"The Apollo landing missions were an unqualified success," said Chirold Epp of NASA's Johnson Space Center in Houston. "Every single time a lunar module headed toward the surface, it landed successfully and its crew got out and did a great job. But that doesn't mean there were not close calls. On four of the six Apollo landings, conditions were such that it gave us pause."

Though the last Apollo mission alighted on the moon more than three decades ago, the story of their landings, and where each of those 24 lunar module footpads settled into the soil, is still something Epp considers worthy of note. He is the manager of NASA's Autonomous Landing and Hazard Avoidance Technology (ALHAT) project - an undertaking designed to provide America's next moon crews with invaluable data that could make the difference between a good day and a very bad day.

The ALHAT system is designed to automatically detect hazards such as craters and boulders and then direct the lander to the safest touchdown location available. It is a job ALHAT must perform - on-the-fly.

"As it was with Apollo, future moon crews will have limited fuel supply and things will happen fast during a landing," said Epp. "With ALHAT, spacecraft can be guided safely, precisely, and repeatedly, to designated landing sites - anywhere on the lunar globe. That's important because the locations we want to explore in the future are going to be more hazardous; the terrain and the lighting will be more challenging than before."

One of the challenging sites under consideration for a future landing is in the vicinity of the south pole of the moon, near the rim of an almost 12-mile-wide, 7.5-mile-deep cavity in lunar crust called Shackleton Crater. There, the sun's slanting rays can throw shadows that minimize or even hide surface features - the kind of surface features that an astronaut piloting a lunar lander would want to know about.

Boulders, craters and sloping hillsides are hazardous surface features that are easily understood. Any one big enough - and in the right place - can leave a lander at a precarious angle or even cause it to topple over. A lunar surface hazard perhaps not so easily understood, but just as important, is the blinding dust that can be kicked up by a lander's rocket exhaust. That dust can block an astronaut's view of the boulders, slopes or craters below.

"ALHAT is the answer to all these challenges," said Andrew Johnson, ALHAT Project Element Manager from NASA's Jet Propulsion Laboratory in Pasadena, Calif. "Not enough sunlight at the landing site? We bring our own light. Moon dust getting in the way of seeing surface features and slopes? We figure out all the important nuances in the surface terrain from a vantage point far enough away that dust is not an issue."

How can ALHAT do all that? Perhaps the easiest way is to take you on a hypothetical lunar landing.

You are aboard a lunar lander headed towards a date with history at Shackleton Crater. The lander's descent stage rocket motor has been firing for over four minutes, bleeding off your orbital velocity and dropping you closer to the lunar surface. You have about five more minutes to go. Peering out the window you cannot see anything. The lander is making its approach to Shackleton from the backside of the moon, and it's pitch-black out there. You consult the spacecraft's computer, which uses sensors and software of the navigation system designed by the ALHAT Project to provide the information needed for a precise and safe landing. A laser onboard the lander points at the moon's surface.

"A laser is why ALHAT can operate just as well in the dark as in the light," said Johnson. "When we zap the surface with laser light, some of it bounces back. We have receivers that can read the signature of this returned laser light. We put that information through specially-designed computer algorithms, and what comes out is a three-dimensional glimpse of the lunar surface. That happens 30 times a second."

With 30 separate three-dimensional glimpses of the lunar surface coming through each second, the result quickly becomes a detailed 3-D map of the moon's undulating surface. Mountains, craters, hills and boulders you cannot see out your window appear on screen. ALHAT automatically compares these surface features with a three-dimensional lunar map stored in its memory.

"That is called Terrain Relative Navigation," said Johnson. "It is very important to know early in the descent that you are on the correct trajectory. And if you are not on the proper course, the sooner you know that, the easier it is to rectify things."

ALHAT's Terrain Relative Navigation is depicting that you are a little left of your planned course and a little low. You quickly modify your angle and rate of descent and are soon back on track. You notice that the lower you go, the more surface detail ALHAT reveals.

