Interplanetary environment

Far from empty, the vast spaces between the Sun and its planets and out to the edge of interplanetary space constitute a dynamic environment suffused with fields of forces, crossed by swiftly moving particles, littered with debris, and penetrated by cosmic rays from outside the solar system. These phenomena endanger human technology, both in space and on Earth, but they also tell scientists much about the Sun and the origin of the solar system.

Overview

The interplanetary environment principally contains materials ejected from the Sun and debris. Space debris comprises particles of great variety in size and composition. Anything smaller than 0.01 millimeter, astronomers call micrometeoroids or interplanetary dust. Anything larger is a meteoroid, of which many are tens of meters in size. By far, the largest proportion of the debris originates when asteroids collide in the Asteroid belt between the orbits of Mars and Jupiter or from comets that swing into the inner solar system and leave behind a trail of particles eroded from them by the solar wind. Accordingly, most dust lies in the plane of the ecliptic (the disk-shaped region of the Sun and planets) in two bands: in the inner solar system, out to about three astronomical units (AU, the average distance from the Sun to Earth), and in the Kuiper Belt, about ten to forty AU. Additionally, a small amount of matter infiltrates from interstellar space as the solar system drifts through galactic gas clouds or from volcanoes on satellites, such as Jupiter’s Io. Found both as chondrites (clusters of particles) and as solid chunks, interplanetary dust, like meteoroids, is rich in carbon, iron, sulfur, nickel, and silicates, but many other elements in mineral combinations have been found, including tiny spheroids of glass embedded with metal sulfides (known as GEMS). In a clear, dark sky of late evening or early morning, interplanetary dust is visible near the horizon in the direction of the Sun as a faint glow of reflected sunlight, called zodiacal light.

Although the interplanetary debris is largely gathered into two clouds, it is not static. Depending on size and location, particles move at different speeds and in different directions. The pressure exerted by solar photons pushes micrometeoroids between 0.5 and one micron slowly outward and eventually out of the solar system altogether. Particles larger than one micron are affected by radiation pressure that creates drag, which causes them to decelerate so that they fall inward toward the Sun until they are vaporized. Asteroid collisions and comets replenish the clouds. It is also the case that debris left behind by comets retains the parent body’s velocity and orbit as a meteoroid stream. When the Earth passes through one of these streams, the vastly increased numbers of particles entering the atmosphere can create spectacular displays of streaking light in meteor showers, notably the Leonid showers each November and the Perseid showers each August (named after the constellations Leo and Perseus, from which they appear to arrive).

In addition to electromagnetic radiation, such as the photons of visible light, the Sun generates the gravitational field that holds together the solar system and magnetic fields. Magnetic fields accelerate a steady stream of plasma (electrons, protons, and ionized atoms) in all directions. That stream of particles is the solar wind. Near Earth, the solar wind has an average density of about ten particles per cubic centimeter, a velocity of about 400 kilometers per second, and a temperature of about 100,000 kelvins, although these properties are highly variable. Particle velocity remains fairly constant beyond Earth’s orbit, but density decreases as the solar wind spreads outward.

The solar wind defines the extent of the Sun’s influence within its galactic neighborhood. As the plasma attenuates in the outer solar system, it eventually is slowed by the pressure of gas between stars and is deflected. The boundary where this occurs is called the termination shock. It fluctuates with the intensity of the solar wind and the density of interstellar gas at between ninety and one hundred AU. In December 2004, the Voyager 1 spacecraft confirmed its existence, passing through it at about ninety-four AU. Voyager 1 then passed into a region of turbulence where the solar wind mixes with interstellar particles, called the heliosheath. Its outermost edge, the heliopause, marks the limit of the heliosphere, the total area of the solar system. The heliosphere is something of a misnomer, however, because it assumes a teardrop shape as the solar system moves through interstellar gas clouds; its forward edge is thought to extend to between 115 and 150 AU.

Occasionally, the Sun hurls outward immense bubbles of matter called coronal mass ejections (CMEs). Also known as solar storms, they typically involve between one hundred trillion and one quadrillion grams from the Sun’s outer layer. By the time the typical CME reaches Earth’s orbit, it has a speed, on average, of about 280 kilometers per second. It also carries with it its own magnetic field. The Frequency of CMEs varies during an eleven-year cycle of solar activity. During solar maximum, Earth happens to be in the way of about seventy-two CMEs per year; at solar minimum, eight CMEs. This constitutes a minority of storms that the Sun ejects in all directions; during solar maximum, the average is 3.5 CMEs per day. They can vary enormously in size and speed, the slowest moving a few dozen meters per second and the fastest 2.5 kilometers per second.

Planets with magnetic fields, such as Earth, are protected from the solar wind and CMEs. Earth’s magnetic field, for instance, deflects the ionized particles so that Earth resides at the center of a tear-shaped bubble of relative calm. The interaction between the solar wind and Earth’s southern and northern magnetic poles produces auroras, eerily rippling sheets of color in the sky. Satellites, asteroids, and planets with little or no magnetic field, such as Mars, are subject to a bombardment of their surfaces, which is deadly to life.

Sudden eruptions on the Sun’s surface called solar flares emit X-rays that can affect the properties of a planet’s upper atmosphere. The Sun also broadcasts a constant blizzard of neutrinos. Some seventy billion of them strike every square centimeter of Earth’s surface every second. However, these particles, nearly without mass, seldom interact with other matter. Although the solar wind deflects low-energy cosmic rays, the high-energy variety penetrates the interplanetary environment from unknown, distant sources; additionally, like everything in space, the solar system is bathed in the cosmic background radiation, the fading glow from the universe’s origin.

