 7 feet per second. The rockets shut off once the lander made contact with the surface.
The Viking 1 lander was 16 days late in joining America’s bicentennial, reaching the Martian surface on July 20, 1976. Its twin lander, Viking 2, touched down on September 3 of the same year. Both Viking landers were unscathed and functioned perfectly upon deployment, functioning into the early 1980s.
After the success of Viking, NASA’s interest in landing on the red planet waned. It was almost 20 years later that the next mission landed on the Martian surface.
Mars Pathfinder was NASA’s next mission to Mars. Unlike its predecessor, Pathfinder was designed and built under a strict budget. Legged landers, like Viking, are fairly delicate. The landing has to be soft and on even enough ground to prevent a leg from snapping off, effectively killing or seriously wounding the mission. This in turn translates to a more difficult, precise, and expensive landing – luxuries Pathfinder couldn’t afford. For Pathfinder to get to Mars, it would have to hit a wider taget area at faster speeds.
The solution from JPL was to mate the entry and descent technology of Viking with a series of airbags. The airbags would allow the lander to free-fall through its final descent and bounce along the surface before coming to its final resting point, thus negating the need for expensive precision landing instruments. (Pictured: the airbags used on Pathfinder. This image is actually from the MER missions, but the airbags are more or less the same.)
Pathfinder also differed from Viking in the design of the lander. Since it was going to be housed in a series of airbags, the lander would have to be stored within the bouncing, rolling mass. Pathfinder was built as a pyramid, the sides of which folded down like petals once it stopped rolling and the airbags deflated.
Pathfinder was packed like Viking for its journey to Mars. The pyramid-shaped lander nested between a cover (also called a backshell) containing a parachute and retrorockets and a base with an ablative heat shield.
Unlike Viking, Pathfinder entered the Martian atmosphere directly – it didn’t spend anytime in orbit. Its entry speed was about 16,600 miles per hour – nearly 80 times the entry velocity of Viking. Between 4 and 6 miles above the surface, Pathfinder’s parachute deployed, followed 20 seconds later by the automatic jettison of the entry craft’s heat shield.
With the heat shield gone, the lander released from the backshell. A 65-foot long cable or bridle kept the two pieces attached. At about a mile above the surface, the airbags around the pyramid-shaped lander inflated explosively. This triggered ignition of the retrorockets on the backshell a safe 65-feet above the airbags.
Prior to impact, the bridle released. The lander fell the rest of the way to the surface, bouncing and rolling until it eventually lost momentum. The retrorockets on the backshell continued to fire to keep it physically distant form the lander. (Pictured: An artist’s impression of the Pathfinder landing. The lander bounces along as the retrorockets get the backshell and parachute clear of the area.)
Once still, the airbags deflated and the petals of the lander opened, revealing Pathfinder’s main cargo – a little rover named Sojourner, the first vehicle to remotely explore another planet. (Pictured: Sojourner on the lander. The deflated airbags are clearly visible.)
Like Viking, Pathfinder was immensely successful. EDL worked according to plan and delivered a perfectly healthy rover to the surface.
Subsequent missions to Mars used variations of these two landing techniques, the technology of Viking making up the backbone of NASA’s Martian landing systems.
The 2007 Mars Phoenix Lander mission mimicked Viking. The principle difference was in the hardware; newer and slightly different technology allowed JPL to build a lander with direction stability in its retrorocket-controlled descent for less money. The principle method, however, remained unchanged. This same retrorocket-powered descent was also used for the Mars Polar Lander mission, which was lost during its 1999 landing attempt. (Pictured: Artist’s concept of Mars Phoenix powered landing, a similar system to Viking.)
The 2004 Mars Exploration Rover missions used the same airbag landing technique as Pathfinder to deliver two 387-pound rovers Spirit and Opportunity to the Martian surface. The methods were so similar that the lander actually had the same dimensions. Spirit and Opportunity were designed to fold up and fit into the same lander capacity as Sojourner. The MER rovers, however, were both larger and about 50 percent heavier than their Pathfinder predecessor. (Pictured: Spirit all ready to go. Those are the packed airbags on the surface of the lander petals.)
The MER landing sites were also more hazardous, a necessary tradeoff when looking for certain things on the planet’s surface. Observation of MER landing sites revealed strong winds and variable surfaces. To combat new challenges, a horizontal control system of attitude rockets was added to the backshell. The airbags were also redesigned and strengthened to withstand the heavier payload landing and bouncing at a faster rate.
The landings of both Spirit and Opportunity were without incident. But the EDL engineers pushed the landing bad system to its limit in the process. The next Mars mission, the Mars Science Laboratory, is centered on the 1,654-pound (0.75 tonne) rover Curiosity. To deliver Curiosity safely on the surface, JPL has had to devise a new way to land a larger and heavier payload.
The solution is the Sky Crane, a landing system that can deliver the rover without the use of a lander. Curiosity will begin its entry and descent like its predecessors, stored in a sphere-cone aeroshell. Within the shell, the rover will be attached underneath a smaller descent vehicle. (Right, the Sky Crane landing sequence.)
4 miles above the Martian surface, the main parachute will deploy to slow the craft. This will trigger the release of the lower heat shield. At this point, radar sensors will determine the rover’s altitude. At about 0.6 miles from the surface, the descent vehicle’s retro rockets will fire, in turn triggering jettison of the backshell and parachute.
The descent vehicle will use its retros to descend towards the surface. When it senses it is about 115 feet above the surface, the descent vehicle will release the rover on a tether. The rover will lower towards the surface as the descent vehicle continues its downward motion.
When the descent vehicle is about 16 feet above the Martian surface, the lander should be on the surface. The two will stay mated briefly before the descent vehicle severs the cord; its retros will carry it away from the rover, which will be free to begin its stay on exploration of Mars.
Landers are unlikely to get smaller any time soon. Technological advances will likely shrink some pieces of hardware, but more scientific instruments will surely fill the leftover space. As landers get increasingly heavier, landing methods are sure to be developed and redeveloped with each new generation of spacecraft. (Left, a comparison of rover wheels is a good indicator of their relative sized. The smallest in the centre is from Sojourner, the medium wheel on the left is from the MER rovers Spirit and Opportunity, and on the right is the wheel of the Mars Science Laboratory.)
This overview of Venusian and Martian landings offers a glimpse of how complex and intricate landing methods are for planetary mission – the story becomes more complicated with landing on even more distant bodies like Saturn’s moon Titan and Jupiter’s moon Europa. In the case of Mars, NASA has developed and redeveloped EDL methods continually over its nearly four-decade long presence on the planet. The Sky Crane or some variation may very well be the future of exploration on the planet. With the MSL landing scheduled for summer 2012, we can only wait and see if it works.
Suggested Reading/Selected Sources
1. Fredric W. Taylor. The Scientific Exploration of Mars. Cambridge. 2010.
2. Steve Squyres. Roving Mars. Hyperion. 2006.
3. Andrew Mishkin. Sojourner: An Insider’s View of the Mars Pathfinder Mission. Berkley Trade. 2004.
4. “ESA – Venus Express – Past Missions to Venus”: http://www.esa.int/esaMI/Venus_Express/SEMS5N808BE_0.html [Accessed March 24, 2011.]
5. “Interesting Facts About Venus”: http://www.universetoday.com/14070/interesting-facts-about-venus/ [Accessed March 24, 2011]
6. “Interesting Facts About Planet Mars”: http://www.universetoday.com/14853/interesting-facts-about-planet-mars/ [Accessed March 24, 2011]
I also have some Suggested Viewing for this topic, all available on You Tube
1. “NASA JPL Mars Phoenix Lander HD Animation”: http://www.youtube.com/watch?v=KHAsRjcLP4o [Accessed March 25, 2011]
2. “Viking First U.S. Mars Landing”: http://www.youtube.com/watch?v=ZRJzDny0l1A&feature=related [Accessed March 25, 2011]
3. “Mars Science Laboratory”: http://www.youtube.com/watch?v=b0A-AgytQY0 [Accessed March 25, 2011]