This past week brought humanity’s farthest and longest-lived spacecraft – Voyagers 1 and 2 – into the picture again for two reasons: it would have been Carl Sagan’s 85th birthday on November 9; and Voyager 2 made a new discovery.
Let’s start from the beginning and talk about what the Voyagers really are. In 1977, just eight years after humans had landed on the moon, another epic journey began. Twin robotic probes Voyager 1 and Voyager 2 were launched in 1977, with the primary mission to study the planetary systems of Jupiter and Saturn. They made game changing discoveries, such as the active volcanoes on Io and liquid-water oceans on Europa (both are Jupiter’s moons), imaging the complexities of Saturn’s rings, as well as studying the atmosphere of its moon Titan. The data provided by them allowed scientists to send further missions to Jupiter, Saturn and their moons
Their journey did not end there. The mission was extended – 3 times; and over 42 years later, the intrepid spacecraft are still sending back data. Their extended mission is called the Voyager Interstellar Mission (VIM) with the new objective: “to extend the NASA exploration of the solar system beyond the neighborhood of the outer planets to the outer limits of the Sun’s sphere of influence, and possibly beyond.”
Voyager 1 began its journey out of the Solar System in 1980 and crossed the heliosphere (a bubble-like region of space, which surrounds and is created by the Sun) in 2012, becoming the third spacecraft to enter interstellar space (Pioneer 10 and 11 had also done this but contact with them has been lost). Voyager 1 however, is still talking to us from a distance of 147.380 AU (22.0 billion km; 13.7 billion mi) from Earth, as of November 4, 2019. It has now become the most distant human-made object from Earth.
Voyager 2 went on to explore Uranus and Neptune; still the only spacecraft to have visited the outer planets. It reached Neptune in 1989, discovered six new moons, photographed its rings (yes Neptune has rings too) and observed a violent storm in its southern hemisphere. And then it also started to move out of the Solar System. It is still communicating with Earth and on November 5, 2018, NASA announced that it had crossed the heliopause (the boundary between the heliosphere and space), and had entered interstellar space. As of November 4, 2019, Voyager 2 is at a distance of 122 AU (1.83×1010 km) from the Sun, moving at a velocity of 15.341 km/s (55,230 km/h).
Both spacecraft are showing us how the Sun interacts with space by providing the first direct measurements of the density and temperature of plasma found in the heliosphere and the interstellar region (just outside of the Sun’s influence bordering the heliosphere). Voyager 1 confirmed that the plasma in local interstellar space is significantly denser than the plasma inside the heliosphere, indicating that it is being compressed. On November 2019, five new papers were published, analyzing data sent by Voyager 2 in 2018. It has now measured the temperature of the plasma in nearby interstellar space and confirmed that it is colder than the plasma inside the heliosphere (although slightly warmer than expected, another indication that it is being compressed). But Voyager 2 also observed a slight increase in plasma density just before it exited the heliosphere, indicating that the plasma is also compressed around the inside edge of the bubble. Scientists don’t yet fully understand what is causing the compression on either side.
There is also leakage of particles. According to Edward Stone, project scientist on the Voyager programme, Voyager 1 recorded an influx of high-energy cosmic rays before it even passed through the heliopause; a leakage from outside in, i.e Voyager 1 recorded what is called the “cosmic ray boundary layer” on the inside of the heliopause – what Stone calls “a connection to the outside”.
On the other hand, Voyager 2 recorded the exact opposite: an increase in low-energy particles (from solar wind) from the heliosphere to interstellar space – a leakage from inside out – showing that the “cosmic ray boundary layer” to be outside the heliopause. Basically, there is a leakage or spillage of particles from both sides of the heliopause. As Stone said, “There appears to be a region just outside the heliopause where there’s still some connection back to the inside.”
Space is indeed extremely bizarre and complex, however, these differences in the boundary where our solar winds meet cosmic rays could be due to the different trajectories of the probes, or due to the Sun’s 11-year solar cycle, during which its activity increases or decreases (currently at a low).
The twins are still beaming out signals to us, which take approximate 16 hours to reach Earth. But consider this for a moment: the technology being used on both is from the seventies and data is transmitted using a 40-year-old, 12-watt transmitter (around the same as the light-bulb in your fridge). The extended mission is expected to continue until 2025, when their radioisotope thermo-electric generators will no longer supply enough electric power to operate their scientific instruments. For now though, the two spacecraft are still in communication with Earth via NASA’s Deep Space Network. Even after we lose communication with them, they will continue on, maybe for billions of years.
Which brings us neatly to Carl Sagan Day, marking his birthday on November 9. Each space probe carries a Golden Record of sounds, pictures, music and messages from Earth, as well as a pulsar map giving directions to our planet, in case some alien civilization happens upon the Voyagers and wants to find us. Carl Sagan conceived the idea of the Golden Records, and a committee chaired by him selected everything on them. He said, “The spacecraft will be encountered and the record played only if there are advanced space-faring civilizations in interstellar space, but the launching of this ‘bottle’ into the cosmic ‘ocean’ says something very hopeful about life on this planet.”
And as always, let’s not forget the photograph of Earth taken on February 14, 1990 by Voyager 1, from a distance of about 6 billion kilometres. As Voyager 1 completed its initial mission and was on its way out of the Solar System, Sagan requested NASA to make it turn around and take one last photograph of Earth. That picture shows our planet as a Pale Blue Dot — a term coined by Carl Sagan — who then wrote one of the most fantastic pieces ever written, in his book of the same name. Let’s end with the first paragraph of that piece: