Interesting Facts About the Heliopause
- Solar wind particles are streaming away from the Sun at a rate of 300 to 800 km/s or 671 thousand to 1.79 million mph. This constant release of solar plasma creates a bubble-like structure around the solar system called the heliosphere.
- Interstellar space is the space between stars. Our Sun, along with its heliosphere, moves through the interstellar medium (ISM). The boundary between the heliosphere and the interstellar medium is called the heliopause.
- The heliopause is the edge of the solar system based on the influence of the Sun’s magnetic field and the solar wind. However, it is just one type of boundary in the solar system.
- The term “heliosphere” was coined by the space scientist Alexander J. Dessler. Heliophysics, or the study of the heliosphere, includes learning about space climate and space weather.
- On August 25, 2012, Voyager 1 was the first spacecraft to cross the heliopause and enter interstellar space. Voyage 2 followed it in 2018. These are the only two man-made spacecraft that have reached interstellar space yet. Still, it is important to note that they are still inside the solar system.
- The Oort Cloud, which is composed of icy planetesimals, is the cosmographic boundary of the solar system. It is located way past the heliopause and is in interstellar space. Voyager 1 will enter this cloud in roughly 300 and exit in about 30,000 years. When it finally exits, it will be outside the solar system and is closer to another star.
What Is the Heliosphere?
Learning about the heliosphere will help us better understand what the heliopause is.
The heliosphere is a bubble-like structure that surrounds the solar system as it moves through the interstellar medium. It is formed by the constant flow of solar wind pushing against interstellar space or the space between stars in a galaxy. The Sun’s wind and magnetic field dominate and significantly influence this region.
The heliosphere surrounds the solar system and extends way beyond the orbits of the eight planets, and even past the region of Pluto. This bubble-like region protects our solar system from cosmic ionizing radiation.
The heliosphere has different features, these are:
- Termination Shock
- Bow Shock
Features of the Heliosphere
Some people believe that outer space is an empty space. However, it is not. The stuff between the stars in our Milky Way galaxy is called the interstellar medium (ISM). The ISM consists primarily of hydrogen and helium (about 99%), with some trace amounts of heavier elements like carbon and oxygen. Interstellar dust makes up about 1% of it.
Basically, the Sun is moving in relation to the local interstellar medium. Emissions from the Sun send out charged particles outwards called the solar wind. It consists of mainly electrons and protons. This stream of particles is blown billions of kilometers from the Sun, traveling at an average speed of 300 to 700 kilometers per second or 700,000 to 1,500,000 miles per hour.
The solar wind is stronger around the Sun but it starts to lose speed as it goes farther out when it reaches the termination shock. At this point, the solar wind abruptly slows down as it begins to be affected by the interstellar medium.
The region between the termination shock and the heliopause is called the heliosheath. It is like a transition region wherein the solar wind travels slower than the speed of light. As a result, the Sun particles pile up and compress, making the solar wind denser and hotter. The neutral atoms in this region form a “ribbon” structure that is likely caused by solar wind particles being sent back into the solar system by the interstellar medium.
The area beyond the heliosheath is the heliopause. In this area, it is thought that the solar wind is not strong enough to push back against the interstellar wind, creating a balance in pressure between the two. This balance causes the solar wind to deflect and go back creating the “heliotail” in its wake—just like how comets look like.
NASA’s Interstellar Boundary Explorer (IBEX), a satellite in Earth’s orbit, found out that the shape of the heliotail is more like a four-leaf clover. The four-lobed tail result was released in 2013.
Another feature called the bow shock is in front of the heliosphere. It forms as the solar wind plasma pushes against the interstellar wind, just like how waves form as a ship plows through the ocean.
How Far Is the Heliosphere From the Sun?
It is believed that the closest boundary of the heliosphere is around 90 to 100 times AU. It means that it extends nearly three times the distance of Pluto and about twice the distance of the inner part of the Kuiper belt from the Sun.
What Is the Heliopause?
The heliopause is the edge of the heliosphere. There are two things that determine this boundary: the effect of the Sun’s magnetic field and the solar wind.
The solar wind is fluid, a plasma of charged particles or ions that escape the Sun’s surface and is continuously heading outward. Because of this, the heliosphere also has a fluid shape that is influenced by the changes in the solar wind.
For instance, changes in the solar wind pressure affect the heliosphere and its boundaries, especially during the 11-year solar cycle. An example of this was the deep solar minimum in 2009 when there was least solar activity and a very high level of cosmic rays was detected. As a result of these changes, there are also changes in the distance and shape of the heliopause over time.
