The scientifically validated theory on the evolution of the solar system and its constituent bodies figures prominently in space research. This will make it possible - within the framework of comparative interplanetary studies - to get a better idea of how the earth came into being and more, predict its future. Planets Mars and Venus are of the utmost interest to us in this respect. Somehow our planetary scientists have had a poor run of luck where the Red Planet, Mars, is concerned. But they have been more fortunate with regard to Venus, the Morning Star, also known as the "Russian" planet. Indeed, our planetologists have amassed a wealth of information on Venus. RAS corresponding member Lev Zeleny, heading the RAS Institute of Space Studies, has told the Russian language scientific and technical journal Polet (Flight) about progress in Venusian studies and what has been accomplished toward unlocking the planet's mystery. Here is an English version of his interview.

Venus is our closest space neighbor. As, moving along its orbit, it passes between the earth and the sun, it comes

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closest to us at a distance of about 40 million kilometers. The Morning Star is a bit smaller in mass and size than our globe, and gets as much solar energy.

Cosmogonically, the similarity of the basic features of Earth and Venus is understandable by and large: according to present theories, the planets of the solar system emerged from a primeval nebula condensed from interstellar matter; its solidification (hardening) process caused the greater part (99.9 percent) of the mass to be congregated in the center, where the sun was born. The rest came to be compressed in a compacted disk that took in solid particles of different dimensions. Colliding, they merged into planetary embryos called planetesimals that in time, over hundreds of millions of years, grew into regular planets. This process came to an end about five billion years ago.

The Earth and Venus are located in the middle of the belt of four "terrestrial", or solid, planets closest to the sun (Mercury, Venus, Earth, Mars, with Mercury and Mars much smaller in size than Earth and Venus). Since the mass and size of both Earth and Venus are nearly equal, they were thought to be alike in their chemical composition and internal structure as well. Accordingly, even 50 or 60 years ago many astronomers believed that the atmosphere and basic characteristics of the surfaces of these planets were not much different either. Some even did not rule out the probability of biological life on Venus.

The first data on the physical characteristics of the Venusian atmosphere were obtained back in the 1920s with the aid of infrared radiometry telescopes. It became possible to measure the temperature of the upper boundary of the Venusian cloud cover.

The discovery of carbon dioxide absorption bands on Venus came as the next step. They were believed to be but a smaller part of the Venusian atmosphere. Next, planetary scientists detected radio-frequency thermal radiation showing that the Venusian surface and the lower layers of the planet's gaseous envelope are very hot. At first one questioned the validity of the interpretation of these measurements. Yet all doubts were dispelled after the momentous flight of our Venera-4 (1967) space probe that, entering the Venusian atmosphere, radioed signals back to Earth. Venera-4 ushered in a new stage in the Morning Star's exploration carried on by space vehicles Venera-6 (1969), Venera-7 (1970) and Venera-8 (1972).

These spectacular voyages allowed to determine the principal characteristics of the planet's atmosphere and its components: carbon dioxide and nitrogen. The pressure near the Venusian surface was found to be around 90 atm, and the temperature there, 735 K (that is, on the Kelvin scale; it decreased at higher altitudes up to 55 km). It became clear that, though Earth and Venus are alike in major planetary characteristics, their atmospheres are most unlike.

As a matter of fact, everything is interconnected in the gaseous envelope of planets. Let us take, for instance, the chemical composition of the lower atmosphere. It depends, in particular, on what gases in particular can escape from the upper atmosphere into the interplanetary void as a result of thermal dissipation*; in turn, the density and chemical composition of particles in the clouds depend on the presence of condensation-capable gaseous components. Even though the amount of such elements could be very small, down to ppm fractions, this would be sufficient for the formation of particles. At the same time even minor components can increase the atmospheric temperature by hundreds of degrees if they absorb much radiation in the infrared region of the spectrum. Furthermore, the wind velocity and direction depend on a temperature differential in different parts of the planet, though these parameters are leveled off by atmospheric currents.

