<p>Most of my information is from <a href="https://science.sciencemag.org/content/357/6355/1026" rel="nofollow noreferrer">this link</a>. So here's a watered-down version.</p>

<p>When the Solar System first formed, numerous concentrations of mass, called 'planetesimals', formed over time from slowly accreting material. Some of these would go on to combine with other planetesimals and form planets in their own right, but some would simply float in space. Over time, these planetesimals would shatter apart into primordial asteroids from impacts with smaller bodies. However, these remnant asteroids would originate from a single body, so they would have about the same physical properties. In addition with our modern orbital calculation techniques*, we would able to identify groups of these primordial asteroids, and flag them as such. However, an asteroid created from a planetary impact would likely be ruled out as non-primordial, since there would be no "brother asteroids" to compare it with.</p>

<p>*To clarify, we can trace orbits of asteroids back over billions of years to see where the bodies might have been in the distant past. Asteroids that originated from a single parent body would be observed to converge to a single point.</p>

CC BY-SA 4.0
2019-05-16T20:12:28.370Z
I think the composition and texture (inferred from the spectrum of reflected sunlight, radar and other clues) suggests it hasn't been melted at any point, which tends to rule out any violent history.
Steve Linton
CC BY-SA 4.0
2019-05-16T20:01:57.983Z
2019-05-16T20:12:45.053Z
2019-05-16T20:12:45.053Z
Cloudy7
<p>As mentioned <a href="https://www.theguardian.com/science/2019/feb/22/japans-hayabusa-2-successfully-touches-down-on-ryugu-asteroid" rel="nofollow noreferrer">here</a>, that Ryugu is a part of the Solar System material that did not get swept into forming a planet.</p>

<p>But how can we be sure that it was not created later for example by an asteroid striking Mars or some other planet? Why do we think its "primordial"?</p>

<asteroids>
How can we be certain that Ryugu is a remnant of the material that did not form planets?
safe_malloc
# How can we be certain that Ryugu is a remnant of the material that did not form planets?

(Asked by: username_0 on 2019-03-02)

As mentioned [here](https://www.theguardian.com/science/2019/feb/22/japans-hayabusa-2-successfully-touches-down-on-ryugu-asteroid), that Ryugu is a part of the Solar System material that did not get swept into forming a planet.

But how can we be sure that it was not created later for example by an asteroid striking Mars or some other planet? Why do we think its "primordial"?

**Tags:** asteroids

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## 1 answers

### Answer by username_1 on 2019-05-16. Score: 1 | Accepted answer

Most of my information is from [this link](https://science.sciencemag.org/content/357/6355/1026). So here's a watered-down version.

When the Solar System first formed, numerous concentrations of mass, called 'planetesimals', formed over time from slowly accreting material. Some of these would go on to combine with other planetesimals and form planets in their own right, but some would simply float in space. Over time, these planetesimals would shatter apart into primordial asteroids from impacts with smaller bodies. However, these remnant asteroids would originate from a single body, so they would have about the same physical properties. In addition with our modern orbital calculation techniques\*, we would able to identify groups of these primordial asteroids, and flag them as such. However, an asteroid created from a planetary impact would likely be ruled out as non-primordial, since there would be no "brother asteroids" to compare it with.

\*To clarify, we can trace orbits of asteroids back over billions of years to see where the bodies might have been in the distant past. Asteroids that originated from a single parent body would be observed to converge to a single point.

> **Comments:**
> **username_2**: I think the composition and texture (inferred from the spectrum of reflected sunlight, radar and other clues) suggests it hasn't been melted at any point, which tends to rule out any violent history.  

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