Health risks of space tourism: Is it responsible to send humans to Mars?
About 60 years ago, humans acquired the technological ability to travel in space. Currently, sci-fi franchises like “star trekinspired entrepreneurs like Jeff Bezos to turn their wealth into space tourism businesses. Bezos recently voiced the desire to send 1 trillion humans into space in the far future, as Earth will not be able to accommodate them all. Unfortunately, humans were not selected by Darwinian evolution survive a long time in space.
The dangers of energetic particles have been known since the earliest days of space exploration. On Earth, humans are protected from these charged particles, which come from the Sun and our galaxy, the Milky Way. The Earth is protected by its magnetic field and its atmosphere. Mars has no magnetic field or atmosphere to protect humans from damage caused by cosmic radiation.
Human astronauts outside of Earth’s magnetic “belly” are zapped by solar energetic particles, mostly in sporadic solar flares that last from minutes to hours. These eruptions are significant when the sun is “active”, i.e. during the solar maxima in its 11 year cycle of surface activity. The most energetic solar particles can be deadly. Humans have a better chance of surviving on Mars when the Sun is least active, i.e. during solar minima.
But even if humans avoid 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 also be deadly. The potential cumulative effects of space radiation should be thoroughly investigated before sending humans on missions that last more than a few years. Protection could potentially be offered in caves deep below the lunar or Martian surface.
Our solar system receives only a fraction of galactic cosmic rays, thanks to magnetic shielding by the so-called heliosphere, located a hundred times the Earth-Sun separation, where the Solar wind meets the interstellar medium. The heliosphere has been traversed by NASA Traveler 1 spaceships in 2012 and by Traveler 2 in 2018. Instruments aboard these missions revealed that the heliosphere blocks about three quarters of galactic cosmic rays.
At present, scientists are unable to reliably predict levels of galactic cosmic radiation throughout the solar system. The very region that shields galactic radiation is the one that is least understood.
Space missions, such as Voyager, New Horizons, Interstellar Frontier Explorer and Cassini-Huygens, revealed the frontal extent of the heliosphere and the influx of hydrogen atoms from the galaxy, but the basic characteristics of the heliosphere remain unknown. In particular, the global shape and distribution of cosmic radiation is uncertain.
Before sending humans on long space journeys, more resources should be devoted to studying the radiation filtered by the heliosphere. A better understanding of our own environment will also help us to predict if life exists on Earth-like planets around other stars.
Some habitable planets are shielded from energetic particles by their atmosphere and magnetic field, as well as by the analog of our heliosphere, called the “astrosphere” for other stars. We currently know very little about the astrospheres in general. Studies of the heliosphere would help us understand the critical mechanisms that control the properties and shielding of energetic particles that pose a threat to extraterrestrial life.
The human body is fragile. Humans cannot safely venture on long journeys beyond our immediate vicinity near Earth. Before sending human astronauts on long expeditions, we need to make sure that we don’t send them to their deaths. Before dreaming of a large human population on Mars, like advocated by Elon Muskwe need to understand the radiation environment throughout the solar system.
A safe bet, for now, is to send our technological children, in the form of robots like the Rover of Perseverance or futuristic AI astronauts. Artificially made hardware is made to be much more resistant to damage from energetic particles than the human body. And we should be proud to launch our technological products into space as we are to send our biological children to explore the world.
Merav Opher is a professor in the Department of Astronomy at Boston University. She is currently the William Bentinck-Smith Scholar at the Harvard Radcliffe Institute. She is the first SHIELD, a NASA DRIVE Science as Principal Investigator. SHIELD is a multi-institutional effort with more than 45 leading scientists at a dozen institutions. She was elected chair of the APS Thematic Group in Plasma Astrophysics; member of the Decadal Survey in Space Physics of Solar and Heliospheric Panel and of the last three NASA Heliophysics Mission Senior Review Panels.
Avi Iwhereb is professor of science at Harvard University, leader of the Galileo project, founding director of the Harvard University – Black Hole Initiative, director of the Institute for Theory and Computation of the Harvard-Smithsonian Center for Astrophysics and former chairman of the department in Astronomy at Harvard University from 2011 to 2020. He chairs the advisory board for Project Breakthrough Starshot and is a former member of the President’s Advisory Council on Science and Technology and a former chair of the Council for Physics and astronomy from national academies. He is the bestselling author of “Extraterrestrial: The first sign of intelligent life beyond Earth» and co-author of the manual «life in the cosmos.”
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