We can learn about the makeup and workings of a particular atmosphere only in the course of comprehensive investigations. That is why the possibilities of our first automatic Venera probes ceased to satisfy us soon: Venera stations had no room enough for complex research instruments, and they conveyed but all too little information through communication channels.

From 1975 on came a second generation of Venus stations (Venera-9 and subsequent probes) equipped with descent packages sent to the Venusian surface and capable of operating there for a sufficiently long time. Such descent modules and orbital satellites circling about the Morning Star were a stride in Venusian studies. In the 1980s Venera-15 and Venera-16 made radio surveys of our neighbor's surface at low latitudes.

The fact that Venus is devoid of a magnetic field came as a poser: how to explain the interaction of a conducting nonmagnetic body like Venus with a supersonic flux of

* Dissipation (scattering) - here, the gradual escape of gases from a cosmic body's atmosphere (e.g. from that of Earth) into ambient cosmic space; dissipation is caused by chaotic heat motion of atoms and molecules. - Ed.

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solar plasma? Taking a closer look, our planetary scientists were the first to prove that the Morning Star had a magnetic "tail" growing - like it is with Earth. Yet the earth magnetic tail is formed by the lines of force of the terrestrial magnetic field, while the Venusian "tail" - by the solar wind lines of force. Flowing around Venus, the solar wind lines submerge into its gas envelope and capture heavy ionospheric ions (those of oxygen and carbon dioxide for the most part). And so Venus grows a "tail" in which the direction of the magnetic field is determined by the transverse component of the solar wind field. This is what makes it different from the earth magnetic field.

The most difficult job in Venusian research was tackled by the automatic stations Vega-1 and Vega-2 sent in 1984 - 1986 towards Halley's comet. But first they headed for the Morning Star on the initial leg of their voyage. Their descent packages were composed of two parts: one, a classical landing module and the other - something quite new, an aerostat (balloon) probe.

Unlike the former, orthodox module, giving a vertical profile (section) of the Venusian atmospheric characteristics, the other one - balloon - flowing at a particular altitude, moves at wind velocity and provides a horizontal profile and data on the atmospheric dynamics. It came out that all of Venus is enveloped by a powerful zonal (i.e. latitudinal) circulation. At altitude 60 to 70 km the wind velocity is ca. 100 m/s. It decreases at lower altitudes closer to the surface and becomes equal to ca. 1 m/s at altitude below 10 km. Consistent balloon measurements (equatorially they were being made at altitude 54 km or so) showed the mean wind velocity to be about 70 m/s. It is hard to explain a powerful circulation like that on a slowly rotating planet; the same is true of other phenomena discovered by automatic Venusian probes. Thus further data and subtler experiments were needed for answering these and other questions. But it came to pass that, having sent as many as 18 unmanned vehicles to Venus, our planetary science turned its looks to the Red Planet, Mars...

Venusian studies were resumed in 2005 by the European station Venera-Express* that went on with studying the atmosphere, circumplanetary plasma and surface of the Morning Star. Russian planetary scientists joined this project at its earliest stages, and they made a significant contribution to research program and equipment (there were seven Russian-made devices on board).

Venera-Express made detailed pictures of the southern polar whirlwind (much to their surprise, scientists saw that it was dual in form - two "eyes" of the hurricane were clearly visible). Another discovery concerned one of the elements of the atmospheric dynamics, what we call the southern polar dipole. In the course of the previous missions - of Venera-16 in particular - only the northern hemisphere's dipole had been detected. But it turned out that a dipole like that is also above the southern hemisphere.

In another pioneering study, Venera-Express monitored Venus in radio transparency windows in the near infrared region of the spectrum. It identified structures at different altitudes in the atmosphere. These structures are yet to be explained. In addition, an optically dense haze was seen on the southern part of Venus. A suchlike time-dependent variable phenomenon was earlier observed on its day side only.

* See: O. Korablev, "Another Voyage to Venus", Science in Russia, No. 2, 2006. - Ed.

