Underground astronomy

We put our radio telescopes in valleys that screen them from manmade interference, but where there is a good view of the sky.

Our optical telescopes get put in dark places, often on top of mountains, above the part of the atmosphere that makes the images shimmer, and of course, once again with a good view of the sky.

It therefore sounds counterintuitive to put astronomical instruments under ground, usually as far down as possible, or deep in the Arctic or Antarctic ice.

We have also put instruments in the deep ocean.

As we move around in our daily lives, huge numbers of high-energy particles are sleeting through us, producing no noticeable effect, and doing us no harm.

These particles are called neutrinos, since they have no electrical charge. They also have almost no mass and travel close to the speed of light. Since they pass through matter as though it does not exist, it is very hard to make anything that can detect them.

However, on very rare occasions, a neutrino does interact with an atom. In this case, "rare" means one in 1E36 (one followed by 36 zeroes) neutrinos passing through the Earth may hit something.

When this happens, the result is a tiny flash of light.

Considering their extreme elusiveness, why bother trying to observe neutrinos at all?

The reason we want to do this is also the reason they are hard to detect: they pass through anything, and are almost totally unaffected. For example, they provide the only direct way we can look into the core of the sun and other stars.

The energy produced in a star's core has a long, circuitous trip to the surface of the star (its photosphere) before being radiated into space. It gets passed from one atom to another, so that after bouncing from atom to atom for around 200,000 years, it radiates off into space.

By that time, it carries very little information about the processes that produced it, or even perhaps whether that energy process has changed. 

However, energy production in stars also produces neutrinos. They escape from the star with no problem at all, so that when we observe them, we are seeing things produced mere minutes ago.

There are other high-energy processes taking place in space that produce neutrinos; these include supernova explosions and things taking place close to black holes. If we have a scientific need to detect them, how can we do that?

Typically, we want a huge number of atoms with detectors watching them to pick up those faint flashes produced when a neutrino gets caught. A really huge volume of clear liquid or solid, surrounded by detectors would do the job.

For example, the Sudbury Neutrino Observatory, located 2.1 km underground in a mine near the Northern Ontario city consists of a big, spherical tank containing over 1,000 tonnes of heavy water, with the inside wall covered with around 9,600 light sensors.

Other neutrino detectors consist of arrays of sensors buried in ice, or at the bottom of the sea. We have to put our neutrino telescopes far underground, under ice or under water to block the other particles arriving from space. 

Even then, our preferred observing direction is downward, through the rest of the thickness of the Earth. Anything managing to reach the detector through the diameter of the Earth will almost certainly be a neutrino. Moreover, the light flash can tell us the direction in which the neutrino is moving.

These elusive particles are proving to be a powerful tool for studying places we cannot observe in any other way.

It is intriguing to consider that we make these observations from deep under the ground, ocean, or ice caps, looking downward.

  • Mars lies low in the predawn glow
  • Jupiter and Venus low in the southwest in the post sunset glow.
  • Saturn is low in the southwest after sunset.
  • The moon will reach Last Quarter on the 19th and will be New on the 26th.


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About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]

The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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