An interstellar instrument bows for the last time | MIT News

They planned to fly for four years and reach Jupiter and Saturn. But nearly half a century and 15 billion miles later, NASA’s twin Voyager spacecraft has far exceeded its original mission, flying past the outer planets and breaking out of our heliosphere without the sun’s influence. The probes are currently making their way through interstellar space, traveling further than any human-made object.

On their unlikely journey, Voyager made the first observations of its kind on all four giant outer planets and their moons, using just a handful of instruments, including MIT’s Plasma Science Experiments – identical plasma sensors designed and built in the 1970s in Building 37 by MIT scientists and engineers.

The Plasma Science Experiment (also known as the Plasma Spectrometer, or PLS for short) measured charged particles in the magnetosphere of planets, in the solar wind and in the interstellar medium, the matter between stars. Since the launch of the Voyager 2 spacecraft in 1977, the PLS has discovered new phenomena near all outer planets and in the solar wind throughout the solar system. The experiment played a crucial role in confirming the time at which Voyager 2 crossed the heliosphere and moved out of the solar regime into interstellar space.

Now, in order to conserve Voyager 2’s little remaining power and extend the mission’s lifespan, Voyager scientists and engineers decided to do this shut down MIT’s plasma science experiment. It is the first in a series of scientific instruments that will gradually disappear in the coming years. On September 26, Voyager 2 PLS sent its final message from 12.7 billion miles away before receiving the order to shut down.

MIT News spoke with John Belcher, the Class of 1922 Professor of Physics at MIT, who was a member of the original team that designed and built the plasma spectrometers, and John Richardson, senior research scientist at MIT’s Kavli Institute for Astrophysics and Space Studies, the experiment’s principal investigator. Both Belcher and Richardson expressed their thoughts on the retirement of this interstellar piece of MIT history.

Q: Looking back on the experiment’s contributions, what are the major achievements in terms of what MIT’s plasma spectrometer has learned about the solar system and interstellar space?

Richardson: A key PLS finding at Jupiter was the discovery of the Io torus, a plasma donut surrounding Jupiter formed from sulfur and oxygen from Io’s volcanoes (discovered in Voyager images). At Saturn, PLS found a magnetosphere full of water and oxygen that had been chipped off from Saturn’s icy moons. For Uranus and Neptune, the tilt of the magnetic fields caused PLS to see smaller density features, with Uranus’ plasma disappearing near the planet. Another important PLS observation involved the termination shock, which was the first observation of the plasma in the largest shock in the Solar System, in which the solar wind stopped producing supersonic. At this limit there was a huge drop in speed and an increase in the density and temperature of the solar wind. Finally, PLS documented Voyager 2’s crossing of the heliopause by detecting a stop in the outflowing plasma. This signaled the end of the solar wind and the beginning of the local interstellar medium (LISM). Although PLS is not designed to measure the LISM, it has continuously measured interstellar plasma flows outside the heliosphere. It is very sad to lose this instrument and this data!

Belcher: It is important to emphasize that PLS was the result of decades of development by MIT professor Herbert Bridge (1919-1995) and Alan Lazarus (1931-2014). The first version of the instrument they designed was flown on Explorer 10 in 1961. And the latest version flies with the Solar Probe, which collects measurements very close to the Sun to understand the origins of the solar wind. Bridge was the principal investigator for plasma probes on spacecraft that visited the Sun and every major planetary body in the solar system.

Q: How have the plasma sensors done their job during their time aboard the Voyager probes over the past 47 years?

Richardson: There were four Faraday cup detectors designed by Herb Bridge that measured currents of ions and electrons entering the detectors. By measuring these particles at different energies, we were able to determine the plasma velocity, density, and temperature in the solar wind and in the four planetary magnetospheres that Voyager encountered. Voyager data was (and still is) sent to Earth every day and received by NASA’s space antenna network. Keeping two 1970s-era spacecraft running for 47 years and counting was an amazing feat of engineering at JPL – you can google the most recent rescue when Voyager 1 lost some memory and stopped sending data in November 2023. JPL figured out the problem and was able to reprogram the flight data system from 15 billion miles away and now everything is back to normal. Shutting down PLS requires sending a command that arrives at Voyager 2 about 19 hours later, providing enough power for the rest of the spacecraft to continue.

Q: How much more could Voyager do with the plasma sensors turned off, and how far could it go?

Richardson: Voyager will continue to measure galactic cosmic rays, magnetic fields and plasma waves. Available power decreases by about 4 watts per year as the plutonium that powers them decays. We hope to keep some of the instruments running until the mid-2030s, but this will be challenging as performance levels decline.

Belcher: Nick Oberg of the Kapteyn Astronomical Institute in the Netherlands has conducted a comprehensive study of the spacecraft’s future using data from the European Space Agency’s Gaia spacecraft. In about 30,000 years, the space probe will reach the distance to the nearest stars. Because space is so vast, there is no chance that the spacecraft will directly collide with a star during the lifetime of the universe. However, micro-collisions with huge interstellar dust clouds will erode the surface of the spacecraft, although this will happen very slowly.

By Oberg’s estimate, the Golden Records (identical recordings placed aboard each probe containing selected sounds and images representing life on Earth) will likely survive for a period of over 5 billion years. After these 5 billion years, things are difficult to predict because that is when the Milky Way will collide with its massive neighbor, the Andromeda Galaxy. This collision has a one in five chance of ejecting the spacecraft into the intergalactic medium, where there is little dust and little weathering. In this case, it is possible for the spacecraft to survive for trillions of years. A trillion years is about 100 times the current age of the universe. The Earth will cease to exist in about 6 billion years when the Sun enters its red giant phase and engulfs it.

In a “poor man’s” version of the Golden Record, Robert Butler, the plasma instrument’s chief engineer, wrote the names of the MIT engineers and scientists who worked on the spacecraft on the collector plate of the side-facing cup. Butler’s home state was New Hampshire, and he put the state motto “Live Free or Die” at the top of the list of names. Thanks to Butler, New Hampshire won’t last a trillion years, but its state motto will. The flight spare part of the PLS instrument is now on display at the MIT Museum, where you can see the text of Butler’s message by looking into the side-facing sensor.