Hypomagnetic field, an ignorable environmental factor in space?

All living organisms on the earth have been exposed to the geomagnetic field (GMF, ~50 μT), a nature component of the habitability environment, throughout the evolutionary history. However, the environmental magnetic field (MF) in space decreases geometrically with the distance from the ground. The MF magnitude (|B|) at the International Space Station (ISS; near-earth orbit) is similar to the GMF (~80% GMF with limited variation), except that the radial component (Br) exhibits significant variations both in direction and magnitude [1]. The environmental MF in outer space is extremely lower than the GMF, called a hypomagnetic field (HMF), e.g., Martian MF is <5 μT, Lunar MF is <300 nT and interplanetary MF is just a few nanotesla [2] (Figure 1). Thus, besides micro-gravity (μ-gravity) and cosmic radiation, astronauts have to be exposed and adapt to the HMF condition when onboard in the long-term and long-distance space missions. Even though biological effects of the HMF were valued in early studies (1960s), exploration of the space HMF has not rapidly progressed in the following decades. The reasons may be as follows. First, considering the strength of MF in the near-earth orbit is about 80% of the GMF, the effects of μ-gravity and radiation are more observable than HMF in the short-term and short-distance space missions. Significant attention has been attached to the ground-based simulation experiments of μ-gravity and dosimetry of radiation in the previous space projects and missions. Second, the maintenance of a high quality HMF requires sophisticated instrument and expensive materials, such as highprecision magnetometer, magnetic shielding metal and Figure 1 The environmental magnetic field in outer space and the definition of the hypomagnetic field. A weak static magnetic field with |B|<5 μT is defined as a hypomagnetic field (HMF). Lunar magnetic field range: <300 nT; Martian magnetic field range: 300 nT5 μT. The interplanetary magnetic field is lower than 10 nT. The magnetic intensity is plotted in logarithmic scale (modified from the figure in [2]).