The first to the first from the sun

Mercury, the initial information about which is given on the Sumerian cuneiform tablets of the 3rd millennium BC. e., turned out to be one of the most difficult to study planets. And it is not surprising - with an orbit with an average diameter of only 0.387 a. e. At the moments of maximum angular removal (maximum elongation), it is defended from the center of the Sun by only 28 °. Therefore, the naked eye can be noticed only at sunrise and sunset near the horizon or during complete solar eclipses.

The ancient Greeks who observed Mercury at first believed that they saw two different planets - the morning, called Apollo, and the evening, called Hermes. But about 2.5 thousand years ago, it became clear that we were still talking about the same heavenly body, behind which the name of Hermes in honor of the Greek God, the patron saint of trade and (which is less known) thieves, eventually entrenched. The name "Mercury" came from the Roman pantheon.

The appearance of telescopes made it possible to conduct observations of Mercury in the daytime, but only during periods close to the dates of elongation, that is, no more than forty days a year. The rest of the time to see the planet, located almost at the very sun disk, does not give the scattering of sunlight in the atmosphere, and such attempts are simply dangerous for the eyes.

With a diameter of 4880 km, the angular size of the Mercurian disk when observing from the Earth does not exceed ten angular seconds, which, in combination with the influence of the earth's atmosphere, also does not favors the study of the planet, especially the features of its surface. Historical fact: Galileo Galileo, who first watched Mercury into the telescope at the beginning of the 17th century, failed to find the Mercurian phases, which was seen only in 1639 by Italian Giovanni Batista Zupi. Orbital space telescopes do not greatly improve the situation. So, the legendary Hubble has never been decided to direct to Mercury, since the error in the tip can cost the life of the equipment of this unique apparatus - we recall the dangerous neighborhood with the Sun.

Thus, in order to see the surface of Mercury in all details, there is only one way - to approach it using interplanetary space stations. But here, nature put the barrier before human curiosity: flights to Mercury are extremely complicated from the point of view of implementation.

In order to go to the vicinity of Mercury along the most energetically advantageous trajectory, which is an ellipse tangent to the orbit of both planets (Gomanovsky trajectory), and used in the flights to Mars or Venus, a space apparatus on a near -Earth orbit of 200 km high will need to be informed of a departure pulse of approximately 5.55 km/s. Another 7.55 km/s will be required to inhibition at Mercury itself and an access to orbit around it. And if we want to land, then the second value will be 10.4 km/s. All numbers are given without taking into account gravitational losses, the value of which depends on the specific impulse of the engine used and the time of its operation. Unlike the same Mars or Venus, the atmosphere will not help when braking - it is so discharged on Mercury that it can not even be considered in practical and engineering tasks. By the way, for comparison: when flying along the Gomanov trajectory to Mars, the magnitude of the departure and brake pulses of speed will be 3.61 km/s and 2.10 km/s, in case of planting, the second value will be 5.66 km/s. Even for comparison: the reserve of the characteristic speed of the launch vehicle that takes the satellite into the near-Earth orbit, taking into account gravitational and aerodynamic losses, is about 9.5 km/s.

As you can see, if you set the purpose of the flight along the Gomanian trajectory, then the spacecraft should have a total margin of characteristic speed of at least 13.1 km/s, but in fact more. Which significantly exceeds the energy requirements even for launch vehicles.

In order to get by with lower costs, a more complicated flight scheme is required using gravitational maneuvers, during which the spacecraft accelerates in the gravitational field of the planets that fall on its path without spending precious fuel. But, firstly, in order to implement such a complex ballistic scheme, it is necessary to be able to very accurately control the spacecraft. Secondly, the “circular” path significantly increases the duration of the flight, and if along the optimal trajectory the device would go to Mercury 105.5 days after departure from the near-Earth orbit, then the flight of the only Earth’s Messenger, which had been in the near-Mercurian orbit, lasted 6.5 years.

From this circumstances, extremely high requirements for on -board equipment and its components arise, which should withstand such a long road through outer space and not lose performance. Add to this seven times the larger sunny permanent in the orbit of Mercury and the corresponding increase in thermal loads. Remember the radiation emitted by a close star. The elbow is close, but you will not bite!

