Rocks in Space
By: Thomas Albin
Tags: Astronomy, Space, Education, Science, Physics

Space Science in a Nutshell

Rocks in Space

Our Solar System has planets, asteroids and other bodies. But what are the differences?

Large Meteor (Fireball) over the Atacama Desert in Chile. The radio telescope dishes are part of ESO’s (European Southern Observatory) ALMA telescope (Atacama Large Millimeter/submillimeter Array). Credit: ESO/C. Malin; License: Creative Commons Attribution 4.0 International License

“Last night I saw a meteorite in the sky!” “I thought its called asteroid?” Hmmm …

Astronomy and astrophysics are great research fields that inspire us with fascinating images of exoplanets, galaxies, black holes or try to answer fundamental questions of the universe and our existence. But our very cosmic neighbourhood, the Solar System, has one advantage: we can reach and explore foreign worlds with spacecraft missions. We analyse and catalogue miscellaneous bodies and can see different objects (and appearances) in the night sky. You have probably heard of asteroids, comets, meteors and many more.

But what are the differences between these objects? And why do we need to differentiate between them? Let’s take a look:

Planet

2006 the International Astronomical Union set new definitions for planets and other minor bodies (IAU News: Page 16 following). Three requirements need to be fulfilled to classify an object as a planet:

The object revolves around a star (without being a star, since there are double-star systems and more) The object needs has to have a nearly perfectly spherical shape. The IAU states that “shape of objects with mass above 10²⁰ kg and diameter greater than 800 km would normally be determined by self-gravity, but all borderline cases would have to be established by observation” The orbit of the object needs to be clean from debris and other objects (also here: observations need to determine the cleanness of an orbit. Our planet moves through different dust streams and minor bodies cross the orbit as well)

The result of this new definition: 8 planets; and for all other objects the general term “Small Solar System Bodies” applies.

Dwarf Planet

Not a planet since 2006: Pluto. Colour composition taken by NASA’s New Horizons mission in 2015. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Dwarf planets are like planets, but their orbit is not cleared from other almost same sized objects. This definition applies for (1) Ceres, who revolves the Sun within the main belt between Mars and Jupiter; Pluto, Sedna and other too-small-to-be-a-planet objects.

You may have heard about Plutons or Plutoids, Kuiper Belt Objects (KBOs) or Transneptunian Objects (TNOs). These classifications are based on the orbital elements of the objects; so the orbit-types these objects have.

Asteroids

When I started working in the area of asteroids I thought “Well… I give it a try, but it MUST be boring”. I was wrong.

Today, we know 100-thousands of these primordial objects that inhabit different regions in the Solar System like the Asteroid belt between Mars and Jupiter. Asteroids are smaller than dwarf planets and mostly do not have a round-shape body. Larger asteroids (the transition between dwarf-planets and asteroids is not 100% clear) may have a dense and differentiated structure (iron core in the centre, rocky layers and a regolith surface), but for the majority the so called Rubble Pile model applies.

Asteroids have a way lower density than dwarf planets. This suggests that asteroids are not monoliths (a single rock), but a conglomerate of rocks and boulders that are held together by gravity. The Saturn moons Helene and Pandora (that are captured asteroids) have such a low density that they would float on your oceans.

(21) Lutetia in the main belt between Mars and Jupiter. The asteroid was visited during a brief flyby in 2015 by ESA’s mission Rosetta. Credit: ESA 2010 MPS for OSIRIS Team MPS/UPD/LAM/IAA/RSSD/INTA/UPM/DASP/IDA

Michel, P., et al., editors. Asteroids IV. University of Arizona Press, 2015. JSTOR, www.jstor.org/stable/j.ctt18gzdvc. Accessed 13 May 2020.

Comets

Comets are, like asteroids, remnants of the formation of our Solar System. These objects are mostly referred as dirty snow balls and contain water ice, dry ice, minerals and also complex organic compounds like amino-acids. These life-building-blocks are used by living organisms to create proteins.

