Huygens to test Volta's 200-year old invention at Titan
27 March 2000
The Cassini/Huygens spacecraft has already completed a third of its interplanetary journey to Saturn and Titan. During the journey the Huygens Probe is usually dormant, so it does not require much electrical power to stay healthy. Even when it does need electrical power for the periodic checkout activities, it can always rely on the Orbiter's radio-isotope thermo-electric generators. So the probe is like a baby in the womb, being fed by the mother Orbiter. Once the baby is born, it has to learn to survive by itself and the same applies to Huygens. After the release from Cassini, the Probe will have to rely on its own power electrical generator: 5 LiSO2 batteries with a total capacity of 1800 Wh.Alessandro Volta, the Italian physicist who invented the first battery, would be very proud if he knew that his discovery 200 years ago is playing such an important role in a mission as delicate as Huygens. Volta's name is also linked with Titan exploration, because in 1778 he discovered the gas methane, one of the main components of Titan's atmosphere. But his main claim to fame remains that, on 20 March 1800, on a foggy morning in Pavia near Milan (typical early spring weather for Northern Italy), he announced that he had developed the first electric battery, providing future generations with a constant source of current electricity, storing power with efficiency and economy. Today we call a battery any device used to store electrical energy, essentially a tin full of chemicals that produce electrons. Batteries have three basic components: the anode, the cathode, and an electrolyte system. Electrons flow from the metal cathode through an electrical device to the anode, made of a different metal. This current allows the electrical device to function. The electrons are supplied by a chemical reaction in the electrolyte system.
Modern battery design focuses on finding ideal combinations of metals and electrolytes to simultaneously maximize the amount of electrons produced by the chemical reaction and the speed at which they can pass through the system. The nature of the materials used is critical to maximise portability, versatility, safety and the possibility for recharge in a cell.
In the past 200 years, battery technology has progressed, seeking more flexible, efficient ways to store electric power. When Volta created the first battery, he layered zinc, blotting paper soaked in salt water, and silver, this arrangement was known as a Voltaic pile.
After Volta's success, other scientists sought more efficient energy storage techniques. The first commonly used battery in the 1800s was the Daniel cell, which was used for telegraphs and doorbells. Producing a more powerful current than the voltaic pile, its use was restricted because it had to remain stationary due to the sensitive nature of the electrolyte system.
The Zinc-Carbon battery, the forerunner of most modern-day batteries, was designed in the 1860s. It was durable and inexpensive, and a new, 'dry' cell quickly replaced the early 'wet' cell. Dry cells are superior to wet cells because there is much less chance that the corrosive materials, being paste- or gel-like instead of fluid, will leak and cause damage.
Rechargeable batteries, a more efficient alternative to primary, or one-use, batteries, were designed only a few years after Volta's cell, but were temporarily abandoned because there were no means to recharge them with any degree of efficiency, undermining their advantage. In 1890, Thomas Edison's use of a nickel hydroxide anode and an alkaline electrolyte created the first rechargeable alkaline system. Commercial nickel batteries began targeting the electric car market in 1910. Since then, rechargeable batteries have become more and more widely used.
World War II sparked more battery advances. The mercury cell was devised for use in harsh climates where ordinary zinc-carbon batteries used in flashlights, mine detectors, and walkie-talkies were not durable enough. A German design of a Nickel-Cadmium battery supported the need for lightweight, high-energy batteries, and that design is nearly identical to those used on jet aircraft today.
Battery evolution is still very much an issue in research and design. as engineers try to create a battery that combines all of the best aspects of the current models. The perfect battery has a high energy density, can withstand the rigours of portability, has a long-life span, is safe, allows application flexibility, and is rechargeable. This technology is becoming even more critical than in the past because of increasing dependence on electrical devices and interest in alternative energy sources.
Batteries were therefore the obvious choice for the electrical power system of the Huygens Probe, after separation from the Orbiter. During the cruise phase, no battery energy is used as Huygens relies solely on electrical energy supplied by the main Orbiter during the periodic checkouts. Apart from the minute 'natural' energy loss that occurs in any battery that is stored for a long time, the 1800 Wh of battery capacity is well preserved for the mission phase at Titan. To achieve this, it is important to keep the battery temperature in the right range (better cool than warm); special care was taken during the thermal design of the probe to ensure the desired storage temperature (around 10-15 C) for the Huygens batteries during the seven-year long cruise phase. The five LiSO2 batteries will therefore provide sufficient mission power for the transmission between the Orbiter and the Probe during the descent and an additional surface phase, just after landing. Its nice to know that a 200 hundred year old discovery still has a part to play in one of the biggest technological challenges of the new millenium.
