One of the most interesting radio frequencies for radio telescopes is that emitted by hydrogen atoms when excited by an energy input. Upon returning to its ground state, the atom emits a photon with a frequency of 1.42 gigahertz. This frequency corresponds to the microwave region, which ranges from 300 MHz to 300 GHz. If we focus a radio telescope on a cloud of gas and dust in our galaxy, which is composed mostly of hydrogen, this frequency is easy to detect.
But what happens if we try to detect this frequency in very distant regions of the universe? The expansion of the universe affects these waves by lengthening them (that is, decreasing their frequency), in the same way as the frequency of the cosmic microwave background radiation, which was initially in the visible and infrared regions of the spectrum, is now in the microwave region, with its peak at a frequency of 160.2 GHz.
The cosmic microwave background radiation appeared
about 380,000 years after the Big
Bang. Afterward,
the universe went through a stage of 200 to 400 million years in which no stars
or galaxies had yet formed, so everything was dark. This is why that stage of
its evolution is called the Dark Ages.
During the Dark Ages, the universe was teeming with neutral hydrogen
and helium atoms, so when the former became excited (probably by colliding with
each other), upon returning to their ground state they emitted photons at the
indicated frequency (1.42 GHz). But if one of those photons reaches us, it will
have been subjected to the expansion of the universe, which stretches the
corresponding waves, so its current frequency will have decreased, depending on
the exact moment it was produced. In any case, right now those waves will have
a frequency of less than 50 MHz. In fact, the radio telescope that will attempt to
detect them will be tuned to the frequency range between 0.1 and 50 MHz.
Detecting these waves would provide us with data
about the stage of the universe called the Dark Ages, just as the cosmic microwave background radiation
provides us with data about what happened some 380,000 years after the Big Bang. But if we try to detect them, we have a problem:
these frequencies correspond to the medium wave, shortwave, and VHF bands,
where mobile services, radio broadcasting, and industrial and scientific
equipment operate, so the area near Earth is saturated with noise from these
frequencies, and it would be impossible to detect such faint waves as those
that would come from the Dark
Ages of the
universe. We call noise anything different to the signal we want to get.
Where should a radio telescope be placed to escape
ambient noise? Simply placing it in a desert, as has been done with other
telescopes, wouldn't suffice, because we would encounter another problem: the Earth's ionosphere reflects waves in that frequency range (which is
why shortwave radio stations work), so waves coming from space wouldn't be able
to reach the radio telescope, as they would be reflected outwards.
Consequently, the Earth's surface is ruled out as a suitable location for this
radio telescope.
 |
| Buzz Aldrin on the Moon |
That leaves the Moon. But even on that celestial
body, not just any location will do. The side facing Earth must be eliminated
because it receives too many waves from our planet (in other words, noise). Consequently, we have to place it on the far
side, which is shielded from that noise by the entire mass of our satellite.
NASA plans to place a radio telescope on the far
side of the Moon in early 2027. The telescope is called LuSEE-Night, and has two antennas that point in perpendicular
directions and can extend up to 6 meters in length. The associated equipment
has a volume of one cubic meter and weighs more than 100 kg, a third of which
is the battery.
The instrument will operate only during the lunar
night, which lasts more than 14 Earth days, to eliminate the influence of the
sun, but not continuously, to prevent the battery from running out. During the
lunar day, solar energy will recharge the battery. The instrument must remain
stable under extreme conditions: During the lunar night, the temperature will
drop to -130°C. During the lunar day, it will rise to 120°C. A large portion of
the battery's energy will be used to heat the instruments during the lunar
night.
The site for the radio telescope has already been
chosen. It must be located on a flat plain, diametrically opposite Earth's
position. The most likely location is at 24° south latitude on the far side of
the Moon. Of course, the landing is planned to be fully automated, without a
crew. And how will the radio telescope transmit the data it obtains back to
Earth? Via an artificial satellite located at a certain distance from the Moon,
and visible from Earth.
This post is based on anarticle published in IEEE Spectrum in February 2026.
Thematic Thread on Space Exploration: Previous Next
Manuel Alfonseca
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