---
title: "S-Type Asteroids | Intro to Astronomy"
description: "S-type asteroids are stony, silicate-rich asteroids with nickel-iron and high reflectivity, common in the inner asteroid belt in Intro to Astronomy."
canonical: "https://fiveable.me/intro-astronomy/key-terms/s-type-asteroids"
type: "key-term"
subject: "Intro to Astronomy"
unit: "Unit 13"
---

# S-Type Asteroids | Intro to Astronomy

## Definition

S-type asteroids are silicate-rich, rocky asteroids with some nickel-iron metal. In Intro to Astronomy, you usually meet them when studying asteroid composition, spectra, and the inner asteroid belt.

## What It Is

S-type asteroids are a major asteroid class in Intro to Astronomy, made mostly of silicate minerals like olivine and pyroxene, with some nickel-iron mixed in. The “S” stands for stony, which is a good clue to their overall makeup: they are rocky bodies, not icy ones and not mostly metal.

You usually find them in the inner asteroid belt, the region closer to Mars than Jupiter. That location matters because the inner solar system was hotter when the planets formed, so materials that could condense there were more rocky and metal-rich. S-type asteroids fit that pattern better than the darker, more carbon-rich bodies farther out.

Astronomers identify S-type asteroids by their spectra. When sunlight bounces off the asteroid, the reflected light carries absorption features that show which minerals are on the surface. For S-types, the spectrum shows strong signatures from silicates, especially olivine and pyroxene. That means this category is not based on appearance alone, but on the way the asteroid interacts with light.

They also tend to have relatively high albedo, around 0.10 to 0.22. Albedo is a measure of reflectivity, so S-types usually look brighter than many other asteroid classes. They still are not shiny like a metal mirror, but they reflect more light than dark carbon-rich asteroids, which makes them easier to detect in some surveys.

A useful thing to remember is that S-type asteroids are not just “generic rocks in space.” They tell you something about solar system history. Because they contain silicates and some metal, they preserve clues about the materials that were available in the inner solar system and about how small bodies can become sources of meteorites. Many stony meteorites that land on Earth are thought to come from S-type asteroids or bodies like them.

## Why It Matters

S-type asteroids matter because they connect several big ideas in Intro to Astronomy: composition, spectra, asteroid classification, and the early solar system. When you can identify an S-type asteroid, you are doing more than naming a rock. You are reading a clue about where it formed and what materials were present when the solar system was young.

This term also shows up whenever your class compares asteroid types. S-types are one of the main “rocky” categories, so they provide a middle ground between darker, carbon-rich asteroids and metallic asteroids. That comparison helps you sort asteroid populations by composition instead of just by orbit.

They are also tied to meteorites. If a lab, image set, or homework question asks where stony meteorites might come from, S-type asteroids are a strong answer. That links small bodies in the asteroid belt to physical samples you can hold in a museum or study in a lab.

Finally, S-types are a good example of how astronomy uses light to study distant objects. You cannot walk up to most asteroids and sample them, so spectra and albedo become the evidence. Knowing what S-types look like in a spectrum helps you interpret telescope data, asteroid surveys, and classification questions.

## Connections

### C-type Asteroid

C-type asteroids are usually darker and richer in carbon compounds than S-types. If S-types are the brighter stony bodies of the inner belt, C-types are the more common dark, primitive bodies farther out. Comparing the two helps you see how temperature and solar system distance shaped what different asteroids are made of.

### M-type Asteroid

M-type asteroids are the metallic side of the asteroid spectrum, often associated with higher amounts of nickel and iron. S-types do contain some metal, but their spectra are dominated by silicates. If a question asks you to separate rocky from metallic asteroids, this comparison is one of the cleanest ways to do it.

### Albedo

Albedo is the reflectivity measure that helps astronomers tell asteroid surfaces apart. S-type asteroids usually have a moderate to high albedo compared with darker asteroid classes, which makes them easier to distinguish in observations. On a problem set, you may use albedo together with spectral data instead of relying on brightness alone.

### [asteroid belt](/intro-astronomy/key-terms/asteroid-belt)

The asteroid belt is where many S-type asteroids are found, especially in the inner region closer to Mars. Their location is not random, because the belt contains bodies that formed under different thermal conditions. When you map asteroid types across the belt, S-types help show the inner to outer compositional pattern.

## On the AP Exam

A quiz or image question may show an asteroid spectrum and ask you to identify the type. For an S-type asteroid, you would look for silicate absorption features, a stony composition, and a relatively reflective surface. If the question includes location, the inner asteroid belt is another clue.

In a short answer or lab write-up, you might compare an S-type asteroid to a C-type asteroid and explain why they differ in color, albedo, and composition. You could also explain how S-types connect to stony meteorites, using spectrum data as evidence. If a homework set gives you a graph or reflected-light plot, the move is to link the absorption pattern to olivine, pyroxene, and a rocky surface rather than just saying “it is an asteroid.”

## S-type asteroids vs C-type Asteroid

S-type and C-type asteroids are often mixed up because both are common asteroid classes. The difference is their composition and reflectivity: S-types are stony and relatively bright, while C-types are carbon-rich and usually much darker. If the spectrum shows silicates like olivine and pyroxene, you are looking at an S-type, not a C-type.

## Key Takeaways

- S-type asteroids are stony asteroids made mostly of silicate minerals, with some nickel-iron mixed in.
- Their spectra show absorption features from minerals such as olivine and pyroxene, which is how astronomers identify them.
- They usually have a moderate to high albedo, so they reflect more light than darker asteroid classes.
- Most S-type asteroids are found in the inner asteroid belt, closer to Mars than Jupiter.
- They are thought to be the source of many stony meteorites that reach Earth.

## FAQs

### What is S-type asteroids in Intro to Astronomy?

S-type asteroids are a class of stony asteroids made mostly of silicate minerals with some nickel-iron metal. In Intro to Astronomy, they come up when you study asteroid composition, reflectivity, and spectra. They are common in the inner asteroid belt.

### How do astronomers identify an S-type asteroid?

They look at the asteroid's spectrum and albedo. S-types show absorption features from silicate minerals, especially olivine and pyroxene, and they tend to reflect a moderate amount of sunlight. You do not identify them by color alone, because the spectrum gives the better clue.

### Are S-type asteroids the same as stony meteorites?

Not exactly, but they are strongly linked. Many stony meteorites that fall to Earth are thought to come from S-type asteroids or bodies with similar composition. The connection comes from matching mineral makeup and spectral evidence, not from direct sampling of every asteroid.

### What is the difference between S-type asteroids and C-type asteroids?

S-types are silicate-rich and relatively bright, while C-types are carbon-rich and usually much darker. S-types are more common in the inner asteroid belt, while C-types are more common farther out. If you remember rockier and brighter versus darker and more carbon-rich, you will usually pick the right one.

## Related Study Guides

- [13.1 Asteroids](/intro-astronomy/unit-13/1-asteroids/study-guide/8xkz7hqDoQ2qD3WN)

## About This Document

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