David Raiklen

21st Century grad student uses half-century old spacecraft to explore interstellar space (artists conception based on data) Stella Ocker is a PhD candidate at Cornell University. Last year she filled out an application to be a Student Guest investigator on the Voyager 1 mission. A mission that was begun 50 years ago and took the first close images of Jupiter and Saturn, you've probably seen them. Historic first images taken by Voyager 1 & 2 Stella Ocker "It's pretty incredible how a mission that's so old can still produce such exciting new discoveries.” Stella is interested in the Interstellar Medium, the atoms and plasma that float between the stars throughout the Universe. She had previously studied distant plasma using radio telescopes. Here was a chance to study the region between stars in situ, on location. But what could a 21st century grad student do with a half-century-old spacecraft? Voyager 1 is one of two groundbreaking NASA missions launched in 1977 with a mission to explore the outer planets of the Solar System. Voyager 1 and 2 have obtained stunning pictures and information about the cosmos, and have both been in continuous contact with Earth since their launch. Voyager 1 is the most distant human-made object in space. To accomplish these feats, mission planners took advantage of a particular alignment between the planets that happens only once every 175 years. This alignment allowed the spacecraft to consecutively slingshot between them purely through the use of gravity. Current location of the Voyager craft It turns out Voyager 1 never stopped exploring. 14.1 billion miles from Earth it is continuing on with a new mission. In August of 2012 it became the first human-made object to reach Interstellar Space. Astoundingly, after traveling tens of billions of kilometers through frigid deep space for decades, there are still five scientific instruments functioning normally on the craft. And also on its twin, Voyager 2. The JPL engineers make gifts that keeps on giving. Like the ocean, the interstellar medium is full of turbulent waves. The largest come from our galaxy’s rotation, as space smears against itself and sets forth undulations tens of light-years across. Smaller (though still gigantic) waves rush from supernova blasts, stretching billions of miles from crest to crest. The smallest ripples are usually from our own Sun, as solar eruptions send shockwaves through space that permeate our heliosphere’s lining. These crashing waves reveal clues about the density of the interstellar medium – a value that affects our understanding of the shape of our heliosphere, how stars form, and even our own location in the galaxy. As these waves reverberate through space, they vibrate the electrons around them. Voyager 1’s Plasma Wave Subsystem – which includes two “bunny ear” antennas sticking out 30 feet (10 meters) behind the spacecraft – was designed to hear that ringing. Ocker was given access to years of plasma wave data and she began applying modern computer analysis to tease out the wave structures. For months she found nothing. Then she narrowed the search to smaller frequency bands and there it was. A persistent low amplitude signal around 3 kilohertz. This was unexpected and very exciting. We had earlier seen plasma wave radiation caused by solar flares. But Ocker found activity when things were quiet, between the flares. The continuous signal, in the kilohertz range, that varies as a craft travels, makes astronomers think that they've discovered something new: intrinsic activity of interstellar plasma. The interstellar medium of our Milky Way Galaxy is diverse No one knows what is causing this, although a guess is local heating and cooling. Stella has assembled a team of theorists to model the phenomena. The Voyager data can also be used to calculate the local density of the plasma. “This is really exciting, because we are able to regularly sample the density over a very long stretch of space, the longest stretch of space that we have so far,” said Ocker. “This provides us with the most complete map of the density and the interstellar medium as seen by Voyager. It opens up a new pathway for us to explore the structure of the very nearby interstellar medium” Soon-to-be-doctor Ocker wrote a good commentary here, in the issue of Nature where her breakthrough paper is published: https://astronomycommunity.nature.com/posts/voyager-1-hears-the-hush-of-interstellar-plasma The famous Golden Disk The Voyager missions carry on board the Golden Disk to communicate with extraterrestrial life. The disk was a project of scientist-educator Carl Sagan. Stella Ocker works in The Carl Sagan Institute, connecting her work to the earlier generation that built the craft. In fact there are now 3 generations of astronomers working on Voyager, they belong to a new trend in science of multi-generational projects that take several decades to complete. Voyager 2 also is now in interstellar space, here's a video about that: https://youtu.be/MGPM58S5Njg -30-

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