Deep Space Darkness
When in deep space, beyond the point at which the
sun's light is significant, are objects easily visible? For
instance, how visible would a passing object be if the only light
on it are the stars?
Would this be similar to being on earth at night, during a new moon?
Without any sunlight, an object will be virtually black and
invisible. It is not likely that you would see it alt all.
One needs to generalize the "visible" to include wavelengths (or
equivalently, frequency) of electromagnetic radiation outside the
very narrow range (about 400 to 700 nanometers) that the human eye
can see. Every object gives off radiation whose frequency
distribution depends upon its temperature -- so called "black body
radiation". For "hot" objects such as stars, this is approximately
what produces their "brightness". An object that is too cool to
radiate energy due to its temperature, can reflect radiation that
falls upon it. The obvious examples being the moon and planets that
reflect sunlight. The amount of radiation emitted will depend upon
what is between the source and the observer. So an intervening
object can cause the amount of energy that can be detected to vary.
An eclipse of the moon is a classic example of the Earth blocking
radiant energy from the Sun from reflecting off the Moon. There are
other objects that may be faint, or even invisible, between 400 and
700 nanometers (the nominal visible range) but may be more intense
at greater wavelengths. The wavelength of radiation is also
influenced by the Doppler shift. For distant objects that are
moving away from us rapidly, this can distort the wavelengths that
are in the visible region. The bottom line is that your question is
not easy to give a short simple answer because there are a lot of
factors that come into play.
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Update: June 2012