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If I were hypothetically wearing a spacesuit and sitting on one of the Voyager space probes at their current positions in space, how much light would I have? (Intermediate)

The Voyager 1 and 2 space probes are rushing further away every day, but they are currently about 22 billion kilometres (13 billion miles) and 18 billion kilometres (11 billion miles) away from the Sun, respectively [1]. For comparison, the Earth is about 150 million kilometres (93 million miles) away from the Sun, so the Sun’s light will certainly be much dimmer at the Voyager space probes than at the Earth.

The apparent brightness of a light source decreases in proportion to the square of the distance from the light source. This means that a light source viewed from a distance of three metres away will appear to be 32 = 9 times fainter than if it were viewed from a distance of 1 metre away. Using this reasoning, we can calculate the strength of light at the Voyager space probes!

A unit for quantifying the brightness of light, as seen by the human eye, is the lux. The brightness of a sunny day is about 10,000 lux, while twilight is about 10 lux. A dark night with a full Moon is about 0.1 lux, while a dark night with only starlight is about 0.001 lux [2].

The brightness of the Sun at the Voyager 1 and 2 space probes is about 6 lux and 9 lux, respectively. So if you were sitting on one of the Voyager space probes, the Sun itself would appear to be roughly as bright as a point on the sky at twilight.

However, it would actually seem to be much darker than twilight on Earth. For one thing, the Sun will only appear to be a pinprick point source of light. While it is dangerous to look directly at the Sun, it actually has an angular size of about 1900 arcseconds (where 1 arcsecond is 1/3600 of 1 angular degree). Coincidentally, the Moon also has a similar angular size, which makes it possible for it to completely cover the Sun during a total solar eclipse. The smallest angular distance that the human eye can resolve is about 15 arcseconds, so anything smaller this this will appear to be a point source of light. The angular size of the Sun as seen from both Voyager 1 and 2 is about 7 arcseconds, which is well below the limit of what the human eye can resolve. So, the Sun would appear as a tiny pinprick point of light that is no larger than any other star! However, you would be able to identify it as the Sun because it will would be much brighter than any other star.

Furthermore, on Earth during the day, the sky appears to be bright in all directions because molecules in Earth’s atmosphere scatter light in all directions. However, there is no atmosphere surrounding the Voyager space probes, so you will only see sunlight if you are looking directly at the Sun (which would be safe to do from that distance) or if the Voyager probe that you were sitting on reflected some sunlight back into your eyes.

Only a fraction of the sunlight that shines on the Voyager space probe will reflect back into your eyes (with the fraction depending on how reflective the surface is), but you would likely be able to faintly see at least the most reflective parts of the space probe.

So in conclusion, if you were sitting on a Voyager space probe, it would be very dark. However, the pinprick point source that is the Sun would be much brighter than any other star (roughly as bright as a point on the sky at twilight) and you would likely be able to see some faint sunlight reflecting off the Voyager space probe that you were hypothetically sitting on!




Last updated: May 25, 2019

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