Artist rendering of proposed landers approaching Pluto. Image credit: Anatoly Zak / (probes), NASA (background)

Jul 14, 2015 Pluto's missing lander

Sen—When the New Horizons spacecraft pierced through the Plutonian system Tuesday, it missed the mysterious icy world by mere 12,500 kilometers—a hair-thin distance when compared to the colossal divide crossed by the probe on its nine-year journey. At the press conference following the flyby, Principal Investigator for the New Horizons mission, Dr Alan Stern, said he would like another mission to Pluto and that he had worked on a proposal for a lander mission, noting that the technology to land on Pluto would be much harder than a flyby, but that technology was being developed that could make it possible. In fact a mission concept to land on Pluto can be traced back to the archives of scientists who stood at the roots of the New Horizons project where designs are gathering dust for a lander concept that would have directly probed Pluto's tenuous atmosphere and even touched its surface. More surprisingly, some of these unrealized dreams were conceived in Russia!

Pluto was one of the few destinations shortlisted for the joint U.S.-Russian project known as Fire and Ice. It was a fruit of one of the earliest space agreements between the U.S. and Russia aiming to replace the Cold War competition with a peaceful cooperation in space.

Besides its symbolism, NASA had good practical reasons to cooperate with the Russians at the beginning of the 1990s. Pluto had already become a reachable target for exploration by spacecraft back in the time of the Voyager project in the 1970s, however the orbital mechanics forced the mission planners to trade off a once-in-a-generation opportunity to reach Pluto for a closer examination of the cloud-covered Saturnian moon Titan.

Then came the end of the Cold War and with it a financial ban on mega-expensive planetary missions. With its space budget tightening, NASA saw no hope for funding a powerful but expensive rocket, such as Titan-4 with a price tag of $400 million, which would be needed to accelerate a space probe fast enough to reach remote Pluto during the lifetime of scientists who designed it! Instead, there was now a prospect to get a free ride on Russian rockets, such as a medium-class Soyuz or even a heavy-lifting Proton.

On June 17, 1992, the U.S. Vice President Al Gore and the Russian Prime Minister Viktor Chernomyrdin, signed a historic agreement on the peaceful cooperation in space. The first project resulting from the deal had a self-explanatory name "To Mars Together", followed by "Fire and Ice."

The "Fire" part of the plan envisioned an unmanned probe heading to the Sun, while the "Ice" would target Pluto. At the Moscow-based Space Research Institute (IKI), which traditionally leads the nation's planetary exploration program, Vladimir Gotlib and Vecheslav Linkin took charge of the effort. They worked with their colleagues at the California-based Jet Propulsion Laboratory (JPL), Robert Staehle, the project manager, and Alan Stern, head of the scientific program.


The Pluto lander as envisioned at Space Research Institute (IKI). A pair of scientific instruments installed on the same deck with cameras but perpendicular to their line of sight are not shown.

The lander was first proposed as an umbrella-shaped vehicle less than a meter in size. The tiny spacecraft was expected to carry an energy-mass analyzer, which would take advantage of tremendous speed during the plunge into Pluto's weak atmosphere to sniff its chemical composition. Moreover, based on the Doppler shift of the radio signal from the descending craft relative to the flyby vehicle, it would also be possible to estimate the density of the Plutonian atmosphere, Gotlib explained.

No doubt outside of scientific circles people would be most excited about a video camera transmitting back Plutonian vistas as its host spacecraft zoomed toward the surface.

By August 1994, Russian engineers streamlined the design of the lander to just six kilograms in mass, not counting the separation hardware. However, the probe still included a 0.6-kilogram dual camera system, a half-a-kilogram mass-energy analyzer and a 0.3-kilogram mass spectrometer. The team expected that one or even a pair of landers would be released from the "mothership" around a month ahead of the closest approach to Pluto with a spin to give them stability. The probes would hit the Plutonian atmosphere with a mind-boggling velocity reaching 19 kilometers per second. The descent was expected to last 25 minutes. During this time between 80 and 100 images discerning details from 100 to 50 meters across could be produced by two cameras shooting through a rotating mirror facing a narrow hole in the center of the heat shield.

Images and scientific information would be sent to the main spacecraft via a pole-like antenna. A miniature heater powered by radioactive plutonium with a mass of just 0.2 kilograms would empower the work of all the instruments and electronics in the extremely cold conditions. A 0.7-kilogram battery would power the probe.

In the meantime, astronomers on the ground forecasted that the Plutonian atmosphere at the time of the probe's arrival would be strong enough to cause the lander to burn up completely before reaching the surface, even despite its beryllium-laden heat shield. Determined to reach the surface, IKI scientists conceived a small but massive impact probe, which would be dense enough to survive the entry into the Plutonian atmosphere.

"The lander could release a massive but small-size projectile made of silver which would be guaranteed to reach the surface at high velocity. The resulting explosion would create a cloud, which would be observed by spectral instruments onboard the main flyby vehicle, allowing to determine composition of the surface," Gotlib said.

A veteran of Venus exploration at IKI Vasily Moroz, who made the proposal, jokingly called the experiment "bolvanochnaya (dumb down) spectrometry," Gotlib remembered.

Unfortunately, as the Russian and American scientists were working out the technical details of the lander in the mid-1990s, the economic situation in Russia was getting progressively worse.

Squeezed to the limit, the Russian space agency had no choice but to ask its international partners to pay for rockets and other services provided by its institutions. As a result, NASA lost its main motivation for cooperating with Russian scientists.

According to Gotlib, after 1995, the U.S.-Russian discussions on the Pluto mission were reduced to a possibility of using Russian-built power and heat generators, which relied on radioactive plutonium. In the 1970s, similar devices proved their efficiency onboard of the famous Soviet moon rovers, Lunokhods, providing heating under harsh conditions of the lunar night.

In the following years the Americans faced their own budget crunch and the Pluto mission came to a brink of cancellation at the turn of 2000s. It was eventually reborn as the New Horizons project, which finally made it to the launch pad in 2006, but without landers or impact probes. As a result, a historic opportunity to touch Pluto was lost.

"I would like to congratulate the participants of the current multi-year project with a great success and with the beginning of the exploration in the vicinity of Pluto," Gotlib wrote in email to the author.