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 traveled to the heliopause. Therefore, scientists are unclear about the extent of the heliopause and its other properties.
Some scientists want to learn more about this region and are developing a mission concept to explore it.
The heliosphere plays a crucial role in the solar system. The Sun's heliosphere is a protective shield against incoming galactic cosmic rays such as those emitted by powerful supernovae. The heliopause marks the extent of the protective power of the heliosphere. Furthermore, galactic cosmic rays are unhindered.
“We want to know how the heliosphere protects astronauts and life in general from harmful galactic radiation, but that's difficult when we don't even know the shape of our shield.”
Marc Kornbleuth, Boston University
There is no comprehensive understanding of the shape and extent of the heliosphere and heliopause. A new study aims to address this problem by designing a probe that will travel beyond this region to find the necessary answers.
The study is titled “Complementary Interstellar Detections from the Heliotail” and was published in Frontiers in Astronomy and Space Sciences. The lead author is Sarah Spitzer, a postdoctoral researcher 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 inside of the fishbowl,” Spitzer said.
The heliopause protects everything within it from galactic cosmic rays, including our astronauts as they leave Earth's protective magnetosphere. “We want to know how the heliosphere protects astronauts and life in general from harmful galactic radiation, but that's difficult when we don't even know the shape of our shield,” said Marc Kornbleuth, a researcher at Boston University and co-author of the study.
According to simulations, this image shows three models of what the heliosphere might look like. Left: a comet-like shape. Middle: The croissant model. Right: Another, more streamlined comet-like shape. Image credits are listed in the image.
The shape of the heliosphere arises from the interaction between the Sun's solar wind and the local interstellar medium (LISM). The LISM consists 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 for scientists is that we can't learn much more about the shape of the heliosphere and its relationship to the LISM and its clouds without leaving the heliosphere. Although Voyager 1 and 2 have far exceeded the wildest expectations with their long lives and departure from the heliosphere, they are nearing the end. Their instruments don't work like they used to, and even then, these spacecraft were built in the 1970s. It goes without saying that technology has evolved since then.
What we need is a purpose-built spacecraft that can leave the heliosphere when and where we want. Of course, this is an extremely long journey, and other scientific goals would also be fulfilled along the way. But unlike the Voyager probes that were sent to explore the planets and only reached the LISM through sheer persistence, this probe would be designed primarily to explore the heliopause.
This illustration shows the position of NASA's Voyager 1 and Voyager 2 probes outside the heliosphere, a protective bubble created by the Sun that extends well beyond Pluto's orbit. Voyager 1 left the heliosphere in August 2012. Voyager 2 left the heliosphere at a different location in November 2018. Image credit: NASA/JPL-Caltech
“A future interstellar probe mission will be our first opportunity to truly see our heliosphere, our home, from the outside and better understand its place in the local interstellar medium,” said lead author Spitzer.
The idea has been around for some time. 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 journey into the LISM. They said it would “…provide the first real view of our vital system from the outside.” It could launch in 2036 and reach a top speed of 7 AU per year. That's around a billion kilometers per year.
The cover page of the 2021 proposal for a heliosphere exit mission. Photo credit: Interstellar Probe/JHUAPL
The phase-out point is a key difference between the 2021 proposal and this one. The 2021 proposal stated that the probe should “acquire a side view of the heliopause to characterize its shape, preferably about 45° from the direction of the heliopause nose at (7°N, 252°E) in Earth's cliptic coordinates.” “
The authors of this new paper say the Interstellar Probe team misunderstood the exit point. “However, this report assumes that a probe trajectory near 45 degrees from the nose of the heliotail, or the front of the Sun's directional motion, is optimal,” they write. Spitzer and her colleagues examined the problem and came to a different conclusion. They examined six different trajectories of a probe, from nose to back. They concluded that a side view was best.
“If you want to find out how far back your house is, walking through the front door and taking a photo from the front sidewalk is probably not the 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 offers the best science and insight into the shape of the heliosphere.
“To understand the shape of the heliosphere, one must understand the heliotail, as the shape depends heavily on the heliotail and its LISM interactions,” the authors write in their paper. “The Interstellar Probe mission is an ideal opportunity for measurements along a trajectory that passes through the heliotail over the flank…”
There is another compelling reason to follow this path. Researchers believe that plasma from the LISM could enter the heliosphere through its tail due to magnetic reconnection. If true, the probe could sample the LISM twice: once inside the heliosphere and once outside.
The team also proposed sending two probes beyond the heliosphere. One would have a forward trajectory, the other would have a backward trajectory. This would “… provide a more complete picture of the shape of the heliosphere and help us better understand its interactions with the LISM,” they explain in their paper.
Recent research suggests that the solar system is on a path that takes it out of the Local Interstellar Cloud (LIC). It may already be in contact with four different clouds with different properties. Photo credit: Interstellar Probe/JHUAPL
“This analysis required a lot of perseverance. “It started small and has grown 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 traveling 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.
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