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Skywatching

Asteroid dust helps us on earth understand the Solar System

Visiting an iron asteroid

One of the most exciting astronomical photographs (not the picture above) in a while shows a handful of grey gravel in a metal container.

What makes that gravel so interesting is it comes from an asteroid. It is a sample of left over primordial material from the birth of the Solar System. Now, another spacecraft is off on a voyage to another asteroid, 16 Psyche, which is made mainly of iron and related metals.

The birth of the Solar System was a very dramatic process. Dust from the cloud forming the Sun and planets coagulated into lumps. Sometimes the lumps grew by sticking onto other lumps. However, there were also occasions when lumps collided at high speed, smashing themselves back into gravel and dust.

On occasion, the colliding bodies were large. For example, we believe the lump that was in the process of forming the Earth was hit by a lump the size of Mars. The resulting debris collapsed to form the Earth and a new body, the Moon.

The energy released by the impacts of incoming material onto a growing body releases a lot of heat. The result is a ball of molten rock, which cools very slowly. Even after 4.5 billion years, the centre of the Earth is still very hot and partially molten. Part of the heat dates back to the Earth's formation, and the rest from the gradual decay of heavy, radioactive elements.

The dust forming the planets was a mixture of various rock minerals, ice and a significant percentage of iron, nickel and related metals, together with traces of heavy elements, including some that are radioactive. The presence of these elements is a consequence of energy production in stars. These obtain energy by fusing lighter elements into heavier ones. However, this chain of events ends with iron and its relative elements. These are very stable and fusing them into anything else requires energy.

The result is production of energy in the star ceases; it collapses and explodes, sending out all that iron to mix with the cosmic dust clouds. The heavier elements, including gold, silver, lead and the radioactive ones, were formed in the explosion. The result is that any planet forming after the first few generations of stars have come and gone will receive a ration of iron and related metals.

The ingredients of the molten mixture making a new planet do not mix well. Silicon and aluminium minerals are lightest and rise to the surface, floating on top of the heavier iron and magnesium minerals. The molten metallic iron, nickel, related metals and the heavy elements sink down to the middle of the planet.

If a planet gets this far in its growth and is then destroyed in a collision with another planet, it is possible that the metallic core could survive the impact. This, stripped of the overlying rock layers, would be left as an asteroid, consisting of mainly nickel and iron, together with small amounts of other heavy elements.

There are at least two reasons we would be interested in having a closer look at a nickel/iron asteroid.

Firstly, it is an accessible sample of the stuff we have in the inaccessible centre of the Earth, providing detailed information on the birth of our world and the other rocky planets. Secondly, when we are establishing bases on the Moon and other worlds, it would be far easier if key resources were locally available.

Water is widely available out there. However, we will need enormous amounts of construction material, including steel, which is heavy. When we compare the size of the rocket that lifts off the launch pad with the size of the spacecraft that actually gets into space, we can appreciate that having to ship water and construction materials into space from the surface of the Earth will be hard and expensive.

Things will be far easier if we can find what we need already in space, or where we are building our base.

•••

• Saturn lies in the southeast after sunset, with Jupiter shining in the east.

• Venus rises four hours before the Sun.

• The Moon will be full on Oct. 28.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.



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About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]



The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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