About 60 years ago, humans acquired the technological ability to travel to space. By now, science fiction franchises like “Star Trek” inspired entrepreneurs such as Jeff Bezos to translate their wealth into enterprises of space tourism. Bezos recently expressed the desire to send 1 trillion humans into space in the distant future, because Earth will not be able to accommodate all of them. Unfortunately, humans were not selected by Darwinian evolution to survive for long periods of time in space.
The hazards from energetic particles have been known since the early days of space exploration. On Earth, humans are protected from these charged particles, which originate from the Sun and our Milky Way galaxy. Earth is shielded by its magnetic field and atmosphere. Mars has no magnetic field or atmosphere to shield humans from the damage caused by cosmic radiation.
Human astronauts outside the Earth’s magnetic “womb” get zapped by solar energetic particles, mostly during sporadic solar flares that last from minutes to hours. Such flares are prominent when the sun is “active”, namely during solar maxima in its 11-year cycle of surface activity. The most energetic solar particles can be deadly. Humans have a better chance of survival on Mars when the Sun is least active, namely during solar minima.
But even if humans avoid the radiation from the Sun, there is an additional risk from Galactic cosmic rays. During a space journey that lasts more than three years, these Galactic particles would be life-threatening as well. The potential cumulative effects from space radiation must be studied thoroughly before sending humans for missions that last more than a few years. Protection could potentially be offered in deep caves under the Lunar or Martian surface.
Our solar system receives only a fraction of the Galactic cosmic rays, thanks to magnetic shielding by the so-called heliosphere, located at a hundred times the Earth-Sun separation, where the Solar wind meets the interstellar medium. The heliosphere was traversed by NASA’s Voyager 1 space craft in 2012 and by Voyager 2 in 2018. The instruments onboard these missions revealed that the heliosphere blocks about three-fourths of the galactic cosmic rays.
As of now, scientists are unable to forecast reliably the levels of Galactic cosmic radiation throughout the solar system. The very region that shields the galactic radiation is the one that is least understood.
Space missions, such as Voyager, New Horizons, Interstellar Boundary Explorer and Cassini-Huygens, revealed the frontal extent of the heliosphere and the incoming stream of hydrogen atoms from the galaxy, but the fundamental features of the heliosphere remain unknown. In particular, the global shape and distribution of cosmic radiation are uncertain.
Before sending humans to long space journeys, more resources should be allocated to studying the radiation filtered by the heliosphere. Better understanding of our own environment will also help us forecast whether life exists on Earth-like planets around other stars.
Some habitable planets are protected from energetic particles by their atmosphere and magnetic field, as well as by the analog of our heliosphere, labeled “astrosphere” for other stars. We currently know very little about astrospheres in general. Studies of the heliosphere would help us understand the critical mechanisms that controls the properties and shielding of energetic particles that pose a threat to extraterrestrial life.
The human body is fragile. Humans cannot safely venture to long journeys beyond our immediate vicinity near Earth. Before sending human-astronauts to long expeditions we must ensure that we are not sending them to their death. Ahead of dreaming about a large human population on Mars, as advocated by Elon Musk, we must understand the radiation environment throughout the solar system.
A safe bet, for the time being, is to send our technological kids, in the form of robots like the Perseverance rover or futuristic AI-astronauts. Artificially-made hardware is manufactured to be far more resilient to damage by energetic particles than the human body. And we should be proud of launching our technological products to space as we are of sending our biological kids to explore the world.
Merav Opher is a professor in the Astronomy Department at Boston University. She is currently the William Bentinck-Smith fellow at the Harvard Radcliffe Institute. She is the leading SHIELD, a NASA DRIVE Science as principal investigator. SHIELD is a multi-institutional effort with more than 45 leading scientists across a dozen institutions. She was the chair-elect of the APS Topical Group in Plasma Astrophysics; member of the Decadal Survey in Space Physics of Solar and Heliospheric Panel and the last three NASA Heliophysics Mission Senior Review Panels.
Avi Loeb is a professor of science at Harvard University, head of the Galileo Project, founding director of Harvard University’s – Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University from 2011-2020. He chairs the advisory board for the Breakthrough Starshot project and is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos.”