Moonlight window to the Universe

In a conversation with astrophysicist Yuri Kovalev (Lebedev Physical Institute) about the achievements and prospects of the RadioAstron project, we touched on the topic of the moon. It turned out that she promises radio astronomy new opportunities that will allow you to look where no one else has looked. Leonid Gurvits from the Joint Institute for VLBI in Europe and Mikhail Mogilevsky from IKI RAS joined in the discussion of the benefits of the lunar program for radio astronomy.

Yuri Kovalev , Head of the Laboratory of Extragalactic Radio Astronomy of the Astro Space Center of the PN Lebedev Physical Institute of the Russian Academy of Sciences



- What do you think of the idea of ​​building a base on the moon?

- In short, you need to build a radio observatory on the moon, but going to the moon just for that is not serious - it's too expensive. However, if a strategic decision is made that Russia is building a lunar base, I’ll say: not to put a long-wave radio telescope on the moon is a crime. The ultra-long wavelength range is the only window in the electromagnetic spectrum that has not yet been opened. Ultra-long waves do not pass from space to Earth; they are reflected from the ionosphere; accordingly, a telescope must be placed outside the Earth. You can put a free flyer on a satellite, you can put on the moon. In principle, the moon will be more expensive. On the other hand, if there is already a base there, then it’s possible and necessary to install a telescope that will work for a very long time.

- And why do not they put on satellites?

- You can put satellites on, but you understand that we are talking about large sizes and a large number of radiation receivers. The wavelength is about twenty meters. And how long will that satellite live? At the same time, the telescope on the moon doesn’t even need to be serviced. Just “scatter” the wire over the surface. Why the moon? The problem is not only to put on the satellite, but that we need to protect ourselves from Earth interference - it is very hot in this range.

- That is, you need to build on the back side?

- Yes, the Moon is considered here as a protection of an ultra-long-wave radio telescope from Earth radiation. It should be placed either on the back or in craters at the poles. Naturally, the task will be to transfer data to Earth. If you put on the far side of the moon, you need a repeater, and most likely it will be a satellite. If you put on the poles, then you can put the repeater on the edge of the crater. Options are discussed, including the implementation of a telescope on a satellite flying around the moon. Then, part of the time, the telescope is closed by the Moon from the Earth and makes observations. And, accordingly, when it opens, it dumps data to Earth.

While this is being discussed at the level of ideas that need to be thought out and worked out. This is the last remaining unopened window of the electromagnetic spectrum in the study of the Universe.



A few words about possible scientific problems. We begin with a study of the so-called era of secondary ionization. This is another potential Nobel Prize, because of which the long-wave radio astronomy in recent years has received a strong impetus in the development and interest of the world community. The study of hydrogen radiation from different cosmological distances in the universe allows you to build a three-dimensional map of the universe in a line of neutral hydrogen. The farther the hydrogen, the correspondingly longer the wave. By value for cosmology, this is comparable to relic radiation.


ESA Planck Space Telescope and its CMB.

LOFAR, SKA and other projects are studying the era of secondary ionization and mapping neutral hydrogen at shorter wavelengths. For this task, it will also be useful to have an ultra-long-wave radio telescope.

Do not forget that each new window in the electromagnetic spectrum brought its surprises - results that cannot be predicted in advance. I hope the last “window” we are discussing will not be an exception.

- You mentioned the lunar satellite. For me, this topic is close. In your opinion, on the microsatellite scale, is it possible to implement at least a prototype of such a telescope?

- If your lunar microsatellitemight bring out some kind of dipole or a trickier measurement system, it would be potentially useful. An important question is how much the internal interference of the apparatus will interfere with the operation of the telescope. Analysis required.

- We’ll think about what happens. Thanks!
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Head of the Space Research Division of the Joint Institute for VLBI in Europe
Leonid Hurwitz.



- The equipment necessary for microwave radio astronomy is relatively simple. A radio astronomy receiver for frequencies in this range (frequencies below 10-15 megahertz) can be soldered by a school radio amateur. It is pretty cheap. And the amount of information, roughly speaking, is proportional to the carrier frequency. The data stream is relatively small, a modern digital system can handle it. Antenna systems, despite their size - rather large - are very simple. It can be just wires laid out on the surface.

- And how long should they be?

- Yuri Kovalev already mentioned: the wavelength is about 20 meters, so the size of the antenna should be no less. At the same time, there are modern technologies that make the antenna much smaller. For example, cell phones operate at a wavelength of 20 cm, although antennas of this size are not carried. They have active antennas, and the same technologies can be used in space. But for the lunar surface, it may not be necessary to unwind on the surface of the moon or even in outer space, a coil with a very thin wire will not be difficult. There is no fundamental difference between the satellite and the surface.

