I know you’ve heard it before, but it’s now official: last week NASA announced that Voyager 1 has finally entered interstellar space – the space between the stars. While still within the gravitational influence of our Sun, and therefore still inside the Solar System (as is the Oort cloud, the home of the long period comets), NASA scientists have announced that Voyager 1 has pierced the heliopause and is now in a transition region just outside the solar bubble.
The heliosphere, or “solar bubble”, is a region surrounding the Sun where the solar wind plasma dominates over the interstellar medium. There are three important regions inside the heliosphere: the termination shock is a thin region where the supersonic solar wind is slowed down to subsonic speeds. The heliopause is a bullet-shaped boundary that separates the hot solar plasma from the cooler interstellar plasma – the “other” side of the heliopause is interstellar space. The heliosheath is a region of shock-heated solar plasma between the termination shock and the heliopause. In 2004 Voyager 1 crossed the termination shock at 94.0 AU (while Voyager 2 crossed the termination shock at 83.4 AU in 2007). According to Ed Stone and collaborators, Voyager 1 crossed the heliopause into interstellar space on 25 August 2012.
The twin spacecraft, Voyager 1 and Voyager 2, were launched in 1977 and after 36 years they have far exceeded their original 5-year mission! Both spacecraft send information via the Deep Space Network, taking more that 17 hours for the signals to reach Earth. And none of this would have been possible without the three batteries onboard the spacecraft that are fuelled by plutonium-238. Indeed, all of our interplanetary explorers — including Saturn’s Cassini spacecraft, the Galileo spacecraft that visited Jupiter, the Curiosity rover currently exploring Mars, and the New Horizons spacecraft which will flyby Pluto in 2015 — are all powered by plutonium-238.
Plutonium-238 is a radioactive isotope of plutonium, and decays because its atomic structure is unstable. When the nucleus spontaneously decays, it releases an alpha particle (two protons + two neutrons, bound to form a particle identical to a helium nucleus) and leaves behind a uranium atom. The alpha particles collide with and heat the surrounding plutonium and this heat is converted into electricity in radioisotope thermoelectric generators (RTGs) that power the spacecrafts – for decades! The three RTGs on Voyager 1 are expected to continue supporting the mission until 2025.
The problem for future space travel, however, is that we’re running out of plutonium-238.
Plutonium-238 is a radioactive isotope of plutonium, which naturally occurs in exploding stars. However, with a half-life of only 88 years, any naturally occurring plutonium-238 trapped in the Earth during its formation has long vanished. However, humans have created plutonium-238, which is a byproduct of plutonium-239, which was produced for making nuclear weapons. With the fall of the Iron Curtain in 1988, both the USA and Russia have dismantled their bomb-making nuclear facilities. The Russians were processing their nuclear reactor fuel to produce plutonium-239 which they sold to the USA, but this stopped in 2009. It is now thought that the Russian’s too have run out of plutonium-239. The USA’s scientific stockpile ear-marked for NASA is apparently now only at about 36 pounds (or 16 kg). That only leaves enough fuel for a few small missions before 2020. In its current design, the proposed Jupiter Europa Orbiter, which aims to searching for signs of life in Jupiter’s moon Europa, would require 47 pounds of plutonium-239. Apparently at NASA the issue of the lack of plutonium-239 is simply referred to as “The Problem”.
For more information, see:
- NASA’s Plutonium Problem Could End Deep-Space Exploration, Dave Mosher, WIRED
- NASA Spacecraft Embarks on Historic Journey Into Interstellar Space, NASA/JPL news
- How Do We Know When Voyager Reaches Interstellar Space?, NASA/JPL news
- In Situ Observations of Interstellar Plasma With Voyager 1, Gurnett et al. (2013), Science Express, 1241681
- Voyager 1 Observes Low-Energy Galactic Cosmic Rays in a Region Depleted of Heliospheric Ions, Stone et al. (2013), Science, 341, 150