While it is believed that the heliosphere is shaped like a comet, observations from 2009 suggest that it is more spherical and more like a bubble. This finding has also allowed for changes in how we understand the nature of the heliopause.
There are a couple of factors that indicate when a spacecraft has crossed the heliopause. One is that the temperature of solar wind particles significantly drops while there is an increase in the galactic cosmic rays.
These changes were observed in 2012 when Voyager 1 was approaching the heliopause. In May 2012, the spacecraft detected a rapid increase in cosmic rays while a significant decrease in Sun particles was observed from August to October of the same year. In 2013, NASA announced that Voyager 1 has already crossed the heliopause on August 25, 2012.
How To Determine the Heliopause
The heliopause is a boundary wherein the solar plasma is in equilibrium with the interstellar plasma. We can think of it as the edge of the solar system in terms of the solar wind and the solar magnetic field influence.
Another boundary that determines the edge of the solar system is the Oort Cloud. It is located past the heliopause and is already in interstellar space. This theoretical region indicates the cosmographic boundary of the solar system.
The heliopause is located approximately 123 AU from the Sun or 123 times the Sun-Earth distance. This distance is equal to about 18 billion kilometers or 11 billion miles. To compare, the farthest planet, Neptune, is 30 AU from the Sun while the dwarf planet Pluto is 39 AU away.
The heliopause has a fluid shape and is influenced by the Sun’s activity as it moves through space. According to Richard Marsden, a scientist of the European Space Agency (ESA) Ulysses project, “the distance of the heliopause from the Sun changes as the heliosphere breathes in and out on the timescale of the solar cycle.”
The location of the heliopause was discovered when NASA’s Voyager 1 and 2 crossed it in 2012 and 2018, respectively. This boundary was defined when the Voyagers detected a significant decrease in the Sun’s solar wind plasma and an increase in galactic cosmic particles.
Many years before, in 1992, the first evidence of this boundary was detected in powerful radio signals. According to Dr. Don Gurnett, investigator of the Voyager plasma wave subsystem, the signals are created as solar plasma interacts with interstellar gas. Their idea was finally confirmed when the Voyagers reached the milestone some 20 years later.
Another important discovery made by the Voyager mission is that it was found out that the interstellar magnetic field is actually aligned with the Sun’s magnetic field.
Voyager 1 and Voyager 2 in Interstellar Space
The twin space probes Voyager 1 and 2 are making history as the first man-made spacecraft to ever cross the heliopause. They are now traveling through the interstellar medium.
Voyager 1 launched on September 5, 1977, and Voyager 2 followed it 16 days later. These two have been operating for more than 40 years. According to NASA and JPL, Voyager 1 is already 155.8 AU away from the Sun, as of January 2022. It is the most distant man-made object from Earth. As of February of the same year, Voyager 2 is around 130 AU from the Sun.
These two spacecraft are heading out of the solar system in different directions which helped us understand the outer parts of the solar system.
Voyager 1 traversed the termination shock in 2004 at a distance of 94 astronomical units (AU). Voyager 2 followed suit and crossed it in 2007 with a lesser distance of 84 AU. This can mean that the heliosphere has an irregular size and shape. One theory is that it must be caused by changes in the Sun during the solar cycle.
Voyager 1 crossed the heliopause and into interstellar space in 2012 and Voyager 2 also reached the milestone in 2018. These two spacecraft have left the heliosphere but they are still in the solar system. They are yet to cross the large Oort Cloud—the outer edge of our solar system.
Five spacecraft have achieved the solar escape velocity and are now leaving the solar system. Following the milestone of Voyager 1 and 2 are Pioneer 10, Pioneer 11, and the New Horizons. The Voyagers and the New Horizons spacecraft are still sending signals back to Earth through radio communication. The Pioneer spacecraft, however, are already in derelict condition.
The New Horizons probe, the first spacecraft to explore Pluto, is predicted to cross the heliosphere and enter interstellar space around the 2040s.
One of the most famous features of the Voyager spacecraft is the “golden records.” These are identical phonograph records that were included aboard the twin probes. The records are like time capsules that contain images and sounds that portray life on Earth. In case other forms of life in the Universe find it, it will give them an idea of what Earth is like.
How Did Voyager 2 Pass Through 89,000°F Heliopause?
Voyager 2 encountered an increase in temperature and galactic cosmic rays when it crossed the heliopause in 2018. Scientists have predicted this increase but the plasma temperature spiked as soon as the probe reached interstellar space. It was recorded to be around 53,000 to 89,000 degrees Fahrenheit or 30,000 to 50,000 kelvin.
