What is needed for a detailed study of another planet, asteroid or comet?
First, launch a spacecraft closer. And equip this probe with instruments so that they tell as much as possible about the subject of study, based on the restrictions on volume and mass. Today we will see how a person studies the solar system using optical means.
Many cosmic bodies revolve around the Sun, which are very different from each other. Gas giants do not have a solid surface, and rocky planets have atmospheres of different densities, from negligible to superdense. Asteroids are stone, and there are iron, and comets greatly change their activity depending on the distance to the Sun.
It is clear that different instruments will be required to study objects with different properties. At the same time, scientists have already accumulated considerable experience in the application of many types of research methods, they were able to understand what gives the maximum of useful information with a minimum mass. Now we can look at such a "gentleman's set" of robotic space explorer.
Shooting in the visible range
The eyes continue to be our main research instrument, which is why astronomers on Earth are investing billions in giant telescopes, and special cameras are being created for space. They try to make a scientific chamber double, i.e. launch two cameras: one wide-angle, the second long-focus. Wide-angle will allow you to capture large areas with your eyes, but all objects in it will be small. The long-focal one is a "long-range weapon" that allows you to view fine details from a considerable distance.
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This principle holds true both in space and on the surface of planets. So, the Curiosity rover has a wide-angle color lens of 34 mm, and a long-focus lens - 100 mm.
For orbital modules, the ratio between long and wide is usually much more significant. Instead of a long-focus lens, a full-fledged mirror telescope is installed.
The largest mirror telescope outside of Earth's orbit is now working in orbit on Mars, with the MRO satellite - 50 cm in diameter. The HiRise camera captures heights of 250-300 km in phenomenal detail up to 26 cm.
This allows scientists to study Mars and track the movement of rovers, and enthusiasts like us to do Martian archeology.
In addition to scientific cameras, spacecraft are often equipped with navigation cameras. They allow operators to better orientate themselves "on the ground" and to choose targets for scientific cameras. Navigation cameras can cover even wider viewing angles, and can also be created double, but for increased reliability or for stereo photography.
The difference between scientific and navigation cameras is not only in the width of the viewing angle. Scientific cameras are also equipped with replaceable color filters that allow you to analyze some spectral characteristics of the surface of the objects under study. Filters are usually located in a special wheel that allows you to change them on the optical axis of the camera.
By default, scientific cameras shoot in panchromatic range - black and white mode, in which the photomatrix receives all visible light, and even slightly invisible - near infrared. This kind of shooting allows you to get the highest resolution and see the finest details, which is why most images from space are black and white. Although someone thinks that some kind of conspiracy is connected with this.
In panchromatic (black and white) mode, the detail is higher.
Color images can be obtained by repeatedly shooting with alternating color filters by combining the images. A single frame taken with one color filter will also be black and white, so the images need to be combined three at a time. And it is not at all necessary, the resulting color in the image will be what our eyes would see. For human vision, the world consists of combinations of red, green and blue. And the "real" color of the image can be obtained using red, green and blue filters.
Curious is the difference in surface reflectivity in different ranges.
But if the frames are made through, for example, blue, red and infrared filters, then the color of the image will turn out to be "false", although the physical principles of its receipt are exactly the same as real.
When publishing color images on official websites, they sign which color filters are used in the image. But these photos appear in the media without any explanation. Therefore, all sorts of speculations about the hidden color of Mars or even the Moon are still circulating on the Internet.
In ordinary terrestrial cameras, shooting through multi-colored filters is used in the same way, only they are glued to the elements of the photo matrix (Bayer filter) and the automation, not scientists, is engaged in color reduction. The Curiosity rover has already installed Bayer filters, although a separate filter wheel has been preserved.
Infrared shooting
Our eyes do not see infrared light, and the skin perceives it as heat, although the infrared range is not less than visible light. Information hidden from the eye can be obtained by infrared cameras. Even the most ordinary photo sensors can see near-infrared light (try, for example, shooting the light of the TV remote control with a smartphone). To register the middle range of infrared light, separate cameras with a different type of sensors are placed on space technology. And far infrared already requires cooling the sensors to a deep minus.
Due to the higher penetrating power of infrared light, it is possible to look deeper into deep space, through gas and dust nebulae, and into the soil of planets and other solids.