However, in 1962, the astronomers found that after ten years the location of three planets - land, Venus and Mercury - will allow to carry out a flight in the vicinity of the planet closest to the Sun using the maneuver in the vicinity of Venus, and during the time not much exceeding the duration of the flight along the Gomanian trajectory. True, it was only about the flight past Mercury, but not about going out into orbit around him. However, the use of the presented opportunity was very tempting, especially since the next opened only in the mid-1980s. And in 1968, the idea of ​​such a mission was supported by the Committee on Scientific Problems of Space Development, and a year later the development of the mission was approved by the US Congress.

Already in 1969, it was reported about the plan for the parcel to Mercury in 1975 of the Meso spacecraft (Mercure Sonde), while the device was supposed to fly at a distance of 5000 km from the planet. It was assumed that the satellite made in Germany would be brought into the orbit of the atlas-centaur bearer with the Burner-2 acceleration unit, and the main object of the study will be the surface of the planet and its atmosphere. The resolution of the pictures obtained from the span of the trajectory was supposed to be 200 m.

But in reality, the first spacecraft that visited the first planet from the Sun was the Mariner 10 (“Mariner-10”), launched on November 3, 1973 from the Cape Kanveral-ladder “Atlas-3D/Centaur D-1A). It sets tasks for photographing the parts of the surface inaccessible from the ground, clarifying the mass of the planet, and studying the composition of a possible atmosphere, and the properties of a planetary environment. The on -board equipment included an ultraviolet range, an ultraviolet spectrometer, an infrared radiometer, a solar plasma detector, a heiger - muller meters for registering charged particles and two magnetometers.

During the flight, for the first time in interplanetary missions, an already mentioned scheme of flight with gravitational maneuver was used (the very first gravitational maneuver in the practical cosmonautics was carried out by the Soviet Lunar Luna-3 Soviet).

On February 5, 1974, Mariner 10 passed Venus at a distance of 5768 km. During the span, the device handed over to more than three thousand pictures of the planet to Earth. Having accelerated in the field of the Venusian gravity, he went to the trajectory of the flight to Mercury and on March 29, 1974 took a flight at an altitude of 703 km above the Mercury surface, successfully performing the shooting and various measurements. On this, the mission, according to the original plan, was supposed to end. But when the ballistics calculated the station trajectory after meeting with Mercury, the results were so amazed that they were first interpreted as erroneous, it was unthinkable to believe in such luck! It turned out that Mariner 10 enters a heliocentric orbit with a period of 176 days. Since the period of circulation of Mercury itself is 88 days, after two Mercury years, Mariner 10 was supposed to re -become close to the planet at the same point, near Afelia. As a result, despite a number of problems that arose in the operation of the on -board systems of the spacecraft, two more additional flights were taken - on September 21, 1974 at an altitude of 48,069 km and March 16, 1975 at an altitude of 327 km. Perhaps it would have been more, but on March 24, the reserves of the working body of the orientation engines ended, and the mission was completed.

According to the results of three spans, Mariner 10 transferred about 3,500 pictures to Earth, on which approximately 45% of the planet area with a resolution of up to 1 km were captured, while the maximum resolution in some areas reached 50 m.

What limited the possibilities of the surface to shoot the surface? As we have already noted, the interval between spans was 176 days. During this time, Mercury made not only two turns around the sun, but also three revolutions around the axis, that is, during each of the spans, he was in the same position regarding the sun and the spacecraft. As a result, in front of the Mariner chambers 10 each time the western hemisphere of the planet lit by the sun, from which it was possible to shoot the sectors between 120–190 ° W. d. and 0-50 ° s. The eastern side all three times remained unlit, and it was possible to look at it only after almost forty years, already during the flight of the Messenger mission. The surface of Mercury in the pictures obtained turned out to be similar to the surface of the moon and is covered with numerous shock craters, which differed in size. At the same time, it was also found on it that was also found for the lunar relief of Escarpa (cliffs) up to 3 km high and a length of up to a hundred kilometers. The most interesting was that the device found one ring structure similar to the lunar seas, but very large. It was called Caloris Planitia, or a plain of heat. True, during the span it was possible to shoot only a smaller half; The second due to the above circumstances remained shaded.