A satellite is more profitable because landing on the surface of the moon of any kind of payload is a complicated and expensive business. The moon has a major advantage - it is protected from man-made interference. A satellite in low orbit around the moon in a certain part of its orbit is obscured by the moon from the earth, and this can be used. Therefore, a satellite is cheaper than a stationary lunar observatory. On the other hand, since we will fly to explore the Moon, this work will require the delivery of tons of cargo, adding to them a few tens of kilograms of payload for radio astronomy is not difficult. And the effect will be colossal. I think one or two Nobel Prizes are hidden in super-long-wave radio astronomy. This is already practically the law: if some parameter of the research facility improves by an order or orders of magnitude, or work begins in a completely new, an undeveloped area, it is guaranteed there will be discoveries that are difficult to predict. This fully applies to our topic.

If there are manned flights to the moon, and they definitely will, it will be inexcusable not to use this opportunity to deploy an extra-long-wave radio telescope there. So in the future lunar program, radio astronomy will be a passing passenger, and in outer space you can create an independent observatory. The scientific motivation is the same, but the advantage of the satellite is that it is possible to create a swarm of microsatellites in which each device carries one antenna element, simple, light. This swarm of satellites can be placed somewhere in the shadow of the moon, for example, at the Lagrange-2 point of the Earth-Moon system. This point is located beyond the moon, there the satellites will be protected from man-made interference. Placing at this point has a number of limitations. A point is such an ideal position, in fact, a satellite at this point performs the so-called



You can place this swarm in a low lunar orbit. Then this swarm will spend some part of its orbit in the cone of the Earth’s shadowing. Such options are being considered, a specific project is being developed in China. You might think about launching a super-long-wave observatory somewhere very, very far away. So far that man-made interference from the Earth will be negligible due to distance. But you have to fly quite far. Remember, in the splash screen of the “Contact” movie, the spacecraft flies away from the Earth, and at first there is a lot of interference around it, then it moves farther and farther and the signal takes longer to propagate. Finally they intercept the first broadcast from the Berlin Olympics of 1936, then the first radio signals - and silence. So, silence comes due to two factors: firstly, they flew so far, which practically moved in time, and secondly, the distance factor affects. The distance is proportional to the square in the denominator. That is, if you retired ten times, then the intensity of the interference decreased by a hundred times. Having retreated ten times further than the moon, we will reduce the interference by a hundred times. If we leave a hundred times, we reduce the interference by ten thousand.

Accordingly, we have three methods: on the surface of the far side of the moon, on a lunar satellite, or very far away. There is no option to fly very far in real development right now. And there is an option on the moon and on the lunar satellite. This is done by Chinese colleagues in the framework of the Chang'e program. In Russia, the lunar program is also being revived now, there are projects of the “Moon-25”, -26, -27, -28 ... Unfortunately, so far no one of these devices has a payload for ultra-long-wave radio astronomy. This is regrettable, as it would allow “cutting the corner” in the race for discoveries in this range. In terms of technology and cost, this is relatively uncomplicated. To build such an ultra-long-wave radio telescope is much, much cheaper than Radioastron or others from the Spectrum series. The payload is simple, cheap,

If we talk about a test satellite that would be suitable for our purposes, then a device weighing several kilograms would be enough. We developed an antenna that is suitable for our purposes, which is not a dipole, but a tripole. This is a symbiosis of a conventional dipole and an active antenna with an amplifier, an analog-to-digital converter, and a processing and transmission system pulled several kilograms. Dimensions were obtained like a small photo tripod. If such a thing is thrown in a lunar orbit, then it is able to function there independently.

Do you know the Chinese organization Harbin University of Technology? They are working actively with CNSA on a similar project, and they are trying to insert this into Chang'e. In addition, as part of the Chang'e program, antennas are being developed on two Chang'e devices. One antenna on the satellite repeater, and the other on the landing stage, landing on the far side of the moon.


Illustration: chinaspaceflight.com.

The possibility of delivering super-long payload on both devices is being considered. Then they will get a radio interferometer with extra-long bases, similar to RadioAstron, only in the extra-long wavelength range. China is going to locate the repeater just at the Lagrange-2 point of the Earth-Moon system with the known advantages and disadvantages of this point, which we have already talked about. This work is already underway.

Thank you for your help in preparing the material of Mikhail Mogilevsky, head of the laboratory of physics of magnetospheric processes at the Space Research Institute of the Russian Academy of Sciences .