As a kid, I believed that the moon is a land of fairies, thanks to my mother’s bedtime stories, and if I behave well, then Chandamama would come down on earth to take me to the fairyland. The fairies popped up in my dreams quite often until I grew up and learnt that the lunar surface was too hostile to support any life as concluded from the Apollo missions. My fantasy of going to the moon had rested in peace until I recently came across a preprint paper titled, “A very high energy hadron collider on moon” submitted for publication to the highly reputed journal Nature Physics.
Let me explain why this paper is interesting. Though the idea of building a discovery machine on the moon sounds too futuristic, it is worth a thought since it comes at a time when NASA is aiming to establish a permanent presence there and space exploration has gathered pace like never before. Moreover, a lot of fundamental questions about our universe might be answered with such a gigantic particle collider that is supposed to be a 1,000 times more powerful than the most powerful collider on earth, the CERN Large Hadron Collider (LHC). This proposed collider is named Circular Collider on the Moon (CCM) by the authors, one from Duke University, US, and the other from CERN, Switzerland, who have laid out the siting, construction and operation plans for it.
Before moving on to the plan of the CCM, let’s understand why we need high energy particle colliders and how a collider like the LHC works. The LHC is a ring of superconducting magnets placed underground in a circular tunnel of circumference 27 km. Inside it, beams of protons travelling at nearly the speed of light in opposite directions collide with each other and the resulting events are recorded. The study of these events sometimes leads to the discovery of new particles and helps us understand the origin of the universe better. However, some secrets of the universe can be addressed at an energy scale much higher than the LHC’s. Though a number of more powerful colliders on earth are planned like the Future Circular Collider at CERN and the Super Proton-Proton Collider in China, the moon collider would be far more superior compared to these in terms of energy. According to the authors, the CCM would be wrapped around a great circle of the moon of circumference 11,000 km and can have a collision energy of 14 Peta electron volt.
The major challenges in building such a gigantic structure, keeping the logistics aside, are the lunar surface temperature, the length of lunar days and nights, and frequent meteoroid strikes on the moon. To avoid these challenges, the scientists have proposed three different scenarios. To accelerate protons, the CCM needs superconducting magnets that can operate at low temperature compared to room temperature on earth, whereas the temperature on the lunar surface can reach nearly 127 degrees Celsius during a lunar day. Thus, the CCM can be built on the lunar surface and operated only during lunar nights when the temperature drops to -173 degrees Celsius. However, this would lead to long stretches of non-operation since the length of a lunar day is equivalent to 13.5 days on the earth. The second choice is to build it by digging and covering a trench a few meters below the surface so that it can be operated both during lunar days and nights, since according to the measurements from Apollo 15 and 17 missions, the temperature fluctuations due to lunar days and nights are negligible 50 cm below the moon’s surface.
But this scenario would still not make it damage-proof against meteoroid strikes. The third choice that can protect it from these strikes is to bore it a few hundred meters under the lunar surface.
Now coming to the power source that can propel such a huge machine, the most promising would be solar power, which is abundantly available on the moon due to the absence of an atmosphere. This can be harnessed by building a Dyson belt around the equator of the moon that can not only provide power to the collider but also potentially send power to earth. The scientists are hopeful that if the progress on the necessary preceding technologies like transport between earth and moon, and solar powering of the moon continue apace, the construction of the collider on the moon can start as early as the 2070s.
The next few decades are going to be exciting in the context of lunar exploration as NASA plans to set foot on the lunar surface again with its Artemis Program, China and Russia plan to build a moon base together, and the private space industry continues to grow. With nations, industries and academia working together towards making the moon our next laboratory, we stand at the edge of a new age and I am awestruck to see how our research has moved into the realm of fantasy.
Prativa Pritimita
Research Scientist at the Department of Physics, IIT Bombay
(prativa@iitb.ac.in)