To observe the sky, you need a favorable location. Where is the best home for a telescope?

Produced by: Science Popularization China

Produced by: Su Zian

Producer: Computer Network Information Center, Chinese Academy of Sciences

There was an interesting story in the history of science.

In 1998, Australian scientists used a radio telescope to receive an unusually strong pulse signal. This signal has been observed more than 40 times in more than a decade, an average of only a few times a year, but the time of its appearance is irregular.

At that time, everyone thought it was a historic discovery, so they put forward various conjectures. Some even thought it was a communication signal from aliens. It was not until 17 years later that a scientist discovered that the received signal was actually sent by a microwave oven in the laboratory, and the mystery was solved.

Astronomy is a discipline of observation and discovery. Among the nine Nobel Prizes related to astronomy, seven are for discoveries based on observation. Telescopes are an important means for humans to observe and explore the unknown. The radio telescope just mentioned is a type of astronomical telescope.

According to the different observation bands and means, astronomical telescopes can be divided into optical telescopes, radio telescopes, space telescopes, etc. In addition to the performance of the telescope itself affecting the results of astronomical observations, human activities also have a huge impact on astronomical observations. This is why the site selection and construction of astronomical telescopes are very particular, because if you are not careful, you will be misled.

So, how do you choose a suitable location for an astronomical telescope? Today we will introduce it in detail.

Image source: veer gallery

Optical telescopes: Too bright is not good, and being disturbed by atmospheric activity is not good either

When observing the starry sky, "light pollution" is the last thing an optical telescope wants to see.

"Light pollution" refers to the adverse effects of unnecessary or redundant forced light on humans and the earth's natural ecology, including daytime white light pollution (such as buildings reflecting sunlight) and artificial daylight pollution and colored light pollution at night.

The impact of light pollution on astronomical observation is mainly manifested in two aspects: on the one hand, it will make the night sky background (the brightness characteristics of the night sky background) brighter and brighter, resulting in a decrease in the contrast between the brightness of celestial bodies and the night sky background, which in turn affects the observed limit magnitude (that is, the magnitude of the faintest star that can be seen). Generally speaking, when artificial light pollution causes the sky brightness to increase by 5 times, the magnitude of the faintest observable star will brighten by nearly 2 magnitudes.

On the other hand, it will reduce the signal-to-noise ratio (the ratio of the signal to noise obtained when observing a celestial body) during astronomical observation. Generally speaking, the higher the signal-to-noise ratio, the better the observation effect and the higher the reliability. If a fainter star wants to achieve the same signal-to-noise ratio as a brighter star, it will take more observation time, which means that the observation efficiency is relatively lower. Therefore, if the brightness of the sky increases by 5 times due to artificial light pollution, the signal-to-noise ratio of the observed celestial body will be reduced to about 40% of the original, which is almost fatal for the observation of fainter stars.

For astronomical observation, light pollution will seriously affect the quality of observation, weaken the ability to observe faint celestial bodies, and even force some observatories to "move" or build new observation bases to avoid the impact of light pollution. For example, in 1947, the famous Greenwich Observatory moved 70 kilometers south.

In my country, in order to avoid the increasingly serious light pollution, the Sheshan Observatory of the Shanghai Astronomical Observatory built a new observation base in Anji, Zhejiang; the Purple Mountain Observatory of Nanjing built a new station in Xuyi, Jiangsu; and the Yunnan Observatory also built a new observation point in a remote mountain village in Lijiang.

At present, all the large observatories in the world have formulated relevant laws and regulations to control the impact of surrounding light pollution on astronomical observation and research. Newly built observatories in my country, such as the Ali Observatory in Tibet, have also established dark sky nature reserves nearby to protect their site conditions. The International Astronomical Union (IAU) has also established a special B7 committee dedicated to protecting existing and potential observatory sites to avoid them from being polluted by electromagnetic radiation of various bands (including visible light pollution and radio wave interference, etc.).

Night views of Beijing and Tianjin taken from the International Space Station (Image source: NASA)

In addition to considering light pollution, there are many considerations when choosing a site for an optical telescope.

