Expert: Courtney Seligman - 12/12/2010

Question
Hello, again.

I've just finished reading about 'zodiac lights', and I was wondering...

I know that asteroids aren't visible to the naked eye because the human eye can rarely distinguish objects above a magnitude of 6, and most asteroids have a magnitude between 11 and 19 so... really hard to spot without a telescope or such.

But, if cosmic dust can be seen (even if only under certain conditions)... perhaps if it weren't for all the stars in the sky we could see the asteroid belt as a lighter band in the sky. What do you think? Would it be 'possible' (assuming that all stars in the sky would suddenly disappear)?

Sorry for the strange question and thanks for your insight,
Sara

Answer
The key to seeing the zodiacal light (and the counterglow, or gegenschein, which is opposite the Sun) is the fact that there are a very large number of very small motes scattering the light. And although there are a large number of asteroids, and each of them is much brighter than a small mote, the light they scatter or reflect is only a tiny fraction of what the dust motes scatter and reflect.

To understand this, suppose we combined a million motes into a single larger object. With a million times the mass, the larger object would have to have a million times the volume, which would make it about a hundred times larger, and ten thousand times brighter than an individual mote. But if there were one such object, and a million such motes, the combined light of the motes would be a hundred times brighter.

In other words, although the individual asteroids are much larger (and brighter) than the motes which cause the zodiacal light, it is the immense number of such small objects that produces the effect. Since a typical asteroid is thousands of feet across (or larger, depending upon how small you're willing to count them), and a zodiacal mote is a hundredth or a thousandth of a foot in size, it would take millions of millions of millions of motes to make an asteroid size object; and in combining them, the total light reflected would be reduced by tens of thousands of times.

So no, if we could see the light of the asteroids as a sort of continuous glow, we wouldn't have much hope of noticing it, because it would be tens or hundreds of thousands of times fainter than the zodiacal light.

In closing, I might note that a similar situation occurs when we look at "nebular" clouds of (gas and) dust surrounding (very young) bright stars. Most of the light (or more frequently, infrared radiation) given off by the dust in the disks surrounding comes from nearly microscopic bits. Larger bits, although individually brighter, aren't as numerous, and their radiation would be less, because of their smaller surface area, even if they were as numerous. So even if most of the mass in such circumstellar disks is in the form of larger objects, most of the radiation which reveals the disks comes from the smallest bits.