When the Mars Science Laboratory's Curiosity rover landed on Aug. 6, it was another step forward in the effort to eventually send humans to the Red Planet. Using the lessons of the Apollo era and robotic missions to Mars, NASA scientists and engineers are studying the challenges and hazards involved in any extraterrestrial landing.
The technology is known as "vertical takeoff-vertical landing." According to a group working in NASA's Engineering and Technology Directorate at the Kennedy Space Center in Florida, the best approach requires a landing pad already be in place.
"One of the greatest challenges to Apollo astronauts landing on the moon was dust, rocks and debris obscuring their vision during the final part of the descent," said Rob Mueller, a senior technologist in Kennedy's Surface Systems Office and Lunar Destination co-lead for NASA's Human Spaceflight Architecture Team. "When the Apollo lunar modules reached the 30-meter point (about 100 feet), the dust was like a fog making it difficult to see their landing site. Similarly, photographs show there were some rocks and dust kicked up by the rocket engines on the sky-crane lowering the Curiosity lander onto the Martian surface."
As the Mars Science Laboratory's descent stage used rocket engines to hover, its sky crane lowered the Curiosity rover with a 25-foot tether to a soft landing on the surface.
Mueller and others are working on ways to develop landing pads that could be robotically constructed in advance of future human expeditions to destinations such as the moon or Mars. These specially constructed landing sites could greatly reduce the potential for blowing debris and improve safety for astronauts who make the trip to Mars or another destination.
"Our best estimates indicate that descent engines of the Apollo landers were ejecting up to one-and-a-half tons of rocks and soil," said Dr. Phil Metzger, a research physicist in Kennedy's Granular Mechanics and Regolith Operations Laboratory. "It will be even more challenging when we land humans on Mars. The rocket exhaust will dig a deep hole under the lander and fluidize the soil. We don't know any way to make this safe without landing pads."
Building a landing site in advance of human arrival is part of the plan.
"Robotic landers would go to a location on Mars and excavate a site, clearing rocks, leveling and grading an area and then stabilizing the regolith to withstand impact forces of the rocket plume," Mueller said. "Another option is to excavate down to bedrock to give a firm foundation. Fabric or other geo-textile material could also be used to stabilize the soil and ensure there is a good landing site."
Metzger explained that one of the ways to ensure an on-target landing would be to have robotic rovers place homing beacons around the site.
"Tracking and homing beacons would help a spacecraft reach the specific spot where the landing pad had been constructed," he said.
Landing pad technology may be perfected on Earth well in advance of its use elsewhere in the solar system.
"Several commercial space companies are already discussing returning rocket stages to Kennedy or Cape Canaveral saving on the cost of sending payloads to low Earth orbit," Mueller said. "Rather than the first stage simply falling into the ocean, the rocket would land vertically back here at the Cape to be reused."
While landing pads will provide a smooth touchdown location, they will also require advanced technology design and decisions on how large the landing pad should be.
"One of the factors we have to consider is the atmosphere where a landing will take place," Metzger said. "The Earth has a dense atmosphere that focuses the rocket exhaust onto the ground, but also reduces how far the ejected material is dispersed. Mars, on the other hand, has an atmospheric density that is 1 percent that of Earth. It still focuses the plume into a narrow jet that digs into the soil, but it provides less drag so the ejected soil will actually travel farther.
"Then compare that to the moon with no atmosphere," he said. "The plume won't be focused so it won't dig a deep hole in the soil, but the ejected material will travel vast distances at high velocity. It is like a sandblaster on steroids. So the requirements for a landing pad are determined by the destination we're landing on."
Metzger envisions circular landing pads from about 50 to 100 meters (about 165 to 330 feet) in diameter.
"The specialized material taking the heat of the engine plume would be in the middle," he said. "The area surrounding the center would be designed to hold up support equipment."
Another issue is what substances to use in building the landing pads.
"Tests with prototype landers show that while pads are safer than touching down on natural surfaces, certain pad materials can produce debris of their own," Metzger said. "A supersonic rocket exhaust becomes extremely hot when it impacts a surface. Asphalt or concrete are out of the question because the temperature causes those materials to break apart, throwing chunks of material in all directions."
During investigations of prototype landers, various materials have been examined on the pads from which the vehicles have vertically taken off and landed.
"We've tested several types of materials and it seems that basalt regolith mixed with polymer binders hold up well," Metzger said.
However, the one substance for landing pads that shows the most promise is the material used on spacecraft heat shields.
"Of all the substances we studied, ablative materials seem to work best," Metzger said.
Ablative substances were used on the heat shields for spacecraft during Mercury, Gemini and Apollo. The heat of re-entry was dissipated by burning off successive layers.
"While ablative materials seem to work well, the layers will eventually all burn away," Mueller said. "So next we may try reusable thermal protection material similar to that used on the space shuttle tiles or the Orion capsules."
A human expedition to Mars is still many years away, but Mueller says now is the time to start planning for how to land on another planet.
"The technology we envision will take 10 to 15 years to develop," he said. "We need to begin verifying that these concepts will work, and that's why we are already involved in the research."
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