"The stars are blinking, and the moon is drawing a question mark." The reason why the stars are "blinking" is that when light passes through the Earth's atmosphere, it is disturbed by atmospheric turbulence, resulting in irregular refraction. At the same time, the Earth's atmosphere also absorbs and scatters light, affecting the optical telescope's observation of celestial bodies. Therefore, when selecting a site for an optical telescope, minimizing the interference of atmospheric activity is the most important selection criterion.

Considering the climatic conditions, the location of an excellent observatory should meet the conditions of dry climate, more sunny days, stable atmosphere (relatively constant temperature), etc. Most of my country's observatories are located near water, because water can play the role of "thermostat", reduce the atmospheric turbulence caused by thermal convection, and improve the seeing (the clarity of the image displayed by the telescope).

National Astronomical Observatory Huairou Observation Base (Image source: produced by the author)

The locations of important optical observatories in the world, such as Mauna Kea in Hawaii, La Palma Island in the Canary Islands, and the Atacama Desert in South America, all have ideal seeing conditions. We can see that most of these observatories are located near water, on mountaintops, or in deserts, where the above conditions can be met as much as possible. However, human activities are also having an increasing impact on these astronomical observation points.

Take the Atacama Desert, for example. Known as the world's astronomy capital, it is home to many observatories. Observation data show that over the past 40 years, the average temperature in the desert has risen by 1.5 degrees Celsius, which will prevent instruments from cooling and increase the possibility of observatory failures.

At the same time, the Atacama Desert is also one of the driest areas in the world, with an average annual precipitation of less than 0.1 mm. However, in recent years, global warming has led to an increase in extreme weather, and the rainfall in the desert has begun to rise. In 2015, a rainfall was the sum of the rainfall in the past seven years. The reduction of sunny days will reduce the number of observable days, which will inevitably affect observations.

At the same time, rising temperatures will also lead to an increase in wildfires, posing a huge threat to observatories. Take the 2020 California wildfire as an example. The Hidden Hill Observatory was burned down, and the Wilson Observatory, which was not far away, was once eroded by the wildfire to a distance of about 150 meters. It was saved by the firefighters' full efforts. At the same time, firefighters also saved the 133-year-old Lick Observatory. At that time, the wildfire had burned all the way to the top of the mountain. Fortunately, all the equipment escaped.

Lick Observatory (Image source: https://www.cnbeta.com/)

In addition to these intuitive conditions, there are many changes that are silent: for example, the increase in atmospheric turbulence caused by rising temperatures will limit the observation capabilities of the telescope; air pollution will cause an increase in the number of aerosols in the air, which will reduce the amount of light observed by the telescope.

Maybe one day, the starry sky seen through the telescope will be gray.

Radio telescope: I want to spend my life in an uninhabited area

After talking about optical telescopes, let's talk about radio telescopes, which seem to be less common. Radio telescopes are divided into many types, such as single antenna, radio array (synthetic aperture), very long baseline interferometry, etc. The most well-known in my country is the Five Hundred Meter Aperture Spherical Radio Telescope (FAST) in Guizhou, which always appears on everyone's holiday check-in list.

Although the scenic spot has made clear regulations, set a daily limit for tourists, and prohibits carrying anything that can transmit signals, these measures cannot prevent the impact of human behavior on FAST. If possible, perhaps FAST just wants to stay in the uninhabited area and quietly serve as a telescope.

Radio telescopes receive electromagnetic radiation signals from distant celestial bodies. FAST has a high sensitivity and can receive extremely weak signals from the universe, which is of great significance for exploring distant unknown areas. However, during its operation, if there are electromagnetic radiation waves nearby, it will cause great interference to its work.

In order to ensure the normal operation of FAST, the local government has issued a regulation that prohibits the installation and use of radio stations and the construction of facilities that generate electromagnetic radiation within the core area of ​​FAST within a radius of 5 kilometers. This means that mobile phones, televisions, microwave ovens, induction cookers, gasoline cars, etc. are not allowed to be used within this range.

