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In addition to the planets and moons in the solar system,
there are a set of small bodies that are nonetheless very interesting.
These small rocky bodies include comets, which exist in
a nearly spherical cloud extending to tens of thousands of astronomical units, and
asteroids between the orbits of Mars and Jupiter.
And a set of rocky bodies beyond the orbit of Neptune.
The small bodies have played a vital role in the history of the solar system, and
they're not chemically uninteresting.
The Murchison meteorite, which fell in Australia several decades ago,
was recovered within hours of its landfall.
Allowing the material to be collected without terrestrial contamination.
Over 100 amino acids were found in the Murchison meteorite
including dozens not used in the form of life on Earth.
Now, amino acids are not purely biological material.
They are building blocks of life but they're the essential building blocks and
if these can exist in small, cold rocks in deep space.
Then life could clearly have been seeded at this level in many cosmic locations.
We even directly sampled material from the outer solar system.
The Stardust Mission sampled the tail of Comet Wild 2 and
found amino acids and tiny interplanetary particles in the tail of that comet.
Comets are important in the story of the solar system because they are presumed to
contain an inventory of icy material that could well have
delivered the oceans to the earth.
Unfortunately, the story is more complex than we thought because having
sampled two comets, the deuterium ratio in
their water does not match the ratio found in the earth's oceans.
So earth's oceans must have come from multiple sources including comets.
When NASA engineers plan these missions they're clearly having a lot of fun.
Several times, now we've sent space probes that deliberately smashed themselves or
part of themselves into a planetary or moon's surface or small rocky body.
And then sample the material that splashes off into the weak gravity.
This is the best way of learning what these objects are made of.
We've also soft-landed on several small bodies in the outer solar system.
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Sometimes, the material out there can get uncomfortably close.
It's a grinder with large rocks smashing together and turning into small rocks.
As a result, interplanetary space has many more pieces of
small debris than large debris, which is a good thing for us.
These pieces of debris follow up Power-Law Distribution.
So if there are a certain number of pieces ten kilometers across,
there are ten times more pieces that are one kilometer across, and so on.
Then these at large impacts, or impacts with large objects are exceptionally rare.
Whereas impacts with smaller pieces of debris, and the Earth, are quite common.
The Earth's atmosphere shields us from most impactors that
are smaller than about a meter across.
They simply burn up in the upper atmosphere.
We see them as shooting stars.
In fact, most shooting stars represent objects traveling at tens of thousands of
miles per hour, that are literally pea sized or as small as a grain of sand.
They'll carrying an enormous wallop because of their kinetic energy.
The very large intruders have altered the course of life on Earth.
It's almost certain that Earth has suffered three to five major impacts that
have caused mass extinctions in the history of biology on this planet.
Most notably 65 Million years ago when the dinosaurs and
many other species were destroyed almost instantly.
The fact that debris from space reaches the earth was made abundantly clear in
mid-February when the skies above Chelyabinsk lit up by
a fireball travelling across the sky.
Blowing out windows and injuring hundreds of people with broken glass.
Fragments of the meteor have now been recovered.
And reconstructed it as being a 50 to 60 feet across object travelling at
tens of thousands of miles an hour.
This object packed a punch of 500,000 kilotons of TNT,
equivalent to a small nuclear device.
This is the largest and scariest event, since a similar event a 100 years,
also in Russia, also in Siberia, at Tunguska.
That larger object flattened 1,000 square kilometers of forest.
Luckily, no one was killed.
Almost an uninhabitant region was affected.
As we reach the edge of the solar system,
we can consider the long term history of the space program.
That has delivered four objects beyond the orbit of the outermost planets.
Voyager 1 is the most distant of these sentinels, and is now officially left
the solar system as a distance of over 11 billion miles from the earth.
These are our outermost sentinels into space,
our messages in a bottle if you like, literally and metaphorically.
The Voyager probe carries a record on it, which encapsulates music and digital
images in analog form and the Pioneer spacecraft has a plaque on its leg.
Showing where the Earth exists in the solar system and
showing silhouettes of human figures.
Even though these objects are headed towards other stars,
it'll be long before they reach them.
In fact, none of the Voyagers or Pioneers is destined to head to a nearby star.
But if they did, it would take 50 to 100,000 years to reach there.
A sobering insight into how far it is to the stars and
how difficult it will be for us to ever reach them.
So as we leave the solar system, let's celebrate the enormous successes of
the space program in learning about these nearby objects.
With the journey to Mars that delivered one of the Mars rovers.
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The outer solar system contains many kinds of small, rocky debris.
And comets, which are mixtures of ice and rock.
These objects are not uninteresting.
They've transported water to the interior of the solar system, and
they contain the ingredients for biology in the form of amino acids.
Also, these rocks can be intruders to the terrestrial environment.
In the history of life on Earth, life has been disrupted dramatically by
mass extinction several times, about once every 100 million years.
And just in the last few months,
we've seen what can happen in the skies over Siberia.
Chris ImpeyDistinguished Professor
