Staying on the Moon

Until the outbreak of the global pandemic there was a good chance that mankind would return to the Moon in the next ten years, even if the deadline of project Artemis to have a manned mission in 2024 was a little ambitious. It will be some time before the effects of the current crisis on the manned space program can be evaluated.

The last humans to visit the lunar surface were the Apollo 17 astronauts back in December 1972, who spent only three days there. It is interesting to consider what it would be like to be on its surface for a period of a least a month and watch at first-hand how the lunar surface changes over time.

The Apollo 17 crew (L to R) Harrison H. Schmitt, Eugene Cernan, Ronald E. Evans – image from NASA.

In 1972 Schmitt and Cernan were the last humans to walk on the Moon.

 

The Moon’s low gravity

The surface gravity on the Moon is only 16.5% of the Earth’s value.  So, a person weighing 80 kg on Earth would only weigh 13.2 kg and because their muscles had developed to carry around an 80 kg body on the Earth, that person could achieve some incredible feats in the Moon’s low gravity. They could jump up to a height of around two metres from the ground, even allowing for the weight of a spacesuit. On Earth the average man can throw a baseball to a height of 15 metres. On the Moon this would be 90 metres.

The diagram below shows the path of a baseball thrown on Earth at velocity of 20 metres per second (72 km/h) at an angle of 60 degrees to the ground.

On the Earth, the baseball reaches a height of 15m, travels a distance of 35m and remains in flight for 3.5 seconds  before hitting the ground. For simplicity air resistance has been ignored,

The diagram below shows the path of a baseball thrown on Moon also at velocity of 20 metres per and at an angle of 60 degrees to the ground.

On the Moon, the baseball would reach a height of 90m, travel a distance of 210m and remain in flight for 21 seconds  before hitting the ground.

Lack of atmosphere

The Moon has no atmosphere so, to survive outside a spacecraft, you would need to a have an airtight spacesuit and oxygen supply. Without this you would be dead in about a minute. This is because water cannot exist in liquid form without an atmosphere.  Water would literally boil away from any moist tissues in the human body including the mouth, nasal passages, eyeballs and the insides of the lungs!

The Lunar Sky

On Earth the blue colour of the sky is caused by scattering of the shorter wavelengths of the Sun’s light, i.e. violet, indigo and blue, by air molecules (I’ve described this in my earlier post Why is the Sky Blue? ). On the Moon there is no atmosphere to scatter sunlight, so the sky is completely black in the daytime. To the human eye the Sun would appear to be white in colour rather than the yellowish colour it appears on Earth. This is because violet, indigo and blue light are not stripped out from the Sun’s direct rays – leaving a mixture of colours that the human eye would see as white.

Despite the black sky, if you were standing on the Moon during the day and gazed up into the sky you would not be able to see many stars, even if you were looking well away from the Sun. This is because the brightness of the lunar surface would saturate your eyes and make them unable to see fainter objects. However, if you were to look at the sky from a place where you couldn’t see any of the bright lunar surface, it would be very dark, as your eyes would be receiving very little light and you could see thousands of stars. This interesting effect was pointed out by Arthur C Clarke in his 1961 novel ‘A Fall of Moondust’

On the bright lunar surface the human eye would see very few stars in daytime

If you could view the sky from the bottom of a deep crater where no sunlight could reach you and you couldn’t see any of the directly lit surface (marked with a small a) you would see thousands of stars in the sky.)

At night time the Moon with its lack of light pollution would offer better viewing than the best of the dark skies on Earth.

How the Earth would look from the Moon

On the nearside of the Moon, the Earth would be a truly striking object in the sky. It would be 3.67 times larger in diameter (13.4 times larger in area) than the Moon appears from Earth,

Image from NASA

An observer on the Moon would see the Earth rotate and as it did so different continents, seas and oceans would come into view. As the Moon travelled along its orbit around the Earth they would see the Earth go through a complete set of phases over 29.5 days in the same way that the Moon goes through phases when viewed from Earth.

 

 

How the Earth would appear over a 29.5 day cycle (from a viewpoint on the nearside of the Moon)

At most locations on the Moon’s near side the Earth would never rise or set. It would always be above the horizon, almost stationary in the sky. At locations close to the near side/far side boundary the Earth would hover near the horizon – sometimes dropping below it and sometimes above it.

Locations on the far side of the Moon would never see the Earth and would also be unable to receive any direct radio transmission from the Earth (or from satellites in low or medium Earth orbits).

 

Absence of clues to distance

On the Earth more distant objects appear blurred and hazy compared to nearby ones.  This is because as light passes through the atmosphere it is absorbed and scattered. On the Moon, the lack of atmosphere means that distant objects appear just as clear as nearby ones. In addition, on Earth we can use the apparent size of natural objects such as trees and large animals as additional clues to estimate distances of faraway objects. On the Moon these clues are lacking. So, when looking at a hill some distance away, it would be difficult to tell If you were looking at a 100 metre high hill one km in the distance or a one km high mountain ten km away.

Large swings in temperature

The Moon’s slow rotation means that the Sun moves slowly through the lunar sky. A day on the Moon lasts on average 29.5 Earth days. It is daylight for 14.75 days and night for 14.75 days. At sunset when the last rays of light from the Sun drop below the horizon, there is no twilight and it gets suddenly dark. The Moon’s slow rotation and the lack of atmosphere – which prevents the retention of surface heat during the long lunar nights – cause large daily swings in temperature. On the equator the peak daytime temperature is 115 degrees Celsius and it drops to minus 180 degrees Celsius just before dawn (Williams 2020).

 

Deadly radiation and other hazards

Future astronauts venturing on the lunar surface could be protected from the lack of atmosphere and large swings in temperature by a pressurised spacesuit with a good heating and cooling system. The main hazard which would limit astronauts’ time on the lunar surface is radiation.

 

The Earth has a strong magnetic field to shield it from cosmic rays. These include energetic electrically charged particles from the Sun. The shielding effect extends well above the Earth’s surface and also protects satellites in Low Earth Orbit, such as the International Space Station (ISS).

The Moon has no magnetic field to protect it and its surface is bombarded with charged particles. When they left the protection of the Earth’s magnetic field, the Apollo astronauts kept seeing flashes of light even when they were inside their spacecraft with their eyes closed. This was due to energetic cosmic rays passing through the wall of their spacecraft, and into their bodies, including their eyes. When the cosmic rays hit their retinas, the light sensitive area at the back of their eyes, these created flashes of light. If there had been a solar storm, when the Sun ejects a huge amount of charged particles, the protection of the thin walls of their spacecraft would have been totally inadequate and the astronauts would have received a lethal dose of cosmic rays.

It is not only cosmic rays that astronauts need to worry about. On Earth deadly electromagnetic radiation (namely gamma radiation X-rays and ultraviolet) are blocked from reaching the surface by its thick protective atmosphere. Although a spacesuit would provide some protection, some of this radiation would get through.  Exposure to this would greatly increase the risk of developing cancer in later life.

 

And finally..

 

I hope you have enjoyed this post and are staying safe during these difficult times

 

References

 

Williams, D. R. (2020) Moon Fact Sheet, Available at: https://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html (Accessed: 12 April 2020).