The edge of the Solar System is defined by the heliosphere and its heliopause. The heliopause marks the region where the interstellar medium stops the outgoing solar wind. But only two spacecraft, Voyager 1 and Voyager 2, have ever travelled to the heliopause. As a result, scientists are uncertain about the heliopause’s extent and its other properties.
Some scientists are keen to learn more about this region and are developing a mission concept to explore it.
The heliosphere plays a critical role in the Solar System. The Sun’s heliosphere is a shield against incoming galactic cosmic radiation, like that from powerful supernovae. The heliopause marks the extent of the heliosphere’s protective power. Beyond it, galactic cosmic radiation is unimpeded.
“We want to know how the heliosphere protects astronauts and life in general from harmful galactic radiation, but that is difficult to do when we still don’t even know the shape of our shield.”
Marc Kornbleuth, Boston University
There’s no overall understanding of the shape and extent of the heliosphere and heliopause. A new study wants to address that by designing a probe that would travel beyond this region to find the necessary answers.
The study is “Complementary Interstellar Detections from the Heliotail,” published in Frontiers in Astronomy and Space Sciences. The lead author is Sarah Spitzer, a postdoctoral research fellow in the Department of Climate and Space Sciences and Engineering at the University of Michigan.
“Without such a mission, we are like goldfish trying to understand the fishbowl from the inside,” said Spitzer.
The heliopause protects everything inside it from galactic cosmic radiation, including our astronauts who leave the Earth’s protective magnetosphere. “We want to know how the heliosphere protects astronauts and life in general from harmful galactic radiation, but that is difficult to do when we still don’t even know the shape of our shield,” said Marc Kornbleuth, a research scientist at Boston University and co-author of the study.
The heliosphere’s shape comes from the interaction between the Sun’s solar wind and the local interstellar medium (LISM.) The LISM is made of plasma, dust, and neutral particles. Two clouds in the LISM dominate our region of space: the Local Interstellar Cloud and the G-Cloud, home of the Alpha Centauri system. Two other clouds, the AQL Cloud and the Blue Cloud, are nearby. The clouds are regions where the LISM is denser.
The problem scientists face is that we can’t learn much more about the heliosphere’s shape and its relation to the LISM and its clouds without getting outside the heliosphere. While Voyager 1 and 2 have wildly exceeded the most feverish expectations by lasting this long and leaving the heliosphere, they’re near the end. Their instruments don’t function as they used to, and even then, those spacecraft were built in the 1970s. It goes without saying that technology has advanced since then.
What we need is a purpose-built spacecraft that can leave the heliosphere when and where we want it to. Of course, that’s an extremely long journey, and it would fulfill other scientific objectives along the way. But unlike the Voyager probes, which were sent to study the planets and only reached the LISM through sheer stubbornness, this probe would primarily be designed to explore the heliopause.
“A future interstellar probe mission will be our first opportunity to really see our heliosphere, our home, from the outside, and to better understand its place in the local interstellar medium,” said lead author Spitzer.
The idea has been around for a while. In 2021, scientists developed a mission concept for such a probe. They called it the Interstellar Probe and said it would embark on a 50-year-long journey into the LISM. They said it would “… provide the first real vantage point of our life-bearing system from the outside.” It could launch in 2036 and travel at a peak speed of 7 AU per year. That’s about one billion km per year.
The exit point is a critical difference between the 2021 proposal and this one. The 2021 proposal stated that the probe should “Capture a side view of the heliopause to characterize shape, preferably near 45° off of the heliopause nose direction at (7°N, 252°E) in Earth ecliptic coordinates.”
The authors of this new paper say that the Interstellar Probe team got the exit point wrong. “However, this report assumes that a probe trajectory near 45 degrees off the nose of the heliotail, or the front of the Sun’s directional motion, is optimal,” they write. Spitzer and her colleagues examined the issue and came to a different conclusion. They investigated six different trajectories for a probe, from noseward to tailward. They concluded that a side view is best.
“If you want to find out how far back your house extends, walking out the front door and taking a picture from the front sidewalk is likely not your best option. The best way is to go out the side door so you can see how long it is from front to back,” said co-author Kornbleuth. This vantage point will give the best scientific results and view of the heliosphere’s shape.
“Understanding the shape of the heliosphere requires an understanding of the heliotail, as the shape is highly dependent upon the heliotail and its LISM interactions,” the authors write in their paper. “The Interstellar Probe mission is an ideal opportunity for measurement either along a trajectory passing through the heliotail, via the flank…”
There’s another compelling reason to follow this trajectory. Researchers think that plasma from the LISM might enter the heliosphere through its tail because of magnetic reconnection. If that’s true, the probe could sample the LISM twice: once inside the heliosphere and once outside of it.
The team also proposed that two probes be sent beyond the heliosphere. One would have a noseward trajectory, and the other would have a heliotailward trajectory. That would “… yield a more complete picture of the shape of the heliosphere and to help us better understand its interactions with the LISM,” they explain in their paper.
“This analysis took a lot of persistence. It started small and grew into a great resource for the community,” said study co-author Susan Lepri.
The team behind the proposal says the Interstellar Probe will be a 50-year mission travelling 400 astronomical units. It could potentially travel much further, up to 1,000 astronomical units. According to the researchers, this would give us an unprecedented view of the heliosphere and the LISM.