This sort of “wall of fire” is called the Very Local Interstellar Medium (VLISM). Before Voyager 2, the temperature of this region was predicted to be around 15,000 to 30,000 K.
While there was certainly a great increase in temperature, the plasma density is actually low. And because of this, the Voyager 2 probe was able to pass through it without a problem.
The high temperature indicated the heat contained in the individual particles—not the total heat energy in the region. Since this is a low-density environment, the molecules are far very apart and therefore have low thermal mass. As a result, the Voyager 2 spacecraft was able to pass through the heliopause easily.
What Is Beyond the Heliopause?
The interstellar space lies beyond the heliopause. “Inter” means “between” while “stellar” means it has something to do with stars. The Oort Cloud is located in this region.
Interstellar space begins outside of the heliopause. It is the space between the stars in a galaxy—and it is anything but empty. About 70% of interstellar space is made up of hydrogen and 28% of it is helium. The remaining 2% encompasses heavier gases and interstellar gas.
While interstellar space is not empty, it is actually close to a vacuum. On average, it only has approximately 1 atom per cubic centimeter. In comparison, a cubic centimeter of air on Earth at sea level has 30,000,000,000,000,000,000 molecules.
The material that make up the interstellar space is very spread out. However, some areas of it have greater density than others. These thicker parts of the interstellar medium are what we call molecular clouds. This is the place where stars form, and because of that, they are also called stellar nurseries.
It is believed that every other star has its own heliospheres or protective bubbles as well. These bubbles do not form solid boundaries and are not entirely impermeable. With that said, star particles can escape through the edges of these heliospheres adding up to the stuff in the interstellar medium.
Where Is the Oort Cloud in Comparison to the Heliopause?
The Oort Cloud is located beyond the heliopause. It is already in the interstellar medium. The heliopause is located 123 AU from the Sun while the Oort Cloud is believed to be around 2,000 AU to 200,000 AU from the Sun.
The Oort Cloud is a theoretical cloud that is thought to be composed of icy planetesimals. It is believed to be the origin of long-period comets. One of the most interesting subjects in astrophysics, this cloud is considered to be the cosmographic boundary of our solar system.
The Oort Cloud is composed of two areas. The inner part is believed to have a disk-like shape while the outer region is a spherical cloud.
Because of the great distance, the outer Oort Cloud is already loosely bound to the solar system. It is already affected by the gravity of passing stars and the Milky Way galaxy. Since it is often stretched and disturbed by outside forces, the icy objects in the cloud can be dislodged from their orbits.
As a result, they get slingshotted into the inner solar system as long-period comets. This event can also cause objects to be thrown out of the solar system. Similarly, it can also capture objects from outside the solar system.
The Voyagers have already gone past the heliopause but no man-made spacecraft have reached and crossed the Oort Cloud yet. Even if Voyager 1 is leading the group in traversing away from the Sun, will take another 300 years for it to enter the inner Oort Cloud. It will exit this cloud 30,000 years after.
What Is the Heliopause?: https://upload.wikimedia.org/wikipedia/commons/thumb/1/16/Ibexheliosphererevised.jpg/1280px-Ibexheliosphererevised.jpg
Voyager 1 and Voyager 2 in Interstellar Space: https://upload.wikimedia.org/wikipedia/commons/thumb/4/43/PIA22835-VoyagerProgram%26Heliosphere-Chart-20181210.png/1200px-PIA22835-VoyagerProgram%26Heliosphere-Chart-20181210.png
Golden records: https://upload.wikimedia.org/wikipedia/commons/thumb/7/7b/The_Sounds_of_Earth_-_GPN-2000-001976.jpg/800px-The_Sounds_of_Earth_-_GPN-2000-001976.jpg
How Did Voyager 2 Pass Through 89,000°F Heliopause?:https://upload.wikimedia.org/wikipedia/commons/thumb/2/29/Voyager_spacecraft.jpg/1024px-Voyager_spacecraft.jpg
What Is Beyond the Heliopause?: https://upload.wikimedia.org/wikipedia/commons/thumb/d/da/PIA17046_-_Voyager_1_Goes_Interstellar.jpg/1920px-PIA17046_-_Voyager_1_Goes_Interstellar.jpg
Where Is the Oort Cloud in Comparison to the Heliopause?: https://upload.wikimedia.org/wikipedia/commons/thumb/5/56/Kuiper_belt_-_Oort_cloud-en.svg/1024px-Kuiper_belt_-_Oort_cloud-en.svg.png