By Anatoly Zak
The Venera missions produced tantalising images of the Venusian surface
Densely clouded in acid-laden mist, Venus used to be the Soviet Union's favourite target for planetary exploration.
Now, after a lull of almost three decades, Russia is making plans for a new mission to the "morning star" and has invited Western scientists to participate.
Last week, Moscow-based space research institute IKI hosted an international conference aimed at luring scientists from Europe and possibly other countries such as the US into the ambitious project, officially scheduled for launch in 2016.
"The goal of this conference was to consolidate our and European scientists' ideas and determine what approaches toward this project we could use," says Oleg Korablev, who leads the work on returning to Venus at IKI.
The Soviet Union last went to Venus - jointly with a number of European nations - with the twin Vega probes of 1985. This followed 16 Soviet-only Venera missions between 1961 and 1983.
Known as Venera-D, the new mission is expected to involve a multi-front scientific assault on the mysteries of Venus with an orbiting spacecraft, multiple air balloons, a surface lander and - possibly - an innovative "wind-flyer".
"We want this mission to be as cutting edge as possible," says Ludmila Zasova, a leading planetary scientist at IKI who helped to organise the Moscow event.
"At this day and age, this is not easy even for a very rich country, therefore we understand that this mission has to be co-operative."
The project is in its preliminary stages, and faces enormous technical and financial challenges. Nonetheless, the initial reaction of the European scientists seemed very positive.
Venera-D would take a suite of modern instruments to the "morning star"
"What surprised me was the level of dedication that Russians [put] into this mission," says Hakan Svedhem, a scientist with the European Space Agency (Esa).
"Questions remain of when exactly it is going to fly and what it is going to contain."
Dr Svedhem works with the Venus Express mission which currently orbits the planet, following its successful launch onboard a Russian rocket in 2005.
He believes Venera-D has a good prospect of getting funded by the Russian space agency, and now discussion is focusing on what elements the mission could include.
"The Russian space science research budget has been expanding quite dramatically for a number of years, and it seems possible that there will be room for this mission," he said.
"We are in the very early stage, but I am sure many Europeans will be involved in the end in this mission."
Venus scientists, unite!
Venera-D would be the second attempt in the last few years to combine efforts to explore Venus by European and Russian scientists.
A previous proposal to merge Venera-D with the European Venus Explorer, EVE, had to be abandoned when the European mission was rejected in the favour of other planetary projects.
EVE scientists still intend to resubmit the project for Esa's next selection round at the end of next year; and according to industry sources, the mission has a good chance of getting funding.
However, even with a successful bid in 2010, EVE's launch date would inevitably be pushed beyond 2020.
"This new launch date does not correlate well with the timeframe of the Venera-D mission," says Dr Korablev.
As a result, he says, a different approach is needed.
The IKI conference in Moscow was timed to coincide with the end of the "paper" phase of Venera-D and the start of actual spacecraft development, scheduled for early next year.
During the last few years, the project has been through several incarnations, as scientists tackled the enormous engineering challenge that going to Venus entails.
Esa's Venus Express found enigmatic patterns of ultraviolet absorption
From the outset they understood that in order to unlock the planet's secrets, their equipment would have to survive on the oven-hot surface beyond the hour-long lifespan of previous Soviet landers.
A longer stay would enable sensors on the surface to listen for tremors, which would give clues about the internal structure of the planet.
However, after sifting through various options for cooling schemes and exotic heat-resistant electronics, an initial 30-day survival goal had to be cut to one day.
"We now understand that it may not necessarily be justified, and may be too expensive, to last 30 days," IKI's director Lev Zeleny told the BBC.
"We did invent some cooling systems; however, we have very limited mass (on the lander), and would have to sacrifice a very interesting experiment studying dynamics of the atmosphere (in order to install it).
"Now we want somewhere around 24-hour survivability... but even in one day we can do a lot."
Navigating the Venusian skies
To compensate for the lander's quick frying death, Russian scientists proposed a number of innovative solutions in the exploration of Venus.
A series of balloons designed to float in the misty atmosphere of the planet would be deployed either from the main lander during its descent to the surface, or from protective capsules capable of penetrating the Venusian atmosphere.
Although balloons did once float over Venus as part of the Soviet Vega missions, a new lightweight generation of sensors promises to revolutionise their scientific harvest.
The "juice" of the future research on Venus would be studying isotopes of noble gases such as xenon and krypton in the planet's atmosphere.
Since they react only reluctantly with other substances, they could help to draw a blueprint of the ancient Venusian atmosphere.
The Venera missions were pioneering in the 1970s and 80s
In turn, such information could help to understand processes involved in the formation of closely located planets such as Earth and Venus.
"These are extremely sensitive measurements which have never been done (on Venus)," Dr Korablev told the BBC.
They would provide a colossal breakthrough in the quality of science compared with the Soviet missions completed in the 1980s.
"Even if we take existing instruments from Huygens (the European craft that landed on Titan in 2004) and send them to Venus, it would be a huge progress," he said.
Along with their primary instruments, Venus balloons could carry "microprobes" - essentially pieces of ballast filled with additional sensors.
As balloons drift in the Venusian sky, microprobes would be dropped one by one, to provide scientific measurements in various locations.
Flying in the wind
In addition to balloons and a lander, NPO Lavochkin, Russia's premier developer of deep-space spacecraft, has proposed a "vetrolet" (a Russian term for "wind-flyer") for the Venera-D mission.
A kite-like device could reportedly use the alien winds of Venus to stay aloft almost indefinitely at altitudes of 45-50km, with lightweight instruments and transmitters onboard.
Lavochkin has apparently conducted some experiments to prove that this exotic idea is workable.
Even if the Venera-D mission will be able to afford the additional mass needed to accommodate the device, the "wind-flyer" would probably be included only as an additional demonstrator rather than as the main platform for scientific instruments.
Still, if successful, it could open a new page in the history of planetary exploration.
As a first sign that Russian efforts to internationalise Venera-D are bearing fruit, the French space agency CNES has expressed preliminary interest in taking full responsibility for developing the balloon part of the project.
However, French representatives told their Russian colleagues that a 2016 launch date for the mission would not be realistic for their contribution, and delay to 2018 would probably be needed.
A CNES delegation is expected in Moscow again in the middle of this month for further discussions.
Given Russia's own situation, IKI scientists might be willing to be flexible with the launch date for the sake of international participation.
A recently announced delay of Russia's flagship Phobos-Grunt mission from 2009 to 2011 is expected to cause a domino effect on all further plans.
Still, following Phobos-Grunt and one or two missions to the Moon, Venera-D is lined up to become the next planetary project.
IKI scientists told their European colleagues that they intended to hold conferences on the Venera-D project annually, in the hope of expanding co-operation as the mission moves toward fruition.