Pat Norris not only changed Apollo's lunar trajectory but the entire course of history. Here is his story, in his words. 

This month, July 2019, is the 50th anniversary of Apollo 11 and has caused me to think back to the work I did on the programme in the 1960s in Houston.

After a solid grounding in maths and science at the Christian Brothers in Stillorgan, Co. Dublin, I graduated with a maths degree from UCD in 1964. A job in England working in a radar laboratory for a company that also made computers (quite a novelty at the time) gave me experience that was of interest to several American companies recruiting in London at the time.

So, a week before England won the World Cup (July 1966) I started work for a company providing support to NASA at the Goddard Spaceflight Center near Washington DC. A year and a half helping to work out the precise orbit of NASA's satellites brought me to the attention of a company supporting NASA’s Apollo work at the Manned Spacecraft Center in Houston, Texas (now the Johnson Space Center).

PatNorris-Houston 1968
Pat Norris in Houston 1968

From 1968 to 1970 I led a small team at aerospace giant TRW helping NASA ensure that Apollo navigated safely to the Moon and back. Most of that work concerned the challenge of predicting the trajectory of Apollo when it was in orbit around the Moon – the Moon’s uneven gravity field and occasional small rocket bursts from the spacecraft meant that Apollo was typically a few miles from its intended position one orbit later.

It was a critical part of the Apollo 11 mission, requiring Neil Armstrong to take over manual control of the Eagle Lunar Module to prevent the guidance computer setting them down on the boulder-strewn edge of a steep crater.

The problem first came to light in 1967 when NASA’s robotic Lunar Orbiter satellites dropped to within 50km of the Moon’s surface in order to get detailed photos of potential landing sites for the upcoming Apollo missions. The robotic satellites seemed to disobey Newton’s Laws of motion making it impossible to predict exactly where they would be in the future. The errors were greatest over the dark circular regions of the Moon – the two "eyes" of the man-in-the-moon and three other easily spotted dark circles. The asteroids that created these dark circular impact structures are thought to have weakened the Moon sufficiently to allow heavier material to rise from the interior to near the surface, thus increasing the pull of gravity as you flew over it. Newton’s Laws were OK, but the mathematical formula we were using to describe the Moon’s gravity field (essentially a spherical globe with a bulge around the equator) was too simplistic. There wasn’t enough information available to work out a precise formula for the Moon’s gravity and even if there was the computers of the day would not have been up to the task of calculating the effects.

This meant that our predictions of where Apollo would be an orbit or two ahead were unreliable – several kms error for each orbit we tried to predict. I led a small team that looked for a way around this. We tried averaging over two or three orbits in an attempt to better predict forward one orbit. We tried mixing in sextant sightings taken by the astronauts with the data from the Earth-bound radars. We tried several formulas for the Moon's gravity field – the formulas tried to describe to what extent the Moon isn’t a perfect sphere and varied from four mathematical terms to 121 terms. None of these approaches improved matters significantly, indeed the gravity model with the most mathematical terms often gave the worst results. It wasn’t any consolation but we knew that the Soviet Union was having the same problem, and their Luna 15 probe, which was orbiting the Moon at the same time as Armstrong and Aldrin were on the surface, crashed into the side of a lunar mountain due to faulty navigation. I had asked NASA to request the Soviet’s to send us their best gravity model of the Moon, and when we got it we realised it was even worse than ours.

Mission Control gave Armstrong and Aldrin the most up-to-date information about their trajectory one orbit (2 hours) before the landing, and before their orbit took them behind the Moon for the last time.

As they emerged from behind the Moon, Mission Control could see that they were a few seconds early and thus would overshoot the landing site, but there was nothing they could do about it.

In fact, they were heading for a point that was just inside the 10 mile by 4mile oval area which had been deemed OK for landing before the flight, but they were down in the bottom left corner of that "landing ellipse". Fortunately, Armstrong wasn’t distracted by the computer alarms that they experienced and at 150m height realised that he needed to continue on for another 400m or so to a flat spot. The rest is history.

Buzz Aldrin on the Moon 

Four months later for Apollo 12, Mission Control monitored how early/late they emerged from behind the Moon and converted that into a correction that they radioed up to the crew to enter into their guidance computer, and this simple change brought Apollo 12 in bang on target. The four later Apollo missions then had the confidence to land in hilly areas that were more scientifically interesting.

With the rundown of the Apollo programme, in 1970 I headed to Holland to work for the embryonic European Space Agency. Ten years alter I joined a UK software company (Logica, now CGI) to head up their space business unit where I stayed until 2018.

Postscript: the Moon’s gravity field was finally worked out in detail in 2012 when NASA placed the twin GRAIL probes into lunar orbit. They were able to measure the pull of gravity on the far-side of the Moon for the first time thus completing the mapping of the Moon’s gravity begun nearly 50 years earlier.

Returning People to the Moon After Apollo: Will It Be Another Fifty Years? by Pat Norris is published by Springer Praxis.

Watch 50 Years: To the Moon and Back on RTÉ One on Saturday, 20th of July, at 6:35pm.