Does radiation danger increase or decrease as you travel farther from the Sun?

I understand that there's a significant radiation hazard to astronauts. Say, traveling from Earth to Mars, due to radiation from the Sun and also from other sources.

Does this hazard decrease the farther you get from the Sun and into the outer solar system? I'm not asking solely about the radiation dangers from the Sun. I'm asking about all radiation dangers, regardless of their source. Do levels of any and all radiation sources increase or decrease the farther out you go?

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    $\begingroup$ As a matter of counterexample: Jupiter's deadly particle belts. $\endgroup$ Aug 22, 2013 at 4:08
  • $\begingroup$ Similar enough to be complementary: physics.stackexchange.com/questions/74762/… $\endgroup$
    – Everyone
    Aug 27, 2013 at 6:25
  • $\begingroup$ @Joe: The interrogative says 'further into the outer solar system'. Is the scope of the question restricted to effect of radiation as a function of distance within the Solar System? $\endgroup$
    – Everyone
    Sep 7, 2013 at 18:05
  • $\begingroup$ @Everyone, yes. I'm interested in the danger of radiation on astronauts, only within the solar system, as a rough function of distance from the sun. $\endgroup$
    – Joe
    Sep 8, 2013 at 5:19

2 Answers 2


Let's start with a few unspoken assumptions

  • Radiation refers to emissions from any body in space. To name a few possible sources
    • Star
    • Nebula
    • Gas giant
  • Danger means danger to humans
  • Sun refers to our very own Sol

The answer may appear to contradict itself at times. This may be because my attempt here is to provide a ... holistic (for want of a better word) view of what little I comprehend, recall, and read.

Background: As already premised, Earth's atmosphere & magnetosphere serve as an umbrella. They shield surface bound Earth life from known sources of radiation. The Sun is perhaps the nearest source of radiation. However even the Solar wind serves to protect planets within the Solar System from Cosmic Rays

Wikipedia writes to say

At distances of ~94 AU from the Sun, the solar wind undergoes a transition, called the termination shock, from supersonic to subsonic speeds. The region between the termination shock and the heliopause acts as a barrier to cosmic rays, decreasing the flux at lower energies (≤ 1 GeV) by about 90%. However, the strength of the solar wind is not constant, and hence it has been observed that cosmic ray flux is correlated with solar activity.

An independent source for the above

Moving onwards - another major source of radiation within the Solar System is Sol itself. The planetary fact sheets available from NSSDC show Solar irradiance fall as one moves within the Solar System from Mercury to Pluto. Of course, this is inclusive of the visible spectrum.

So ...

  • Cosmic ray radiation within the Solar System is affected by Sol's weather.
  • Incident radiation from Sol is directly proportional to distance from the Sun

Still staying within the Solar System, what are the other possible sources of radiation? DeerHunter's comment of Aug 22 provides an answer. The Gas giants are audible in the radio bands; they may be a source, and therefore potentially dangerous. A person standing within a pressure vessel of a reactor would likely be in danger regardless of Cosmic Rays, and Solar radiation; replace 'reactor' with, say, Jupiter and the analogy fits to a 'T'.

Between the known major sources (Cosmic Rays, and Sol) my impressions are

  1. The risk increases as one moves away from the inner Solar System, and
  2. The risk is probably a function of proximity to any alternate sources.

Yes as you move away from our solar system the radiation from the sun decreases because it follows inverse square law.. But other source of radiation is Galactic cosmic radiation : enter image description here

Galactic Cosmic Radiation (GCR) Galactic cosmic radiation originates outside the solar system. It consists of ionized atoms ranging from a single proton up to an uranium nucleus. The flux (rate of flow) levels are the low. However, since they travel very close to the speed of light, and because some of them are composed of very heavy elements such as iron, they produce intense ionization as they pass through matter.

For the most part, the Earth's magnetic field provides shielding for spacecraft from galactic cosmic radiation. However, cosmic rays have free access over the polar regions where the magnetic field lines are open to interplanetary space.

enter image description here

The flux of galactic cosmic radiation is isotropic outside heliosphere , inside it propagation effect results in an isotropic of approximately 1%

The sun is not only the source of radiation to an spacecraft but there are countless stars like sun in and outside our solar system which emit radiation.

The radiation effects decreases to greater extent ( after heliosphere ) unless the spacecraft is hit by the suddenout burst of galactic cosmic radiation.

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    $\begingroup$ Yes, the presence of radiation is ubiquitous throughout the universe, but I'm still not clear if this answers the question: Does "radiation danger" (humans, spacecraft, etc) decrease as you move away from the sun (or leave the solar system entirely)? $\endgroup$ Aug 23, 2013 at 14:12
  • $\begingroup$ @RobertCartaino yes there is a danger of radiation outside the solar system but the radiation from the sun decreases as we move away from the sun :-) $\endgroup$
    – Hash
    Aug 23, 2013 at 14:18
  • $\begingroup$ @Hash, I'm not asking about solely radiation dangers from the sun, I'm asking about all radiation dangers. Do they increase or decrease the further out you go? $\endgroup$
    – Joe
    Aug 25, 2013 at 0:51
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    $\begingroup$ An even more interesting question is, whether the radiation danger (in terms of damage to a human body) proportionally correlates with the dose of radiation. As far as I know, it does not - e.g. there is an unexpected increase in certain low dosage regimes, which causes so minor damages, that a human body would not repair it. In the long run, this is causing more damage than a higher dose of radiation, which triggers the repair mechanisms. $\endgroup$
    – s-m-e
    Aug 25, 2013 at 20:09
  • $\begingroup$ The curve would look like: Quadratic drop with distance + a constant. Sum of the solar radiation, which drops, and galactic radiation which remains constant. $\endgroup$
    – SF.
    Sep 8, 2013 at 9:43

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