Comets consist of different components explained in the following list:

Core : The core is the solid part of the comet; the dirty ice ball with its minerals, ice, and so on. Although comets may appear quite bright on the night sky (see image below), the core has a reflectivity of only a few percent. The bright appearance results from other effects that are explained in a moment. Comet cores that approach the Sun closely (within a few AU) start to evaporate. Gas and dust are leaving the comet creating a …

: The core is the solid part of the comet; the dirty ice ball with its minerals, ice, and so on. Although comets may appear quite bright on the night sky (see image below), the core has a reflectivity of only a few percent. The bright appearance results from other effects that are explained in a moment. Comet cores that approach the Sun closely (within a few AU) start to evaporate. Gas and dust are leaving the comet creating a … Coma : This part is a spherical shaped liked region around the core that consists of dust and gas. The density of this cloud decreases for larger distances from the core until the gas and dust are carried away and appear as …

: This part is a spherical shaped liked region around the core that consists of dust and gas. The density of this cloud decreases for larger distances from the core until the gas and dust are carried away and appear as … Tails: Take a look at the comet below. You see a blueish and white / grey tail. The blue one is the ion tail and is affected by the Solar Wind: A (with respect to the Sun) radially escaping stream of charged particles. These particles interact with the charged particles of the comet, the ions, and are carried away. The grey one consists mostly of dust particles and spreads along the path of the comet.

Gas and dust are responsible for the great and stunning appearance of a comet.

Michel C. Festou (Editor), H. Uwe Keller (Editor), Harold A. Weaver Jr. (Editor). (2004). Comets II. University of Arizona Press. pages 780. ISBN-10: 0816524505. ISBN-13: 978–0816524501

Active Asteroids

It’s a comet! It’s an asteroid! It’s … both? Well, yes, in a way. There are objects in our Solar System that have orbital properties of asteroids, but show cometary activity and vice versa. Further, asteroidal objects, rubble piles, eject particles and cause faint tails of dust, e.g. due to:

Collisions with other objects

Fast body rotation that leads to centrifugal forces that exceed the gravitational pull on the surface

Close encounters with the Sun that leads to thermal stress and cracking

One common active asteroid is the object (3200) Phaethon that ejects dust particles that intersect with our home planet causing falling stars in December, the Geminids.

Images of the active asteroid P/2013 P5, taken by the Hubble Space Telescope at different dates. Credit: NASA, ESA, D. Jewitt (UCLA)

Meteoroids

The International Astronomical Union (IAU) did not only introduced definitions to distinguish between planets and dwarf planets. They also introduced the definition of a meteoroid.

A meteoroid is like a small asteroid, a boulder or rock with a diameter between 30 µm and 1 m. That’s basically it.

Dust

Now, particles with a size that is smaller than 30 µm is defined as (cosmic) dust. By the way: cosmic dust was my main scientific doctorate studies topic and did you know that the guitarist of Queen, Dr. Brian May, also received a doctorate degree in this particular field?

Due to the light pollution of our modern world there are only a few places left on Earth where one can see the night sky without any artificial light sources. Dark skies have become rare and people are missing a phenomena Dr. Brian May was writing his thesis about: The Zodiacal Light. The Ecliptic Plane (the imaginary plane, where all planets revolve around the Sun) is populated with dust particles of only a few micrometers in diameter. These particles scatter the light of the Sun, causing this light phenomena, as shown in the picture below. Dust sources in our Solar System are:

Comets

(Active) Asteroids

Collisions of asteroids, meteoroids, etc.

Vulcanic active moons like Io

Meteor

Dust particles that are ejected from comets disperse along the orbit of their parent body. They form a so called Stream. These Streams cross eventually our home planet. High velocity dust particles evaporate in the atmosphere and due to the friction with the air they light up. The result: a falling star, respectively called meteor. Some meteor showers like the Perseids in August or the Geminids in December can be linked to their parent bodies. Other meteors appear randomly in the sky. Their orbit has been altered for thousands of years due to the radiation pressure and gravitational perturbations. Consequently, a parent body cannot be identified. These ones are called sporadic meteors.

Large Meteor (Fireball) over the Atacama Desert in Chile. The radio telescope dishes are part of ESO’s (European Southern Observatory) ALMA telescope (Atacama Large Millimeter/submillimeter Array). Credit: ESO/C. Malin; License: Creative Commons Attribution 4.0 International License

Peter Jenniskens. (2009). Meteor Showers and their Parent Comets. Cambridge University Press. pages 804. ISBN-13: 978–0521076357

Meteorite

If a meteoroid or dust particle enters our Earth’s atmosphere, a meteor can be seen. If the brightness exceeds a certain level it is called a bolide, respectively a fireball. In such cases, depending on the velocity, the mass, density and composition of the falling object, a remnant may hit the ground. If you find such an object, it is called a meteorite.

Meteorites are classified in several sub-categories, depending on the origin and chemical composition. However, generally two meteorite types are quite common: