by The Sun warms the Earth, making it habitable for humans and animals. But that’s not all it does, and it affects a much larger area of space. The heliosphere, the area of space influenced by the Sun, is more than a hundred times greater than the distance between the Sun and the Earth.
The Sun is a star that constantly emits a steady stream of plasma – a highly energetic ionized gas – called the solar wind. In addition to the constant solar wind, the Sun 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 galaxies. Helophysicists 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 objects beyond Neptune that includes the planet Pluto – all reside within the heliosphere. The helisphere is so large that objects orbiting the Kuiper Belt are closer to the Sun than to the nearest boundary of the helisphere.
Helisphere protection
As distant stars explode, they release large amounts of radiation into interstellar space in the form of highly energetic particles called cosmic rays. These cosmic rays can be dangerous to living organisms and can damage electronic devices and spacecraft.
The Earth’s atmosphere protects life on the planet from the effects of cosmic radiation, but, even before that, the heliosphere itself acts as a cosmic shield from most interstellar radiation.
In addition to cosmic radiation, neutral particles and dust constantly flow into the heliosphere from the local interstellar medium. These particles can affect the space around the Earth and can even change the way the solar wind reaches the Earth.
Supernovae and the interstellar medium may also have influenced the origin of life and the evolution of humans on Earth. Some researchers have predicted that, millions of years ago, the heliosphere came into contact with a cloud of cold, dense particles in the interstellar medium which caused the heliosphere to shrink, exposing Earth to the local interstellar medium.
Unknown shape
But scientists don’t really know what the shape of the helisphere is. The shapes of the molds vary from spherical to comet to croissant shaped. The magnitude of this projection varies between hundreds and thousands of times the distance from the Sun to the Earth.
However, the scientists defined the direction in which the Sun is moving as the “nose” direction and the opposite direction as the “tail” direction. The shortest distance to the heliosphere should be in the direction of the nose – the boundary between the heliosphere and the local interstellar medium.
No probe has ever taken a good look at the outer heliosphere or properly sampled the local interstellar medium. Doing so could tell scientists more about the shape of the helisphere and its interaction with the local interstellar medium, the space environment outside the helisphere.
Helicopter crossing with Voyager
In 1977, NASA launched the Voyager mission: Its two spacecraft flew past Jupiter, Saturn, Uranus and Neptune in the outer solar system. Scientists have determined, after observing these gas giants, that the probes crossed the helipad separately and into interstellar space in 2012 and 2018, respectively.
Although Voyager 1 and 2 are the only probes that the helipad could ever cross, they have far outlived their intended mission lifetimes. They can no longer return the required data because their instruments slowly fail or power down.
These spacecraft were designed to study planets, not the interstellar medium. This means they don’t have the right tools to make all the measurements of the interstellar medium or heliosphere that scientists need.
That’s where an interstellar exploration mission could come in. A probe designed to fly over the heliosphere would help scientists understand the heliosphere by observing it from outside.
An interstellar explorer
Since the heliosphere is so large, it would take decades to find the boundary, even with the help of gravity from a giant planet like Jupiter.
The Voyager spacecraft will not be able to provide data from interstellar space long before an interstellar probe leaves the heliosphere. And once the probe is launched, depending on the route, 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 left without any missions operating in the outer heliosphere or the local interstellar medium.
NASA is thinking of developing an interstellar probe. This probe would measure the plasma and magnetic fields in the interstellar medium and image the outer heliosphere. To prepare, NASA asked for input from more than 1,000 scientists on the mission concept.
The initial report suggested the probe travel on a trajectory roughly 45 degrees away from the direction of the nose of the helisphere. This trajectory would follow part of Voyager’s path, and it would reach several new regions of space. In this way, scientists could study new regions and revisit some partially known regions of space.
This path would only provide a partial, angled view of the heliosphere, and would not be able to see the heliothal, the least known to scientists of the region.
In the heliosphere, scientists predict that the plasma that makes up the heliosphere mixes with the plasma in the interstellar medium. This occurs through a process called magnetic reconnection, which allows charged particles to flow from the local interstellar medium into the helisphere. Just like the neutral particles that enter through the nose, these particles affect the space environment within the helisphere.
In this case, however, the particles have a charge and can interact with solar and planetary magnetic fields. Although these interactions occur at the boundaries of the heliosphere, far from Earth, they affect the interior composition of the helisphere.
In a new study published in Frontiers in Astronomy and Space Sciences, my colleagues and I evaluated six possible nose-to-tail launch directions. We found that, rather than going close to the direction of the nose, a path crossing the helisphere laterally towards the tail would give the best view of the shape of the helisphere.
A path along this direction would give scientists a unique opportunity to study a whole new region of space within the heliosphere. When the probe exits the helisphere into interstellar space, it would get a view of the helisphere from the outside at an angle that would give scientists a more detailed idea of its shape – particularly in the disputed tail region.
In the end, whichever direction an interstellar probe takes, the science it brings back will be invaluable and literally astronomical.
This article is republished from The Conversation, a non-profit, independent news organization that brings you facts and analysis to help you make sense of our complex world.
It was written by: Sarah A. Spitzer, University of Michigan.
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Sarah A. Spitzer works as a research fellow for the Department of Climate and Space Sciences and Engineering at the University of Michigan. She receives funding from the University of Michigan and grants supported by organizations such as NASA. She is affiliated with the University of Michigan and the Interstellar Probe Study Team.