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While on Mars* water was discovered only in the form of subsurface ice, Venus has no water at all. It must have escaped into the interplanetary void. This is testified by data obtained from Venera-Express on the concentration of deuterium and hydrogen in the upper gaseous shell of the planet (a similar indicator for the earth is only 1/150th).

Now, what are we to learn next?

Planet-C, a Japanese-made automatic vehicle, is to be sent toward the Morning Star in 2010. It will concentrate on meteorological studies of the planet from an equatorial orbit with the aid of multispectral mapping hardware. There is no doubt that the data supplied by Veneral-Express and Planet-C will be yet another major stride in learning more about the physics of our close space neighbor. However, due to its imitations, remote sounding cannot clear the picture to the full. Such things as the content of isotopes of noble gases in the Venusian atmosphere will not be elucidated; this knowledge is important for understanding the evolution of Venus and the entire solar system. Precise measurements of the concentration profiles of minor gaseous components and analysis of the composition of aerosol particles is essential for resolving problems related to the chemistry and geochemistry of the Venusian cloud cover. Other question marks will remain as well, i.e. the mineralogy of the planet's surface, its interaction with the atmosphere as well as the geological evolution and, particularly, the internal structure of Venus. These problems could be solved via contact studies that could be carried out on board descent modules (they should be "long-livers"!) and with the help of balloon-borne atmospheric laboratories.

The idea of a long-living station on Venusian surface (for making all kinds of measurements, among other things) harks back to the 1980s after the Venera-9 and Venera-10 missions. The idea was conceptualized at the initial design stage, but then this work was suspended in 1981 and 1982.

The project Venera-D (long-living Venus station) is on the list of the Federal research program for 2006 to 2015. Preliminary work was begun in 2007. The interplanetary station is to be launched between the years 2016 and 2018. The mass of the space vehicle will make up 1.5 tons, and that of the descent module, 1 ton. Its research "fillings" should keep working for about a month at high temperature and pressure. Let us recall that the descent packages of the previous Venusian stations could survive on the Venusian surface for 1.5 h at the most.

The Russian Venera-D mission could be combined with the European program of Venusian studies. This possibility is now under consideration. Optimally, this system could be launched from the Kuru spacedrome in French Guiana by the carrier rocket Soyuz-2 and our acceleration pad Frigate**. Its payload will include: a European orbital module developed on the basis of the Venera-Express station; a new Russian-made landing package containing a long-living element with research equipment; a European aerostat (balloon) - possibly, carrying small probes ejected to the Venusian surface with a minimal kit of instruments - for studying the planet's atmosphere.

Since this is going to be a joint international experiment there could be a mutual give-and-take arrangement, that is European technical facilities could be installed in the Russian descent apparatus, while ours - within the orbital module and in the car slung beneath the balloon.

A small Japanese low-flying balloon might also take part because its European analog is to drift at altitudes having fairly comfortable temperature and pressure conditions. The Japanese balloon is to be employed in the subcloud zone, that is at about 35 km, where the atmosphere is hot enough and the pressure is rather high. Measurements at different altitudes will supply a good deal of extra information.

So the interest of planetary scientists in Venus is still much alive. But they know all too well that they can accomplish their purpose only through mutual exchange of new ideas, original technical solutions and regular contacts and, in the long run, by joint efforts on the part of all people of the earth.

L. Zeleny, " Space Studies and the Planet Venus", Polet (Flight) Journal, No. 8, 2007

Prepared by Yaroslav RENKAS

* See: 1. Mitrofanov, "Unlocking Martian Enigmas", Science in Russia, No. 6, 2002; M. Litvak and I. Mitrofanov, "Martian Seasons", Science in Russia, No. 4, 2004. - Ed.

** See: Yu. Markov, "Frigate About to Set Sail", Science in Russia, No. 2, 1995; Yu. Markov, "Frigate" Sets Sail", Science in Russia, No. 3, 2001; Yu. Markov, "Russia and Israel: a Communication Satellite in Orbit", Science in Russia, No. 3, 2004. - Ed.