Knowledge Gained

Forces and particles of the interplanetary medium provide information about the origin of the solar system, its composition, the structure and behavior of the Sun, the evolution of planets, and the nature of other planetary systems.

The pervasive streams of neutrinos, for example, confirm theoretical calculations about the conversion of hydrogen into helium during fusion, the nuclear reaction that produces the Sun’s radiant power. The gusty solar wind enables scientists not only to sample the constituent elements of the radiation, it also provides clues to the behavior of magnetic fields in the solar corona. The production of high-energy X-rays by solar flares reveals the extent of those fields’ power, as do CMEs. The frequency of CMEs and flares characterizes the Sun’s eleven-year cycle of activity.

Most of the matter left over from the formation of the Sun and planets was long ago ejected from the solar system. However, enough remains mixed in with interplanetary debris, particularly in particles shed from comets, to provide clues about the relative abundance of elements and specific isotopes in the presolar gas cloud. With this information, astronomers can distinguish the unique chemical makeup of the solar system. Moreover, the Earth receives a steady rain of particles from the interplanetary dust clouds. Geologists estimate about 40,000 metric tons fall to the surface yearly.

Because the solar system’s present dust clouds result from collisions between asteroids and material sloughed from comets, it is reasonable to infer that dust clouds around other mature stars evolved from the same processes. Astronomers have detected such dust clouds around about one-third of stars. Planetary systems appear to be common.

Context

Knowledge acquired about the interplanetary medium is of more than scientific concern. Understanding the interplanetary medium helps protect humanity from the dangers posed by its various contents. Meteoroids and micrometeoroids streak past Earth at speeds of eleven to seventy kilometers per second, so even the smallest carries enough kinetic energy to damage space vehicles. Many satellites and the International Space Station suffered minor punctures even though the number of micrometeoroids in near-Earth orbit is low.

The solar wind is also dangerous. Astronauts and sensitive electronic equipment must be shielded from it. Still more dangerous is the sudden hurricane of particles and magnetic fields unleashed in a CME. Astronauts then must retreat to heavily protected areas in their spacecraft, and unprotected satellites may be rendered useless. Especially large CMEs can penetrate Earth’s protective magnetosphere to disrupt communications and cause power outages. Because of the Earth’s tilt toward the Sun and the amount of landmass in the northern hemisphere, power grids there are particularly at risk. A surge of X-rays from a solar flare can likewise affect Earth’s ionosphere and drown out radio communications. Additionally, exposure to cosmic rays increases the risk of cancer for astronauts, a fundamental problem to overcome if there are to be long voyages, such as from Earth to Mars.

The combination of relative particle densities, solar flares, the solar wind, and CMEs is called space weather. As humanity grows more dependent on technology, it also becomes more vulnerable to the vagaries of space weather. Should a fierce solar storm strike unexpectedly, it could cause chaos in civilian and military telecommunications, weather prediction, and global positioning systems. It could also cause massive power outages that would bring modern life to a virtual standstill.

As a result, space agencies in the United States, the European Union, Russia, and Japan launched a variety of space probes and orbiting observatories to monitor the interplanetary medium, particularly as it is influenced by solar activity. Some of these, such as the Solar and Heliospheric Observatory (SOHO), can detect a solar storm before it reaches Earth, giving technicians time to protect or power down sensitive equipment and giving astronauts time to seek shelter. Ground-based observatories offer similar vigilance, some of them watching especially for large meteoroids or meteoroid streams that cross Earth’s orbit.

NASA continued to study the interplanetary environment in the twenty-first century to protect the technology and the astronauts it sends into space, but also to increase insight into far-away regions of the universe and the composition of other planets. Programs, such as Solar, Heliospheric, and Interplanetary Environment (SHINE), sponsored by the National Science Foundation, provide broad-based grass-roots efforts to study scientific questions about the effects of the interplanetary environment.

The perils awaiting in the tenuous space between planets, as well as the wealth of information and potentially exploitable resources there, offer a lesson. Humanity exists not only in the framework of civilization and the physical environment of Earth’s biosphere; its survival also requires an understanding of interplanetary space.

Bibliography

Bradley, J. P. “Interplanetary Dust Particles.” In Meteorites, Comets, and Planets, edited by Andrew M. Davis. San Diego, Calif.: Elsevier, 2005.

“Interplanetary Space.” NASA, 19 Feb. 2021, www.nasa.gov/heliosphere. Accessed 15 Sept. 2023.

Jones, Barrie W. Discovering the Solar System. New York: John Wiley & Sons, 1999.

Lang, Kenneth R. The Cambridge Guide to the Solar System. New York: Cambridge University Press, 2003.

‗‗‗‗‗‗‗. Sun, Earth, and Sky. 2d ed. New York: Springer, 2006.

McBride, Neil, and Iain Gilmour, eds. An Introduction to the Solar System. New York: Cambridge University Press, 2004.

Peuker-Ehrenbrink, Bernhard, and Birger Schmitz, eds. Accretion of Extraterrestrial Matter Throughout Earth’s History. New York: Kluwer Academic/Plenum, 2001.

"Solar and Heliospheric Observatory Homepage." NASA, 27 July, 2020, soho.nascom.nasa.gov. Accessed 15 Sept. 2023.

“Solar, Heliospheric, and Interplanetary Environment (SHINE).” National Science Foundation, 10 Feb. 2022, new.nsf.gov/funding/opportunities/solar-heliospheric-interplanetary-environment-0. Accessed 15 Sept. 2023.