Uncovering the Dark Side of the Moon
02:26 GMT, 3rd January.
Chang’e-4 touches down on the moon’s surface, on the largest known impact crater in the entire solar system. With this, China joins the ranks of the United States and the Soviet Union as the only nations to have successfully carried out a soft landing on the moon.
While certainly motivated by political strategy, this space mission is nonetheless a major milestone in space exploration, as it’s the first time that a probe has successfully landed on the dark side of the moon.
But what exactly does this mean?
When looking at the moon’s trajectory around the earth, its orbital period (the time taken to orbit completely around the earth once) is exactly equal to its rotational period (the time taken to complete one full rotation on its axis), resulting in a phenomenon known as tide-locking. This means that one side of the moon is always facing the Earth, while the other is always obscured – commonly known as the lunar dark side. The main challenge with sending probes to this hidden side of the moon is communication, as with no view of Earth it is completely impossible to establish a radio-link with bases down on Earth.
The Chang’e-4 mission found a way around this obstacle.
Instead of directly connecting the probe with the bases on Earth, a relay satellite named ‘Queqiao’ was launched in late May 2018 towards the Earth-Moon Lagrange point 2 (L2). Essentially, L2 is a point where the gravitational attraction to the Earth and the gravitational attraction to the Sun are equal in magnitude, cancelling each other out. ‘Queqiao’ moves in slow circles around L2, remaining visible to both ground stations on Earth and the probe on the far side of the moon. This allowed a radio-signal to be established between the probe and the Earth using the relay satellite as an intermediary.
Yet, in addition to this mission being both of historical significance and a clear demonstration of human ingenuity, its envisioned goals are also extremely consequential.
Firstly, this space mission could potentially answer several questions about the Earth’s only natural satellite. It has been observed that there are far more floods of lava on the lunar nearside than on the lunar far side, indicating that the crust on the nearside is likely much thinner than that of the far side. However, why this is still remains an open question. With the Chang’e-4 mission, we may finally get to learn the answer.
By the same token, the lunar far side’s lack of lava floods means that all the impact craters on the far side of the moon still remain more or less intact. This means that we can study the geology on the dark side of the moon to determine the number of times that ancient asteroids and other celestial objects had collided into the surface, thus allowing us to learn more about the solar system’s early history.
Chang’e-4 is also going to be used to conduct radio astronomy, as its position on the lunar far side allows it to be shielded from all the radio waves emitted here on earth: by cars, WiFi, GPS satellites and (obviously) by radios. Performing radio astronomy in the absence of all this background noise may allow us to better understand the structure of different galaxies, as each different gas emits a distinct wavelength of light (hydrogen, for instance, emits radio waves with a frequency of 21 cm). Furthermore, by studying the cosmic microwave background radiation, astronomers may be better able to study the early development of the universe.
Finally, aboard the Chang’e-4 space probe are a cotton seed, a potato, a fruit fly and a flowering plant. Though this assortment may seem somewhat strange, they will help determine the feasibility of growing plants on the moon and forming a biosphere in low gravity environments (albeit a very, very tiny biosphere). And, besides being just plain interesting, understanding how to sustain life on the moon is a crucial step in looking at long-term human missions beyond earth.
Ultimately, the Chang’e-4 probe lays the groundwork for future space ventures, missions that are even larger and perhaps more precarious. With China’s long-term goal of colonising the moon and using it as a vast supply of energy, Chang’e-4 is most certainly a large step towards that direction. Therefore, the potential future significance of the Chang’e-4 mission should not be ignored, as it could be the first step in a series of steps that revolutionise how we look at and how we understand space exploration.
Sources:
https://www.bbc.com/news/science-environment-46724727
https://www.nytimes.com/2013/03/22/science/space/planck-satellite-shows-image-of-infant-universe.html?module=inline
https://www.atnf.csiro.au/outreach/education/everyone/radio-astronomy/index.html
https://www.nytimes.com/2019/01/02/science/far-side-moon.html?module=inline
http://www.planetary.org/explore/the-planetary-report/china-new-lunar-missions.html