B What is the true density of the Asteroid Belt?

[saddlestone-man]

TL;DR Summary

Typical 'artist's impressions' of the Asteroid Belt show a large density of matter. Is this view close to the truth?

Hello All

Typical 'artist's impressions' of the Asteroid Belt show a large density of matter, see the attached view.

Many unmanned spacecraft (and hopefully soon some manned ones) have crossed this belt, without encountering any asteroids, so how close is this view to the truth? Is there any point in the belt where this view could be valid?

If this sort of view of the belt is common on the www and in textbooks, why isn't it corrected?

In the thickest part of the belt, is there an estimate for the density, say in kg per cubic km?

best regards ... Stef

 

[Ibix]

Per Wikipedia, the total mass of the asteroid belt is 3-4% that of the moon. That's spread over a ring around the solar system larger than Mars' orbit. Average density, therefore, isn't much different from zero, especially as most of that mass is concentrated in a handful of large asteroids.

Why is it drawn the wrong way? It looks cooler. Why isn't it corrected? It frequently is. Arthur C. Clarke's 2001 mentions it in passing, which is the first time I recall reading it. Even TV Tropes points it out in its discussion of asteroids in fiction. Those pictures still get drawn because of the Rule of Cool.

 

[Vanadium 50]

If this sort of view of the belt is common on the www and in textbooks, why isn't it corrected?

You should report this to the WWW Arbiter of Truth immediately!

 

[saddlestone-man]

The Wikipaedia entry on the Belt seems very reasoned and calm, then it can't resist the temptation to publish this image, which I think originates from NASA:

 

[saddlestone-man]

Here's another one from Asteroid Day Live 2020. Populated by many (supposedly) prominent astronomers ...

Surely the Astronomers themselves should start by setting an example.

 

[Bandersnatch]

I mean, what do you expect? A realistic image would be an empty void. It would serve no purpose.

 

[saddlestone-man]

How about publish the picture and say under it "This is nothing like reality, we've no idea why we published this wrong image".

 

[Ibix]

There's a lesson here: don't trust pop sci unreservedly, even from reputable sources. Its primary objective is to get people enthused about science, to inspire them, but not necessarily to teach it in a useful way. There's usually a lot of truth in there, but it's part way to those "based on a true story" films that often sacrifice accuracy for marketability.

 

[saddlestone-man]

I wonder if the Asteroid Belt is the most mis-represented topic in the Solar System?

 

[russ_watters]

How about publish the picture and say under it "This is nothing like reality, we've no idea why we published this wrong image".

There's a lesson here: don't trust pop sci unreservedly, even from reputable sources.

I have some bad news. The style of many articles on the internet, whether from a news source(even non-academic science magazines) or not, is to have body text under a photo because plain text is boring. Frequently - very frequently - that photo isn't directly connected to the article. It's just a stock photo meant for aesthetics. No, it doesn't need a disclaimer. People should really already know this.

 

[Keith_McClary]

A realistic image would be an empty void.

I've been thinking of asking, if you were in the belt, how many asteroids would you see per day (naked eye, assuming you knew where to look).

 

[russ_watters]

I've been thinking of asking, if you were in the belt, how many asteroids would you see per day (naked eye, assuming you knew where to look).

Somebody asked nearly this exact question recently. Answer is the same as for people on Earth: none.

 

[Klystron]

How about publish the picture and say under it "This is nothing like reality, we've no idea why we published this wrong image".

NASA frequently captions pictures as "artist interpretation". Not only would primary source data confuse most readers, human eyes cannot even see many of the wavelengths used for imaging such as infrared (IR) and radio frequencies (RF).

Spacecraft likely use wide radar to detect asteroids and narrow lidar or maser to identify details; none of which safely register on human senses. Artists work with engineers to create pictures that untrained humans can comprehend.

Note that non-radiating solar system objects remain invisible to the eye until illuminated by the sun or by a transmitter or from occluding background light sources. The entire field of radio astronomy requires interpretation and data analysis to portray images.

 

[russ_watters]

NASA frequently captions pictures as "artist interpretation".

Yes, and a nuance to that (aside from your point about astrophotos); sometimes they are meant to be accurate, sometimes they are pure art, and sometimes a mixture of the two. For the one above, the spacecraft is accurate and the background is art.

What annoys me is corporate press release graphics of (for example) a new plane that looks totally unrealistic/unlikely to even work.

 

[saddlestone-man]

I don't think this is just an Internet problem (it's easy to blame the Internet for everything) ... I bet if you look in any astronomy book, you'll find similar images of the Asteroid Belt.

 

[Vanadium 50]

I bet if you look in any astronomy book

That's twice you made this claim. Can you provide an example?

 

[Frabjous]

”Just how crowded is the asteroid belt?
Let's do the math!

There are about 3500 - 4000 or so cataloged asteroids with diameters over 1 kilometer. The asteroid belt is a band between 2 - 3.3 Astronomical Units wide, give or take, but the asteroids are bunched up into families so are not uniformly spread out in this vast volume some 100 million miles wide and perhaps another 20 million miles thick.

The volume of this space is about 4 million trillion cubic miles. The average distance between the asteroids would be about 100,000 miles. But there are likely to be lots of smaller objects too. Both the Voyager 1 and 2 spacecraft passed through the asteroid belt. They were not hit by anything except interplanetary dust which is very common in the asteroid belt, and that is your biggest problem to worry about.”

from https://image.gsfc.nasa.gov/poetry/venus/a10537.html

 

[sophiecentaur]

Somebody asked nearly this exact question recently. Answer is the same as for people on Earth: none.

One hundred times nearly nothing is still nearly nothing. One problem is that the level of solar illumination is a lot lower out there so you'd need a much bigger one to make it as visible as a smaller one at 1AU orbit.
Old SciFi films (and radio plays) used to suggest that asteroids would be the biggest hazard of any space trip - to almost any destination. They used to make a serious clanking noise when they hit the hull. Terrifying.