Two minutes before scheduled touchdown the lander has placed you about 5,000 feet over the surface and 1 mile to touchdown. The spacecraft pitches over to begin the final approach phase. ALHAT begins targeting the pre-selected landing zone with its laser and fires pulse after pulse. A three-dimensional map of the site begins to take shape on a screen in front of you.

"When you get close to the landing site, ALHAT goes into a mode called Hazard Detection and Avoidance," said Epp. "It will detect all the crater, rock and slope hazards in the landing site and provide a solution for a best landing option. We are confident that by the time a lunar landing crew gets within three-fourths mile of touchdown ALHAT can provide them a map of every boulder, crater and slope of consequence, within one sixteenth square mile - about 32 football fields - the desired location."

The seconds tick by and as you fly over the moon's terminator (the edge of the shadow between light and darkness) you can see Shackleton Crater spread out before you in all its awe-inspiring glory. Other prominent surface features can be seen visually as well.

ALHAT's displays show the massive rim of Shackleton just as expected. Several rocks and a huge boulder are also where you anticipated them based on preflight briefings and simulations. ALHAT also indicates the slope of the landing zone is well within Altair's tolerance levels - so you're good there too. But just to the left of the center point of the landing zone, ALHAT reveals there is a 20-foot-wide crater with steep sloping walls. Land too close to that and it could inhibit your planned lunar surface activities. Land a footpad in the crater and it could be even worse.

"When the crew begins to compare the ALHAT information with what they see out the window, that is called the "human interaction interval," said Epp. "They will use all the information available to them in their decision whether to continue to the original landing target, divert to a new landing target, or abort the landing entirely."

You decide to execute a short-range divert, aiming to land to the right of your initial target - but still well within the planned landing zone. The lander responds to your commands and soon you are 100 feet directly above your newly planned landing site. The plume from its rocket engine is sending dust streaming out in sheets, obscuring your visual of the surface as you begin a final, vertical descent.. But your computer screens are displaying the surface below as ALHAT sees it, and everything is go.

Touchdown. Welcome to Shackleton Base.

"It will happen," said Epp. "Whether it is Shackleton Crater, Aristarchus Plateau or any number of other exceptional candidates, we are going back to the moon. And when we go, ALHAT could help us go just about anywhere we want."

NASA is currently going into the deserts of California and Nevada to test the ALHAT system.

NASA's Autonomous Landing and Hazard Avoidance Technology (ALHAT) projectis led by the Johnson Space Center. The Jet Propulsion laboratory in Pasadena, Calif., is the lead center for field testing and terrain analysis, hazard detection and terrain relative navigation algorithm development. NASA's Langley Research Center, Hampton, Va., developed the ALHAT's lidar sensor. Draper Laboratories, Cambridge, Ma., works on navigation and control algorithms. The Applied Physics Lab, Laurel, Md., is lead for ALHAT optical systems and modeling.

earlier related report
Four Out of Six Apollos
Their names are now part of exploration history - Sea of Tranquility, Ocean of Storms, Frau Mauro, Hadley Rille, Descartes and Taurus-Littrow. They are the sites on the lunar surface visited by America's Apollo astronauts. Six unique locations. each with its own unique set of challenges to those who wanted to explore its secrets.

"To paraphrase an old bromide, those who forget the past are doomed to land like it," said Chirold Epp of NASA's Johnson Space Center in Houston. "Having looked at the Apollo landings I have come to two conclusions. One - those crews did a great job. Two - data from several of the landings support the idea that we must give future moon landers more information to increase the probability of mission success."

To prepare for their missions, Apollo crews were heavily trained to recognize specific large-scale lunar surface features at or near the designated landing site. These features would help the astronauts find their way to a safe area as close to the planned landing site as possible. But sometimes lighting conditions would conspire with local topography to deceive even the most highly-trained eye.

With two minutes left on the descent of the lunar module Falcon, Apollo 15 mission commander David Scott looked out his window and could not find the sequence of four craters as a visual guide to his planned landing spot northwest of the fourth and final crater, Index. Said Scott during a 1971 Technical Debrief: "When we pitched over, I couldn't convince myself that I saw Index Crater anywhere."

On the very first Apollo landing, the surface features were prominently displayed, just not the right kind in the right place. At 1,500 feet above the Sea of Tranquility Neil Armstrong saw the kind of surface features an Apollo commander does not want to find in his landing zone. Said Armstrong during a 1969 Technical Debrief: "...we were landing just short of a large rocky crater surrounded with a large boulder field with very large rocks covering a high percentage of the surface."

The closer each moon crew came to the lunar surface, the more detail they could make out. The hills, valleys and boulders of the local terrain came into view and the craters became more defined. But the astronauts did not have time to appreciate their surroundings. Their fuel supply was limited and getting to the surface under rocket power was mandatory to mission success.

As each of the six crews entered the final minute of their ride to the surface, the hills, valleys, boulders and craters that had just moments ago been there for the viewing had all but disappeared.

Observed Armstrong during the Technical Debrief: "...at something less than 100 feet; we were beginning to get a transparent sheet of moving dust that obscured visibility a bit. As we got lower, the visibility continued to decrease."

Dust problems were not exclusive to the Apollo 11 landing site. All of the moon landing crews encountered some form of vision-obscuring dust. On Apollo 12, Pete Conrad encountered so much dust that his final descent to the surface was done in the blind. Said Conrad in a 1969 Technical Debrief: "The dust went as far as I could see in any direction and completely obliterated craters and anything else... I couldn't tell what was underneath me. I knew I was in a generally good area and I was just going to have to bite the bullet and land, because I couldn't tell whether there was a crater down there or not."

With limited piloting time available and restricted visibility below, the Apollo crews skillfully put their lunar modules down on the ground. But each of the six crews knew that getting to the lunar surface was not the endgame. Staying on the lunar surface long enough to walk around and complete the mission's scientific goals was what they came for. For an Apollo mission's success, attitude was important.

"In one respect an Apollo lunar module is like a pinball machine - it doesn't like to tilt," said Epp. "If a lunar module came to rest at an angle beyond 12 degrees tilt the astronauts might not be able to launch themselves off the surface. So if a crew landed on a hill or with a footpad or two on a large rock or in a crater, that could make for a bad day."

Apollo 15's lunar module Falcon came to rest with its rear footpad on the rim of a 20-foot-wide crater. This caused one of the lunar module's footpads to be off the surface entirely and placed the spacecraft at an 11-degree tilt. Stated Scott in the mission's debrief - "...at the altitudes looking down as we approached the landing, it was very difficult to pick out depressions... as far as the shallow depressions there and the one in which the rear footpad finally rested, I couldn't see that they were really there. It looked like a relatively smooth surface."

Although Apollo 16 lunar module's landing tilt was only 2.5 degrees, if it had come down less than 100 feet in any direction from that point would have placed them on a slope of between 6 and 10-degrees. Apollo 16 commander John Young commented in the mission's Technical Debrief: "I couldn't judge slope out the window worth a hoot, and that's the truth. Even down low. The ground looks flat, but I'm sure it would look flat if it had been a 6 - 8-degree slope too. I don't see any way around that."

Young made his statement soon after he returned from the moon in 1972. The boulders, craters, crevices and dusty slopes of the Descartes Highlands were fresh on his mind. Tomorrow's moon crews will certainly encounter the same challenges. But thanks to a new NASA program that remembers the lessons of Young and the other intrepid men of Apollo, these future lunar explorers may very well have a way to "see any way around that." With NASA's Autonomous Landing and Hazard Avoidance Technology (ALHAT), they could be seeing their lunar landing sites in a whole new light.

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