So scientists Venus Express observed the movement of clouds at medium altitudes in the atmosphere of Venus.
New Horizons recorded the thermal glow from volcanoes on Jupiter's moon Io.
The predator mode survey was used on the Spirit and Opportunity rovers.
Mars Express's view of the poles of Mars showed the difference in the distribution of carbon dioxide and water ice over the surface of ice caps (pink - carbon dioxide, blue - water ice).
To obtain maximum information, infrared cameras are equipped with a large set of filters, or a full-fledged spectrometer, which allows you to decompose all light reflected from the surface into a spectrum. For example, New Horizons has an infrared sensor with 65.5 thousand pixel elements arranged in 256 lines. Each line “sees” only radiation in its narrow range, and the sensor operates in the scanner mode, i.e. the camera with him is “guided” over the object under study.
As already mentioned, infrared light is heat, so shooting in this range opens up another opportunity for exploring solid bodies in space. If you observe the surface for a long time in the process of heating from the sun's rays in the daytime and cooling at night, you can see that some surface elements quickly heat up and cool down, and some heat up for a long time and cool down for a long time. These observations are called thermal inertia studies. They allow you to determine the physical characteristics of the soil: loose, as a rule, easily gains and easily gives off heat, and dense - heats up for a long time and keeps heat for a long time.
On the map: pink - with low thermal inertia, blue - with high (i.e. cools down for a long time).
An interesting observation, in thermal mode, was made by the Soviet probe "Phobos-2". While photographing Mars in thermal mode, he noticed a long strip that stretches across the planet.
In the 90s, the press expressed mystical speculations about an aircraft condensation trail in the atmosphere of Mars, but the reality turned out to be more interesting, albeit more prosaic. Thermal camera "Phobos-2" was able to record a strip of cooled soil, which stretches behind the passing shadow of the satellite of Mars - Phobos.
There are also mistakes. For example, while exploring Gale Crater from the Mars Odyssey satellite, scientists identified an area with high thermal inertia, near the landed Curiosity rover. There they expected to find dense rock, but they found clay rocks with a relatively high water content - up to 6%. It turned out that the reason for the high thermal inertia was water, not stone.
Ultraviolet shooting
With the help of ultraviolet radiation, they study the gas component of the solar system, and the entire Universe. The ultraviolet spectrometer is installed on the Hubble telescope, and with its help it was possible to determine the distribution of water in the atmosphere of Jupiter or detect emissions from the subglacial ocean of its satellite Europa.
Almost all planetary atmospheres were studied in ultraviolet light, even those that are practically absent. The powerful ultraviolet spectrometer of the MAVEN probe made it possible to see the hydrogen and oxygen surrounding Mars at a considerable distance from the surface. Those. to see how, even now, the evaporation of gases from the atmosphere of Mars continues, and the lighter the gas, the more intense this happens.
Hydrogen and oxygen in the atmosphere of Mars is obtained by photochemical dissociation (separation) of water molecules into components under the influence of solar radiation, and water on Mars evaporates from the soil. Those. MAVEN made it possible to answer the question why Mars is now dry, although there was once an ocean, lakes and rivers.
The Mariner-10 probe in ultraviolet light was able to reveal the details of Venusian clouds, see the V-shaped structure of turbulent flows, and determine the speed of winds.
A more sophisticated way to study the atmosphere is by light. For this, the object under study is placed between the light source and the spectrometer of the spacecraft. Thus, you can determine the composition of the atmosphere by evaluating the difference in the spectrum of the light source before and after it is covered by the atmosphere.
Thus, it is possible to determine not only the content of gases in the atmosphere, but also the approximate composition of the dust, if it also absorbs part of the light.
It should be noted that in terms of spectroscopic interplanetary research, Russia is not the last. With the participation of the Space Research Institute of the Russian Academy of Sciences, the European infrared spectrometer OMEGA was created for Mars Express; the same apparatus is the result of joint work of Russian, Belgian and French scientists - infrared and ultraviolet spectrometer SPICAM; together with the Italians, specialists from the IKI RAS developed the PFS device. A similar set of instruments was installed on the Venus Express, which completed its mission at the end of 2014.
As you can see, light provides us with a significant amount of information about the solar system, you just need to be able to look and see, but there are other means already associated with nuclear and radiophysics. And this is a topic for the next review.