In addition, the vibrations or background noise caused by the coming and going of tourists will also have a huge impact on precision instruments, causing the data quality of FAST to decline.

FAST telescope (Image source: created by the author)

Space telescope: Beware of hidden "killers" in the starry sky

The influences mentioned above can all be solved artificially, but space telescopes floating in the universe have to face the influence of some uncontrollable factors, such as space debris.

"Space debris" refers to all man-made but ineffective objects and their components that are orbiting the Earth or entering the atmosphere. In simple terms, it refers to ineffective spacecraft or their fragments orbiting the Earth.

Since humans began space activities, the amount of space debris has increased year by year. Abandoned spacecraft or fragments of spacecraft caused by disintegration are the main sources of space debris. Sometimes, human activities can also create a large amount of space debris. For example, in 1963, in order to develop communications, the United States spread 350 million copper needles in the polar orbit to form a copper needle belt. However, these copper needles did not play the expected role. A considerable number of copper needles deviated from the predetermined orbit after completing the initial radio reflection mission and stayed in space for a long time.

Between 1980 and 1988, the Soviet Union launched 16 nuclear-powered ocean surveillance satellites, using liquid metal NaK as a condensing agent, and ejected the core of the reactor after the satellite finished working. It is estimated that about 264,000 condensing agent particles are stranded in orbit, becoming part of the source of space debris. At the same time, astronauts' living garbage and personal belongings (such as astronaut Ed White's gloves) have also become quite "interesting" space junk.

The average relative speed of space debris and spacecraft in orbit is about 10km/s. Under such high-speed conditions, even the impact of millimeter-sized debris may pierce the outer shell of the spacecraft or even cause the spacecraft to disintegrate.

The surface of many recyclers is full of pits and craters, which are the marks left by the impact of debris. The picture below shows the impact damage of the famous space telescope, the Hubble Telescope. In December 1999, astronauts photographed all visible areas of the Hubble's outer shell and identified 571 obvious high-speed impact features, with an average impact density of about 45 per square meter.

Impact craters from the Hubble Telescope (Image credit: NASA)

Since most space telescopes are directly exposed in space, space debris poses a constant threat to their safety and may even cause them to be completely unable to work. For example, the XMM-Newton satellite failed because its charge-coupled device (CCD) was hit.

Generally speaking, for fragments below the centimeter level, a protective layer should be added to minimize impact damage; for fragments above the centimeter level, they need to be numbered and actively avoided.

Although various countries are also trying to actively respond, and have proposed methods such as robotic arms, lasers, and light gas cannons to capture or burn them. An international cooperation organization, the Inter-Agency Space Debris Coordination Committee (IADC), has also been established to work with major space powers to jointly address the problem of space debris. However, at present, most of these measures have had little effect.

Speaking of flying objects in space, we have to mention the famous "Starlink Project". The project intends to launch a large number of Starlink satellites (12,000 in the initial plan). For optical telescopes, they are equivalent to small bright celestial bodies, which will not only increase light pollution and affect the observation of faint celestial bodies, but also make some astronomical images overexposed and unusable.

For radio telescopes, tens of thousands of Starlink satellites will occupy many radio wave channels, directly affecting radio telescopes that observe high-frequency signals.

Scientists estimate that when the number of Starlink satellites reaches 6,400, the sensitivity of the downlink bands of certain radio telescopes whose frequencies conflict with them will be lost by 70%; if it reaches 100,000, these frequency bands may become completely unusable.

At the same time, because Starlink satellites automatically change their orbits and do not operate in fixed orbits, they also pose a certain threat to other spacecraft in space. In September 2019, the European Space Agency's Aeolus satellite had to make an emergency maneuver to avoid a collision with a Starlink satellite.

There are so many unknowns in the sea of ​​stars waiting for us to observe and explore. Looking up and seeing the starry sky is not only a right for each of us, but also a belief in pursuing the universe. I hope that our "eyes" looking into space can always be clear and bright.