The sun warms the earth, making it habitable for people and animals. But that’s not all, and it affects a much larger area of space. The heliosphere, the region of space influenced by the sun, is more than a hundred times greater than the distance from the sun to the Earth.
The sun is a star that continuously emits a steady stream of plasma – high-energy ionized gas – called the solar wind. In addition to the constant solar wind, the Sun also occasionally emits bursts of plasma called coronal mass ejections, which can contribute to the aurora, and bursts of light and energy called flares.
The plasma coming from the sun expands through space, along with the sun’s magnetic field. Together they form the heliosphere within the surrounding local interstellar medium – the plasma, neutral particles and dust that fill the space between stars and their respective astrospheres. Heliophysicists like me want to understand the heliosphere and how it interacts with the interstellar medium.
The eight known planets in the solar system, the asteroid belt between Mars and Jupiter, and the Kuiper Belt – the band of celestial bodies beyond Neptune that includes the asteroid Pluto – are all in the heliosphere. The heliosphere is so large that objects in the Kuiper Belt orbit closer to the Sun than to the nearest boundary of the heliosphere.
Protection of the heliosphere
As distant stars explode, they emit large amounts of radiation into interstellar space in the form of high-energy particles known as cosmic rays. These cosmic rays can be dangerous to living organisms and can damage electronic devices and spacecraft.
Earth’s atmosphere protects life on Earth from the effects of cosmic rays, but even before that, the heliosphere itself acts as a cosmic shield against most interstellar radiation.
In addition to cosmic rays, neutral particles and dust flow steadily into the heliosphere from the local interstellar medium. These particles can affect the space around Earth and can even change the way the solar wind reaches Earth.
Supernovae and the interstellar medium may also have influenced the origins of life and the evolution of humans on Earth. Some researchers predict that millions of years ago the heliosphere came into contact with a cold, dense cloud of particles in the interstellar medium, causing the heliosphere to shrink, exposing Earth to the local interstellar medium.
An unknown shape
But scientists don’t really know what the shape of the heliosphere is. Models range in shape from spherical to comet-like to croissant-shaped. These predictions range in magnitude from hundreds to thousands of times the distance from the Sun to Earth.
However, scientists have defined the direction in which the sun moves as the ‘nose’ direction and the opposite direction as the ‘tail’ direction. The nose direction should have the shortest distance to the heliopause – the boundary between the heliosphere and the local interstellar medium.
No probe has ever been able to get a good look at the heliosphere from the outside or properly sample the local interstellar medium. This could allow scientists to learn more about the shape of the heliosphere and its interaction with the local interstellar medium, the space environment outside the heliosphere.
Crossing the heliopause with Voyager
In 1977, NASA launched the Voyager mission: the two spacecraft flew past Jupiter, Saturn, Uranus and Neptune in the outer solar system. Scientists have determined that after observing these gas giants, the probes separately crossed the heliopause and interstellar space in 2012 and 2018, respectively.
Although Voyager 1 and 2 are the only probes to ever cross the heliopause, they have far exceeded their planned mission lifetime. They can no longer return the necessary data because their instruments are slowly failing or turning off.
These spacecraft are designed to study planets, not the interstellar medium. This means they don’t have the right instruments to make all the measurements of the interstellar medium or heliosphere that scientists need.
That’s where a potential interstellar probe mission could come in handy. A probe designed to fly beyond the heliopause would help scientists understand the heliosphere by observing it from outside.
An interstellar probe
Because the heliosphere is so large, it would take decades to reach the limit, even with the help of the gravity of a huge planet like Jupiter.
The Voyager spacecraft will no longer be able to deliver data from interstellar space long before an interstellar probe leaves the heliosphere. And once the probe is launched, depending on its trajectory, it will take about 50 years or more to reach the interstellar medium. This means that the longer NASA waits to launch a probe, the longer scientists will be without missions into the outer heliosphere or the local interstellar medium.
NASA is considering developing an interstellar probe. This probe would make measurements of the plasma and magnetic fields in the interstellar medium and image the heliosphere from outside. In preparation, NASA asked more than 1,000 scientists for input on a mission concept.
The first report recommended that the probe travel on a trajectory approximately 45 degrees away from the nose of the heliosphere. This trajectory would follow part of Voyager’s path, while reaching some new regions of space. In this way, scientists could study new regions and revisit some partially known regions of space.
This path would give the probe only a partially oblique view of the heliosphere, and would not be able to see the heliotail, which scientists know the least about.
In the heliotail, scientists predict that the plasma that makes up the heliosphere mixes with the plasma that makes up the interstellar medium. This happens through a process called magnetic reconnection, which allows charged particles to flow from the local interstellar medium into the heliosphere. Just like the neutral particles that enter through the nose, these particles influence the space environment in the heliosphere.
In this case, however, the particles have a charge and can interact with magnetic fields from the sun and the planets. Although these interactions occur at the boundaries of the heliosphere, very far from Earth, they influence the composition of the interior of the heliosphere.
In a new study published in Frontiers in Astronomy and Space Sciences, my colleagues and I evaluated six possible launch directions, ranging from the nose to the tail. We found that a trajectory that intersects the flank of the heliosphere in the direction of the tail, rather than ending close to the nose direction, would provide the best perspective on the shape of the heliosphere.
A trajectory in this direction would provide scientists with a unique opportunity to study a completely new region of space within the heliosphere. As the probe leaves the heliosphere and enters interstellar space, it will acquire an outside view of the heliosphere at an angle that would give scientists a more detailed idea of its shape – especially in the disputed tail region.
Ultimately, whatever direction an interstellar probe is launched, the science he returns will be invaluable and literally astronomical.
This article is republished from The Conversation, an independent nonprofit organization providing facts and analysis to help you understand our complex world.
It was written by: Sarah A. Spitzer, University of Michigan.
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Sarah A. Spitzer is a research fellow in the Department of Climate and Space Sciences and Engineering at the University of Michigan. She receives funding from the University of Michigan and from grants supported by organizations such as NASA. She is affiliated with the University of Michigan and the Interstellar Probe Study Team.
