On July 20 1969 The Apollo 11 Lunar Module touched down on the surface of the moon and Neil Armstrong and Buzz Aldrin began their walk. Many (including me) judge this to be mankind’s greatest single-event achievement so far. Outlined below are the many aspects of this story which provide learning opportunities and potential exam questions across the three GCSE Sciences, particularly Physics.
Despite the enormous sound and visual fury of the launch, the fuel used by the Saturn rockets powering the mission was mainly not fossil fuel, rather it was a mixture of liquid oxygen and hydrogen. Normally gaseous, very low temperatures are required to liquefy them, -219 C and -253 C respectively. Being liquid rather than gas is safer, and occupies much less space because
volume = mass / density and liquid density is higher. Saturn had sections which as fuel was used up were jettisoned to just leave the lunar and command modules. The enormous power was needed to enable the modules to reach the required speed to exit the earth’s atmosphere and escape the main gravitational pull.
The journey there
The distance from the earth to the moon is about 240,000 miles and the maximum speed was just over 24,000 miles per hour as it left earth’s orbit. So a “time = distance / speed “ calculation indicates a ten hour journey time and yet it took 3 days, so what happened to this slow-coach! Well, maximum speed does not mean average speed, and after the Saturn rockets were jettisoned, gravity slowed down the un-powered Module , as required, in order not to fly straight past the moon as it approached. Also, the journey included an orbit of the earth and several of the moon before descending to the moon so the distance was much higher.
If the rockets were jettisoned, how did the modules get to the moon without their powerful fuel? Well, once the modules were propelled out of earth’s orbit at high speed, less force was acting upon them since air resistance was zero. There was still a backwards gravitational pull of earth but it became smaller and smaller. So Newton’s Law would suggest they just carry on in the direction they were pointing, namely towards the moon, even without Saturn rockets’s major fuel source, albeit gradually decelerating from initial 24,000 mph. Small amounts of fuel were needed for lighting, communication and landing/leaving the moon, and these were a mixture of conventional fuels and fuel cells developed in Cambridge University, which were the early versions of the fuel cells we learn about in Physics GCSE. Namely hydrogen plus oxygen combining through electrodes to produce water,and release energy as electricity. The maximum power was around 2000 Watts and the water was not wasted – it was drunk by the astronauts!
“In space, no one can hear you scream”
As the advert for the science fiction classic confirmed, sound cannot travel in space because the longitudinal sound waves, whose vibrations are parallel to the direction of travel, need particles such as air to vibrate – but there is no air in a vacuum. So how come we could hear the astronauts?
As David Bowie memorably told us in Space Oddity, a conversation was possible between Major Tom and Ground Control. Well, the answer is that communication was achieved by Radio waves, which are not sound waves but Electromagnetic waves which as transverse waves vibrate at right angles to the direction of travel. Just like other parts of the spectrum – like light waves from the sun – radio waves can travel through a vacuum at the speed of light namely 300 million meters per second. Since the 240,000 miles is around 360 million meters, then using time = distance / speed, the time for a radio signal to travel from the moon to the earth is only 1.2 seconds. Hence the only-slight delay between Houston asking a question and the astronauts answering.
Note however that Michael Collins, alone in the Command module while Armstrong and Aldrin walked on the moon, could not be contacted on the far side of the moon as radio contact was lost, as expected. Perhaps this was why Bowie’s Major Tom lost contact at the end of the record – “can you hear me Major Tom?”
The physics of an orbit
When the lunar module had jettisoned its rockets it performed an orbit of the earth before heading to the moon. How does this work? If the module is set in forward motion at just the right speed then the force at right angles to its motion – namely gravity – pulls it towards earth and the net result is a bisecting direction along the path of the orbit.
The speed of the orbit remains constant at 25,000 miles an hour but the velocity is constantly changing. How can this be? Well, it’s because velocity is a vector and speed is a scalar quantity and as Vector tells Gru in Despicable Me, a vector has magnitude as well as direction. So the velocity is constantly changing because the direction in a circular path is constantly changing. When a force creates a circular motion, this is a centripetal force. (Gravity is a non-contact force while other centripetal forces are contact forces – the friction when a motor bike turns, and the tension in the spokes of the London Eye)
The diameter of the earth is about 8000 miles and the Module initially orbited the earth at around 100 miles up. So the diameter of the orbit around the centre of the earth was 8200 miles, giving a circumference of approximately 25,000 miles using Pi. At almost 25,000 miles per hour, the initial orbit took 1 hour.
The Moon Landing
When Armstrong and Aldrin’s lunar module separated from Collins’s Command Module above the moon, it reduced its speed but slightly overshot the landing site in the Sea of Tranquility in order to avoid landing in a crater. Armstrong took over control from the Module computer to achieve this ( a computer with less processing power than an I Phone incidentally). Less than 30 seconds of fuel remained, so this was where both of the astronauts’ flying experience, including dog fights with Russian MIG’s in the Korean War, proved invaluable. They stayed impossibly cool, while Houston’s control centre personnel famously were so tense they almost “turned blue”.
Armstrong’s heart beat stayed normal at 70 beats per minute, almost until the “Eagle has landed” but even he succumbed at touchdown to the fight or flight adrenaline hormone at touch down, when his heartbeat reached 150.
After Armstrong stepped down off the ladder – “one small step for man, one giant leap for mankind” – Aldrin soon followed him and began, as the Police would later sing, while Walking on the Moon, to take “giant steps” with his “feet hardly touching the ground”. Why is this? Well ,gravity there is only a sixth of the earth’s gravity ( g is 1.6 rather than 10). So it was easy to hop around. And why is the gravitational force lower? Because the force is proportional to the mass of the two objects, and the moon is lighter than the earth, even if the man has the same mass. So a person of 50 kg faces a gravitational downward force of 500 N on earth but only 80 N on the moon.
They collected rocks and when later analysed they were found to contain the chemical lelements silicon, iron, aluminum, calcium, magnesium, titanium and oxygen. No carbon or nitrogen, so not enough ingredients for biological life. Years later however , hydrogen and iced water were found at the moon’s poles and this opens the possibility, with the presence of hydrogen and oxygen, of creating fuel cells using electrolysis which could mean that the Moon could be used as refuelling stop on the way to Mars.
The journey back
After taking off from the moon, the lunar module docked with the orbiting Command Module and together they returned to earth. Long before the mission, Aldrin had written a thesis on docking in space based on his experience as a scientist and Air Force pilot in Korea. As the Module approached the earth atmosphere the frictional force – this time a contact force – caused the heat shield to reach high temperatures and gradually melt – as planned.
A parachute slowed the Module down further, with air resistance offsetting the weight of the Module, which floated down at a leisurely terminal velocity to the sea.
The crew were kept in quarantine for several days in case they had caught viruses on the moon. A virus – unlike bacteria – is counted as non-living but nevertheless can contain DNA. It is worth recalling that DNA was discovered by Watson and Crick at Cambridge University only 16 years before the Apollo 11 mission.
Further Physics work
All of the above science should be readily understandable by anyone taking Physics or Maths GCSE – if not it’s a definite revision topic! For those carrying on with Physics, the A Level and Physics Aptitude Test for Oxford will contain more advanced Space concepts like eclipses, Kepler’s Laws for orbits and what many consider to be one of the all-time great equations; namely Newton’s formula for the Force exerted by gravity on two objects, of mass m1 and m2: F = Gm1m2/r^2 where r is the distance between the masses and G is the universal gravitational constant.
Scientists are still not sure what Gravity truly is, yet in the 1700’s Newton could already quantify it, and in a sense invented the science behind Apollo.