003: Sun
Going to Mars has long captured the imagination, however there are obstacles in addition to the propulsion system to pull it off. The trip itself is dangerous due to hazardous radiation.
Our Earth, for the most part protects us from all the bad space radiation by way of the magnetosphere. Mars bound astronauts will not have this protection. The principal problems will be galactic cosmic radiation (GCR), random solar flares, and streams of energized particles from coronal holes.
I had a crazy idea while I was reading a news article on CNN.com. The story stated that some solar scientists had determined that the Sun is entering a potentially decades long “Grand Solar Minimum.” The last time this happened the Earth experienced what is now called the “Little Ice Age” between the years 1650 and 1715. The article went on to say that this will likely not be our fate, ironically because of greenhouse gas emission based global warming. However, there still could be an impact in terms of radiation. The Sun has its own magnetosphere if you will, called the heliosphere. So basically, the grand solar minimum can result in both the weakening of the heliosphere and the magnetosphere. The heliosphere controls how much GCR is in the solar system and the magnetosphere controls how much of that reaches the surface of the Earth. According to NASA, “During solar minimum, the sun's magnetic field weakens and provides less shielding from these cosmic rays. This can pose an increased threat to astronauts traveling through space."¹
My crazy idea was, maybe GCR is a long-term variable. All the radiation perils of interstellar travel are ultimately tied to the Sun. What if this was a bad period for the Sun? Maybe in the distant past the GCR, and the corresponding space weather events, were a much smaller problem than they are now. Perhaps at some point it was relatively easy to go to Mars!
Given that Mars doesn’t have a magnetosphere at all, the GCR let in by the heliosphere has a big impact. Currently there is sterilizing radiation on the surface that doesn’t make the search for life particularly easy, nor does it make the idea of growing crops on the red planet a simple task. But what if the GCR was significantly less in the past.
An analogue to this hypothesis would be the Bering land bridge between Asia and the Americas. During the Ice Age, Asian peoples simply walked to America on a land bridge of ice that now no longer exists.
Perhaps there are long cycle seasons of the Sun. Thus, the complexities of interstellar travel are a function of what mood the Sun is in, whether it’s calm or cranky.
Reference:
Strickland, Ashley. (2020). The sun is experiencing a less active phase called 'solar minimum,' but it won't cause an ice age. CNN. Retrieved from https://www.cnn.com/2020/05/19/world/solar-minimum-scn-trnd/index.html.