Scientists justify the need to launch an interstellar probe to study the heliosphere

The sun heats the Earth, making it suitable for human and animal life. But that's not all it does – it affects a much larger area of ​​space. The heliosphere, the region of space influenced by the Sun, is more than a hundred times the distance from the Sun to Earth.

The Sun is a star that constantly emits a continuous stream of plasma – a high-energy ionized gas – called the solar wind. In addition to the constant solar wind, the Sun occasionally experiences ejections of plasma called coronal mass ejections, which can contribute to the appearance of an aurora, and bursts of light and energy called faculae.

The plasma leaving the Sun expands in space along with the Sun's magnetic field. Together they form the heliosphere in the surrounding local interstellar medium—plasma, neutral particles, and dust—that fill the space between stars and their respective astrospheres. Heliophysicists like me want to understand what the heliosphere is and how it interacts with the interstellar medium.

The solar system's eight known planets, the asteroid belt between Mars and Jupiter, and the Kuiper belt—a group of celestial objects beyond Neptune that includes the planetoid Pluto—are all found in the heliosphere. The heliosphere is so large that Kuiper Belt objects are closer to the Sun than to the nearest edge of the heliosphere.

Heliosphere protection

When distant stars explode, they release large amounts of radiation into interstellar space in the form of high-energy particles called cosmic rays. These cosmic rays are 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 earlier, the heliosphere itself acts as a cosmic shield from most interstellar radiation.

In addition to cosmic radiation, neutral particles and dust constantly enter the heliosphere from the local interstellar medium. These particles can affect the space around Earth and even change how the solar wind reaches Earth.

Supernovae and the interstellar medium could also have influenced the origin of life and human evolution on Earth. Some researchers theorize that millions of years ago, the heliosphere came into contact with a cold, dense cloud of particles in the interstellar medium, which caused the heliosphere to contract, exposing Earth to the local interstellar medium.

Unknown form

But scientists still don't know what the shape of the heliosphere is. Models range from spherical to comet-shaped to croissant-shaped. Their sizes vary hundreds and thousands of times compared to the distance from the Sun to the Earth.

However, scientists have defined the direction in which the Sun moves as the “nose”, and the opposite direction as the “tail”. The bow direction should have the shortest distance to the heliopause – the boundary between the heliosphere and the local interstellar medium.

So far, no probe has been able to clearly view the heliosphere from the outside or correctly sample the local interstellar medium. And this could tell scientists more about the shape of the heliosphere and its interaction with the local interstellar medium – space beyond the heliosphere.

Voyager crossing the heliopause

In 1977, NASA launched the Voyager mission: The two spacecraft flew past Jupiter, Saturn, Uranus and Neptune in the outer solar system. Scientists found that after observing these gas giants, the probes individually crossed the heliopause and entered interstellar space in 2012 and 2018, respectively.

Although Voyager 1 and Voyager 2 are the only probes to ever potentially cross the heliopause, they have long since expired. They can no longer return the necessary data as their instruments gradually fail or lose power. [это не совсем так / прим. перев.]

These spacecraft were designed to study planets, not the interstellar medium. This means they don't have the right instruments to make all the measurements scientists need of the interstellar medium or heliosphere.

This is where a potential interstellar probe mission could come in handy. The probe, designed to fly beyond the heliopause, will help scientists understand the heliosphere by observing it from the outside.

Interstellar probe

Because the heliosphere is so large, it would take decades for the probe to reach its edge, even with the gravitational support of a massive planet like Jupiter.

The Voyager spacecraft will stop receiving data from interstellar space long before the interstellar probe leaves the heliosphere. And once a new 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 for the probe to launch, the longer scientists will be left without missions operating in the outer heliosphere or in the local interstellar medium.

NASA is considering developing interstellar probe. This probe will measure plasma and magnetic fields in the interstellar medium and take pictures of the heliosphere from outside. To prepare for this, NASA asked more than 1,000 scientists to submit mission concepts.

The original report recommended sending the probe along a trajectory away from the nose of the heliosphere by about 45 degrees. This trajectory will follow part of Voyager's path, but will also reach new areas of space. In this way, scientists will be able to explore new regions and revisit some partially known areas of space.

This path would give the probe only a partial angled view of the heliosphere, and it would not be able to see the heliotail, an area that scientists know the least about.

According to scientists' predictions, in the heliotail the plasma that makes up the heliosphere mixes with the plasma that makes up the interstellar medium. This occurs through a process called magnetic reconnection, which allows charged particles to pass from the local interstellar medium into the heliosphere. Like neutral particles entering through the nose, these particles affect the space environment within the heliosphere.

However, in this case, the particles have a charge and can interact with solar and planetary magnetic fields. Although these interactions occur at the edges of the heliosphere, very far from Earth, they influence the composition of the interior of the heliosphere.

In the new research, published in the journal Frontiers in Astronomy and Space Sciences, my colleagues and I analyzed six potential launch directions, from nose to tail. We concluded that a trajectory crossing the flank of the heliosphere towards the tail would provide the best insight into the shape of the heliosphere, rather than exiting close to the nose.

A trajectory in this direction will provide scientists with a unique opportunity to study a completely new region of space inside the heliosphere. When the probe exits the heliosphere into interstellar space, it will get an angled view of the heliosphere from the outside, allowing scientists to get a more detailed picture of its shape—especially in the disputed tail region.

After all, no matter what direction an interstellar probe is launched in, the scientific data it returns will be invaluable and literally astronomically important.