Just about every webpage related to asteroids states that, on average, asteroids are about 600,000 miles apart from one another. Have planetary scientists hazarded a guess at how close they might have been 4 billion years ago? 2 billion years ago? 200 million years? I'm not looking for a particular age range, just an understanding of the trend over time. Unfortunately, I'm having trouble looking up an answer to this question on my own because the keywords I've thought of keep grabbing other unrelated things

Thank you!


2 Answers 2


The asteroids formed roughly where they are now, from matter in the sun's protoplanetary disc that didn't form into a larger body (at least partly under the influence of Jupiter's gravity)

Since early in the solar system's story, the asteroids have been stable. Most asteroids haven't changed significantly in 4.5 billion years. And 4.5 billion years ago the separation between asteroids was comparable to how it is now.


Early in the history of the solar system, before all of the surviving objects had stable orbits, there were a number of bodies with orbits tat turned out to be unstable. Many of those bodies were ejected from the solar system or fell into the Sun. And some of those bodies collided with other bodies, which often shattered both objects.

For example, the theory of the origin of the Moon which has the least problems claim that a roughly Mars-sized planet called Theia or Orpheus collided with the Earth about 4.5 billion years ago. Debries from his impact formed a ring around the Earth and eventually combined to become the Moon.

A hypothetical planet V may have existed between Mars and the asteroid belt before being ejected from the solar system.

The Five Planet NICE model of the formation of the solar system involved a fifth giant planet which was ejected from the solar system.

And there have been other such theories:


A number of nickel-iron meteorites have been found on Earth which mostly consist of alloys of iron and nickel. All meteorites are believed to be tiny asteroids with orbits that interested with the orbit Earth. And some asteroids are known to have largely iron and nickel compoosition.

16 Psyche is believed to be the largest mettalic asteroid with an average radius of about 111 kilometers and thus an average diameter of about 222 kilometers.

How can metals be concentrated so that entire asteroids can be made of metals?

the theory is that in the early solar system tiny planetesimals often collided, and sometimes they stuck together after colliding, So gradually the number of plentesimals decreased and the average size of planetesimals increased. In some cases planetesimals grew large enough that their internal pressure and heat liquified part of their interiors and varous liquid materials gradually separated as denser materials fell downwards and lighter materials floated upwards.

Thus the interiors of those planetesimals gradually developed layers of materials, with denser materials at the inner core and lighter materials successively higher. And the liquid interiors of those objects caused their surfaces to assume roughly spheriodal shapes. Thus all the planets, dwarf planets, and large round moons in the solar system were formed.

But in the early days of the solar system objects, even as large as planets, sometimes were ejects from the solar system, or fell into the Sun, or collided with other objects.

And so it is believed the metallic meteorites, and the metallic asteroids, were one part of the iron nickel cores of planetesimals large enough to have liquid cores which became dfferentiated by density before cooling and solidifying. And those large parent planetesimals were shattered by collisions with other bodies, producing vast numbers of asteroids of various types, including metallic asteroids.

In the case of 16 pysche, it is believed it is the largest metallic core fragment of an object with a diameter of about 500 kilometers. Of the remaining large asteroids, Ceres, Vesta, and Pallas have diameters over 500 kilometers and so may have iron nickel cores, and possibly some other asteroids are large enough to have iron nickel cores.

So the evidence shows that at one time asteroids collided often, and that sometimes even large asteroids hundreds of kilometers in diameter were shattered into a number of pieces by those collisions.

The era when that happened may have been the era of the hypothetical Late Heavy Bombardment about 4.1 to 3.8 billion years ago, or even earlier.

No doubt there are occassional collisions happening between asteroids at the present, so that some asteroids shatter into many pieces which become separate asteroids, and some asteroids combine to become larger asteroids. But such events have probably been very rare for 3 or 4 billion years, and the average separation between asteroids has probably changed only very slightly in the last 3 or 4 billion years.


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