Earth's Magnetic Field Shields Moon From Space Radiation
In late summer 1859, spectacular light shows astounded the world, the likes of which have never been witnessed since. Telegraph communications were disrupted for hours while crimson lights bathed the night sky. Contemporary spectators attributed these phenomena to volcanic eruptions or reflections off polar ice caps.
Little did anyone recognize the true source of the disturbances, or how lucky they were to be watching from Earth. Even under quiet space weather conditions, the solar wind, comprised of hot ionized gas called plasma, buffets Earth with dangerous radiation. When it contacts Earth's magnetic field, known as the magnetosphere, its particles interact with the ionosphere and emit light, giving rise to the aurora. The marvelous light shows of 1859 were caused by possibly the largest solar flare ever recorded. It would have carried a massive amount of radiation. We on Earth can revel in these light displays because of Earth's magnetic field, which deflects the radiation even as it stimulates the aurora.
Without the Earth's magnetosphere, the radiation exposure caused by the flare of 1859 would probably have been lethal. Normal solar wind conditions would expose anyone outside the magnetosphere to unusually high doses. Radiation causes biological damage, increasing the risk of diseases such as cataracts and cancer. Exploration beyond the protective bubble of our magnetosphere is a risky proposition. But new initiatives aimed at sending men to the moon and beyond propose just that. How will the astronauts be protected?
Exciting new research by University of Washington scientists suggests that the intrepid lunar explorers will not be left out in the hot plasma alone. In the November 2007 issue of Geophysical Research Letters, Erika Harnett and Robert Winglee in the UW Department of Earth and Space Sciences demonstrated that Earth's magnetosphere can protect the moon from space radiation.
In 2004, President Bush outlined his Vision for Space Exploration. Included in the Vision is the goal of returning men to the moon by 2020. NASA plans to construct a permanent lunar base, allowing for continued human presence on the moon. Lunar missions offer scientists the opportunity to conduct research directly on the moon, rather than relying on data collected from satellite observation. Space exploration, it is hoped, will also stimulate the development of important new technologies that we can utilize on Earth. But first scientists need to characterize the environment the explorers will encounter.
That is where the simulations conducted by Harnett and Winglee come in. "There's this push to send people back to the moon, to do it a little bit more long term, rather than have them hop around for a day and then come back,” says Harnett. The ability of Earth's magnetosphere to protect the moon will have a direct impact on the habitability of the lunar environment.
The previous manned Apollo missions were brief, and they were conducted during a quiet period in space weather. No major solar events, such as the 1859 solar flare, occurred. Extended lunar missions would increase not only the radiation exposure, but also the risk that a solar event would occur during the mission. The particles in the solar wind carry energy ranging from thousands to billions of electron volts. Harnett and Winglee used computer models to calculate the amount of shielding the moon could expect while inside Earth's magnetosphere, as well as which areas on the moon were best protected. They hope that mission designers will be able to use this information when planning exploratory lunar missions.
The researchers caution that the initial calculations used very general models for solar wind conditions and particle trajectories. Further work examining more accurate solar wind conditions and particle tracking is under way.
"I thought it was very interesting work, and it was nice to see someone take some very theoretical work and have it make a direct application to space exploration,” says Timothy Stubbs, of the Laboratory for Extraterrestrial Physics at NASA's Goddard Space Flight Center.
The moon, unlike Earth, generates no magnetic field to protect it from the solar wind's charged particles, or the radiation pummeling it from beyond our solar system. The ability of Earth's magnetosphere to confer shielding is heartening, but even so the moon only spends about seven days of its twenty-eight day orbit within this zone of protection. This is due to the extreme asymmetry of Earth's magnetosphere, caused by interaction with the solar wind.
In the absence of the solar wind, Earth's magnetosphere would "extend to infinity in all directions,” explains Nicola Richmond of University of Arizona's Lunar and Planetary Laboratory. "But the solar wind from the sun consists of charged particles, and because they're charged, and the sun has a magnetic field, the solar wind carries with it a magnetic field.” This magnetic field is called the interplanetary magnetic field, or IMF, because it extends past all the planets in our solar system. When the solar wind crashes into Earth's magnetosphere, the magnetosphere is pushed toward Earth. On the day side the magnetosphere is compressed, while the night side magnetosphere is elongated by the solar wind. It stretches for hundreds of Earth radii into space, creating the magnetotail. The moon spends about a week of its orbit in this magnetotail; during the other three weeks, the radius of the magnetosphere is too small to reach the moon, and the moon is utterly exposed to space weather.
So why go to the moon? What can we learn from such a desolate environment, so unlike our own, so dangerous? To some, the vulnerability of the moon is its selling point. "The great thing about the moon is that it doesn't have much atmosphere, so it interacts directly with the surrounding space environment,” explains Stubbs. Understanding how plasma interacts with the moon could be applied to other bodies in the solar system. Before astronauts set up camp back on the moon, scientists first need to characterize the obstacles they would encounter.
Astronauts hoping to spend long periods in space face two radiation threats: energetic particles originating from the sun and galactic cosmic radiation from outside our solar system. Harnett explains, ”The galactic cosmic radiation is always there. It's just sort of a back-ground rain-drizzle coming at you. So energetic particles from the sun, called SEPs, solar energetic particles, those are not, those are transient. And we can't really predict when those happen.” It is this unpredictability of the SEPs that worried Harnett and Winglee. While a lunar base could be protected by thick walls, or by being buried beneath the moon's surface, astronauts on extended missions beyond base would have little recourse against a sudden gust of radiation.
The researchers used computer models to predict how much shielding Earth's magnetosphere might offer the moon under different solar wind conditions. One character of a magnetic field is its direction. If the IMF's direction aligns with our magnetosphere's direction, the magnetosphere is strengthened and better shields the moon. But even when magnetic fields do not align, the researchers were pleasantly surprised to see significant shielding of the moon.
"I think everybody assumed that it would be inconsequential—either offer no protection, or not add much to the radiation hazard,” says Harnett.
But their models predicted that the moon could be shielded from particles with energies up to billions of electron volts, encompassing both solar and galactic cosmic radiation. Harnett and Winglee hope this information can be used to advantage. While astronauts inside a base could expect shielding from the base itself, astronauts on lengthy excursions away from the base would benefit from planning the excursions for the week the moon spends in the magnetotail.
Different locations within the magnetosphere were tested for their possible shielding ability. The researchers were able to use their simulations to identify locations on the moon where a lunar base would receive the most protection from the magnetosphere.
Stubbs points out that new data will soon be available to validate the models generated at the University of Washington. The first step in NASA's mission to return men to the moon is the launch in late 2008 of the Lunar Reconnaissance Orbiter (LRO). LRO Spacecraft will map the moon's surface, sending back crucial data for planning landing missions and choosing the lunar base location. One of its instruments was designed with the express goal of measuring the lunar radiation environment. Called CRaTER, for Cosmic Ray Telescope for the Effects of Radiation, this instrument will measure the effects of different types of radiation on tissue-equivalent plastics. This will help NASA evaluate the effect that longer-term radiation exposure will have on astronauts based on the moon.
Sabrina Richards is a graduate student in the Department of Immunology at the University of Washington.
Temperature variation on the sun. An image of the sun showing the temperature variation in the sun's outer layer, or corona. Darker regions are coronal holes, cooler areas where high speed solar wind originates. Image: SOHO (ESA & NASA)