The solar system's tilt shield is a large magnetic bubble called the "heliosphere". It is part of the Sun's magnetic field. No one knows the exact dimensions of the heliosphere but it is larger than the orbit of Pluto
Ariel Eisenhandler, Israel Astronomical Society
If you've ever seen "Interstellar Voyage" then you know the importance of shields: when a star explodes or when a Klingon death ray suddenly emerges like a spear from the darkness the captain shouts two words, "raise shields!", and all is well. Tilt shields: Don't leave home without one. Believe it or not but the solar system also has such a shield.
The solar system's tilt shield is a large magnetic bubble called the "heliosphere". It is part of the Sun's magnetic field. No one knows the exact dimensions of the heliosphere, but it is larger than the orbit of Pluto (it is estimated that its length is about 100 astronomical units), all nine planets are inside it.
The heliosphere is important to life on our planet. For example: several million years ago a cluster of massive stars drifted through the part of the Milky Way where we are and the stars exploded, one after the other, like popcorn. Most of the cosmic rays from the explosions were deflected, thus shielding the ancient humans from a radiation bath.
But the bubble is not perfect. Actually "it's leaking", says space scientist Eberhard Moebius from the University of New Hampshire. "There are things that do penetrate." (This also happens in Star Trek. If the shields were impenetrable, there would be no room for drama.) Take the cosmic rays for example: they are atomic fragments that were shattered and accelerated to the speed of light by supernova explosions. The heliosphere covers about 90% of them; The rest, the most powerful 10%, penetrate into the interior of the solar system. The bubble is even more sensitive to electrically charged particles. Magnetic fields can deflect charged particles such as cosmic rays but not opaque and neutral molecules or small pieces of dust and rock - the door is open to those.
So: a stream of neutral helium atoms - "an interstellar breeze," says Moavius - is flowing into the solar system at these moments. "The current comes from the direction of the Sagittarius constellation. Since the atoms in the current are not charged, the magnetic bubble can do nothing to stop them."
Learning the current is important because it can teach us a lot about the heliosphere: what is its size? How leaky is it? In addition, the stream can teach us about interstellar "things" lurking out there, says Moavius.
The stream, which was discovered about 30 years ago, is actively monitored by a small fleet of spacecraft of the American and European space agencies (NASA and ISA), which include SOHO (Solar & Heliosphere Observatory), EUVE (Extreme Ultraviolet) Explorer, Ace (ACE-Advanced Composition Explorer) and especially Ulysses which explores the polar regions of the sun. Each of them measures something different. For example, the EUVE measures ultraviolet light scattered from the stream while Ulysses samples the stream itself by capturing atoms directly from the stream.
For many years the characteristics of the stream were vaguely known. "But the ability we have today, to look closely at the current with the help of these modern spacecraft, makes the difference," says Moavius. Moavius recently led a research team at the International Space Science Institute (ISSI) in Switzerland. By using information from the spacecraft they were able to accurately describe the temperature, density and speed of the current.
The temperature of the current, 6000 degrees Celsius, is almost the same as the surface temperature of the sun. If so, a spacecraft flying through the current won't melt and won't even feel the heat. The gas in the stream is very thin, explains Moavius. "There are 0.015 helium atoms in every one cubic centimeter." For comparison, the Earth's atmosphere at sea level is a thousand billion billion times denser (ten to the 21st power).
And finally, the speed of the current is 26 km/s which is about 93,000 km/h. These numbers confirm what astronomers have long suspected: the solar system is colliding with a massive interstellar cloud.
Most people think that space is empty, but it is not. The space between the stars is filled with gas clouds. The length of clouds in the Bible is several kilometers. The length of clouds in space is several light years. They differ in their characteristics and range from black and very cold to colorful, warm and very radiant. Stars are born in clouds and they send out more such clouds when they end their lives. Interstellar clouds are everywhere so it's no surprise that the solar system encounters one. The question is: what kind of cloud?
This cloud, like most things in the universe, consists mainly of hydrogen. We know this because hydrogen picks up suggestive colors from the light of surrounding stars. Astronomers use this feature to follow the outline of the cloud: it is several light-years wide and is "rough" at the edges.
The abundance of hydrogen in the cloud does not easily penetrate the heliosphere because the hydrogen atoms are ionized by interstellar ultraviolet radiation. The hydrogen atoms, like cosmic rays, are charged and therefore they stop. Helium atoms, on the other hand, are mostly neutral and therefore escape into the solar system.
Although helium is a small component of the cloud, it tells researchers about the cloud as a whole. The temperature of the cloud is 6,000 degrees Celsius, like the temperature of the helium stream. Its speed, 26 km/s, is also the same. If the cloud contains an acceptable mixture of hydrogen and helium - a logical assumption - then its total density should be 0.264 atoms per cubic cm.
mystery? Absolutely not.
These numbers are important. They are essential to the size and leakage of the heliosphere. The bubble is inflated from the inside by the solar wind and compressed from the outside by the cloud. This is a balancing act. If the cloud pressure (a function of temp, density and speed) is high, it overcomes the solar wind, shrinks the bubble and weakens our defense against cosmic rays.
A number of researchers believe that in thousands of years the solar system will completely pass through this cloud and appear in a hole in low-pressure space, which reached such a state by supernovae millions of years ago. The heliosphere will expand and provide improved protection against cosmic rays.
Later.... who knows? Another cloud can pass and shrink the bubble again. Ultimately the ISSI research team can tell us how the heliosphere will react.
raise shields? lower shields? It's not science fiction anymore.