It turned out that the mercurian craters are less in diameter and depth than lunar. Scientists explained this by a stronger gravity of Mercury (3.72 m/c2 versus 1.62 m/c2) - with the same energy of the collapse of the regolit, which was knocked out by the explosion from the Mercurian surface, fell closer to the center of the crater than on the moon, covering about five times smaller area.

The reflective ability of the surface of Mercury was less than that of the moon. At the same time, the most vivid craters of both celestial bodies - the Mercurian coil and the lunar aristarch - had close brightness. In addition, such expressed “seas” were not found on Mercury and its surface was less contrasting compared to the lunar. The similarity of the polarization properties suggested that Mercury, like the Moon, was covered with regolithic-basalt rocks such as anortosites and a Kreep-rock, but with more than on the moon, the content of iron and titanium. From the height of the span orbit, it was also possible to estimate the surface temperature. According to data obtained by the infrared radiometer, at the point of noon, the surface of the planet was heated to +186 ° C, at night the temperature dropped to -183 ° C. Recalculation of the results for the point of perihelia of the orbit of Mercury showed that theoretically the temperature on the midday side can reach 610 ° C! However, according to modern data, the daytime side of Mercury can heat up to +290 ° C in Afelia and +420 ° C with the passage of perihelia - it is curious that the Sun is exactly above the plain of heat or above the opposite point. So the hottest place in the solar system is still on Venus (+475 ° C), however, Mercury retains the championship in terms of daily temperature amplitudes.

The results of the search for the Mercury atmosphere were sad for scientists. In fact, it did not turn out, even though in the vicinity of the planet and it was possible to find traces of helium and hydrogen, and subsequently still Argon, Neon, xenon, sodium, oxygen, calcium and potassium. But the pressure of this gas shell was about 0.2 NPA. That is, it would be more correct to talk not even about the atmosphere, but about the exosphere.

Another major discovery carried out during the first span was the detection of the plasma shock wave near Mercury, as well as the magnetic field, albeit extremely weak. But was it its own magnetic field of the planet or induced as a result of interaction with the solar wind? It was possible to establish this only during the third span that took place at the minimum height, during which it was confirmed that the discovered magnetic field is really generated by Mercury itself. And this discovery in those years has become a real scientific sensation - the fact is that, as the theory said, to excite the field, the planet of a liquid core is necessary, the possibility of which was doubtful: such a small planet as Mercury was supposed to cooled at the first quarter of its age. As a result, the existence of magnetism was associated with the presence of a partially molten metal (iron-nickel) nucleus, supported in a liquid state due to the action of a tidal effect due to the pronounced eccentricity of the orbit of Mercury. Based on the very high average density of Mercury (5.44 g/cm3), it was possible to estimate the size of this core, which should occupy up to 50% of the planet and extend to 0.75-0.80 of its radius.

Thus, the main results of the Mariner 10 mission were photographs of half of the surface of Mercury (which, before the Messenger flight in 2011-2015, were, in fact, the only map of this planet), measurement of the surface temperature, detecting the magnetic field and the collection of the first information about the gas environment of the planet.

What is the main density of Mercury? What is its geological history and the nature of the magnetic field? What is the structure of the nucleus? What is the composition of the atmosphere? What happens at the poles? Finally, what is on 55% of the surface of the planet, which has not been possible to see from the Mariner 10 side?

There were no answers to these and other questions all the next forty years. And about how scientists tried to find answers to them during the next mission to Mercury, we will talk in continuation of the material.

In the meantime, we recall that the first earth intelligence officer of the perisheular outskirts of Mariner 10, apparently, still moves along its path, once every 176 earthly days, making a meeting with the first planet from the Sun, strictly after it makes two revolutions around its axis. And under his lifeless lenses, the same area of ​​the surface arises once again-the same one that first appeared to his eyes in March 1974 ...

Ivan Sobolev,
Technical Director of LLC "Selelenok"