Voyager 1 and Voyager 2

 Voyager 1 and Voyager 2

Launched in 1977 and carrying the same suite of ten instruments, Voyager 1 and Voyager 2 reached interstellar space in 2012 and 2018, respectively. It is little wonder that Voyager 1 and Voyager 2 are operating on dwindling power supplies. After all, the two spacecraft have travelled a combined 29 billion miles to become the farthest human-built objects from Earth. Both passed Jupiter in 1979 and went on to Saturn a few years later, after which Voyager 2 also continued on to Uranus and Neptune. Eventually both entered interstellar space, leaving the heliopause, where the Sun’s solar wind and the interstellar wind of the Milky Way Galaxy meet and are in balance, in August 2012 and November 2018, respectively. Voyager 1 is farther from Earth at nearly 168 astronomical units (AU) from the Sun because it is moving faster (11 miles [18 km] per second, or 3.6 AU per year) than its sister ship (which is moving at 10 miles [16 km] per second, or 3.3 AU per year). One AU is defined as the mean separation between Earth and the Sun, or 93 million miles (150 million km's). For reference, Earth’s orbital speed around the Sun is about 6.3 AU per year (19 miles [30 km] per second). NASA engineers are turning off two instruments to ensure that the twin spacecraft, Voyager 1 and Voyager 2, can continue exploring space beyond the limits of the solar system. "The Voyagers have been deep space rock stars since launch, and we want to keep it that way as long as possible," Voyager project manager at NASA Jet Propulsion Laboratory Suzanne Dodd said. "But electrical power is running low. If we don’t turn off an instrument on each Voyager now, they would probably have only a few more months!"

Currently considered outside the solar system, the spacecrafts will nonetheless take close to 300 years before reaching the Oort Cloud. The chances of either craft coming into contact with one of the comets located within the cloud is less likely than winning the lottery. To save energy for further interstellar exploration, mission engineers at NASA’s Jet Propulsion Laboratory (JPL) deactivated Voyager 1's cosmic ray subsystem experiment on 25 Feb. On 24 March, they shut down the low-energy charged particle instrument on board Voyager 2. Both Voyager spacecraft have a power system based on generating electricity from the heat emitted by the decay of a radioactive isotope of plutonium. This radio isotopic power system loses around 4 watts of power from Voyager 1 and Voyager 2 each year. In the 1980s, several instruments aboard both spacecraft were turned off. This was because the Voyager twins had both completed their investigation of the solar system's giant planets, which boosted the longevity of both probes. To conserve this power, NASA operators turned off Voyager 2's plasma science experiment. The experiment aimed to measure how much plasma flows past it and in what direction. The Voyager 2 instrument had been collecting limited data in the years before its shutdown due to its orientation of Voyager 2 in relation to the flow of plasma beyond the solar system.

In 1950, Leiden University astronomer Jan Hendrik Oort theorized the existence of a cloud of comets beyond some 10,000 AU from the Sun, possibly filling the volume dominated by the Sun’s gravitational influence in the Milky Way Galaxy. This volume is an almost spherical bubble stretching as far as 100,000 AU. In comparison, the distance of the nearest star, Proxima Centauri, is about 270,000 AU (4.24 light-years). So far, the Oort Cloud has not been observed directly, but many visiting comets originate from this enigmatic region. Oort estimated that the cloud would contain some 100 billion comet-sized objects (meaning a radius of 6.2 miles [10 km]). The total mass would be roughly comparable to Earth’s. Some comets, such as Hale-Bopp, are much larger and more massive, but smaller comets are most common. With a 6.2-mile (10 km) radius, a comet has a surface area of about 116 square miles (300 square km). If we spread out all 100 billion comets, they would cover a surface area roughly comparable to the Sun’s surface. Now imagine that each of these comets is lifted from the Sun’s surface to a distance of 10,000 AU, the inner edge of the Oort Cloud. Each comet will then become separated from its neighbors and the surface area covered by them drops by a factor of about 1/1013. If this is difficult to imagine, here’s a more earthly example: Compare the comets in the Oort Cloud to the 96 million black styrofoam balls dumped in 2015 in the Los Angeles reservoir to prevent evaporation. If we spread those styrofoam balls over the entire Earth’s surface, the probability of finding one of them in your garden is comparable to winning the jackpot in a lottery. These odds are still 100,000 times larger than the probability of Voyager hitting a comet. 

The data from the suite of three telescopes designed to study cosmic rays was integral in the Voyager science team's determination that Voyager 1 had exited the heliosphere, the sun's sphere of influence at the edge of the solar system. Whereas, Voyager 2’s low-energy charged particle instrument is to measure the various ions, electrons and cosmic rays originating from the solar system and our galaxy. "The Voyager spacecraft have far surpassed their original mission to study the outer planets," Voyager program scientist Patrick Koehn said. "Every bit of additional data we have gathered since then is not only valuable bonus science for heliophysics, but also a testament to the exemplary engineering that has gone into the Voyagers, starting nearly 50 years ago and continuing to this day." The fact that the Voyager spacecraft are the only two human-made objects to make it to interstellar space means that the data they collect is unique. Thus, the decision to switch off any instruments on either Voyager 1 or Voyager 2 isn't taken lightly. Shutting down these two instruments should grant both spacecraft another year of exploration before more instruments have to be turned off. Both spacecraft have three operational instruments, dropping to two in 2026. It is hoped that Voyager 1 and Voyager 2 will carry one operational instrument into the 2030s. Unforeseen circumstances could arise and change these plans, though.

"Every minute of every day, the Voyagers explore a region where no spacecraft has gone before," Voyager project scientist at JPL Linda Spilker said. "That also means every day could be our last. But that day could also bring another interstellar revelation. So, we’re pulling out all the stops, doing what we can to make sure Voyagers 1 and 2 continue their trailblazing for the maximum time possible."