Date: February 9, 2010

Title: Things That Go Bump In the Night


Podcaster: Patrick McQuillan

Organization: Incorporated Research Institutions for Seismology (IRIS) –

Music Credit: “What is a Shooting Star?” by They Might Be Giants from the CD Here Comes Science.

Description: The Universe is mostly empty. That’s why it is called space and not stuff. However, collisions do occur. Let’s take a look at some of the more notable collisions that have occurred in the history of the Universe.

Bio: Patrick McQuillan earned a B.S. Physics and an M.S. in Museum Education from the College of William and Mary. His senior research project involved determining the period of variable stars, most notably Alpha Auriga. In the twenty plus years since then, he has explained astronomy to the general public as a Planetarium Director, the Education Manager for Challenger Center for Space Science Education, a NASA Solar System Ambassador, and currently explains Earth Science as Education and Outreach Specialist for IRIS. You can view current earthquake activity using the Seismic Monitor located on the IRIS website.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored anonymously because studying the stars promotes science and reason.


Patrick: Welcome to the February 9 edition of the 365 Days of Astronomy Podcasts. Hello, I’m Patrick McQuillan, the Education and Outreach Specialist with IRIS, the Incorporated Research Institutions for Seismology, a NASA Solar System Ambassador and a former Planetarium Director.

The Universe is huge. It’s the largest thing we know of and is perhaps infinite. But even given all its size, the Universe is mostly empty. That’s why it’s called space and not stuff.

On average the Universe has only one hydrogen atom per cubic meter. Pretty empty. So you would think collisions would be very rare. If that were the case you wouldn’t be listening to this podcast. Lots of matter had to collide to form the Earth so that we would have a chance to exist. Luckily one of the fundamental forces of the Universe, Gravity, helps bring matter together.

One of the most recent collisions that we know of occurred on January 18. A small chondridic meteorite ended its journey around the solar system when it crashed though the roof a doctor’s office in Lorton, Virginia (just outside Washington D.C.). The doctors gave the half-pound meteorite to the Smithsonian Museum of Natural History. They thought it would be great if it could be added to the Smithsonian’s meteorite collection and go on display for everyone to enjoy. Unfortunately things are never that simple.

The Doctor’s do not own the building or the land where their office is located. They rent. The landowner is claiming, that since the meteorite fell on his property, he legally owns it and is demanding it be returned. Typically meteorites belong to the landowner. And since meteorites can fetch a high selling price, they are interested in getting their piece of the universe.

The Lorton meteorite is a fairly common type of chondrite. Chondrites make up about 85% of the 27,000 or so known meteorites. So it isn’t intrinsically very valuable. But the Lorton meteorite may well be the best-documented meteorite fall. The fireball created on its entry into the atmosphere was seen by lots of people and its final resting spot is well documented. So it may be worth the eventual lawyers fees that will resolve the ownership dispute. For now the Smithsonian is holding on to the meteor for safekeeping.

It is not rare for a meteorite to impact the Earth. Estimates for the mass of material that falls on Earth each year range from 37,000-78,000 tons. Most of this mass would come from dust-sized particles. A study done in 1996 (looking at the number of meteorites found in deserts over time) calculated that for objects in the 10-gram to 1-kilogram size range, 2900-7300 kilograms per year hit the Earth. Over the whole surface area of the Earth, that translates to 18,000 to 84,000 meteors larger than 10 grams per year. The meteorite that may have contributed to the demise of the dinosaurs 65 million years ago was over 10 km in diameter and might have weighed over 90 million kilograms.

Ironically, it may have been a collision in the asteroid belt that put the Lorton meteorite on a course that led to a doctor visit. Most chondrites formed from the material of the early solar system in the region between Mars and Jupiter known as the asteroid belt. Over 200 asteroids are known to be larger than 100 km, while a survey in the infrared wavelengths shows that the main belt of asteroids has 700,000 to 1.7 million asteroids with a diameter of 1 km or more. The total mass of the asteroid belt is about 4% the mass of the moon.

This doesn’t mean that if were in the asteroid belt you would have to constantly dodge rock chunks ala the Millennium Falcon in Star Wars the Empire Strikes Back. The volume of space that the asteroid belt occupies is large enough that even with 1 and a half million asteroids you would be lucky to see even one. The asteroid belt is mostly empty space. Many NASA spacecraft have traveled safely through the asteroid belt enroute to their final destination. Voyager 1 and 2, Galileo, Cassini, and New Horizons have all made it safely.

But collisions due occur. Collisions between main belt bodies with an average radius of 10 km are expected to occur about once every 10 million years. A collision may fragment an asteroid into numerous smaller pieces. After more than 4 billion years there have been tons of collisions.

Hubble Space Telescope images taken on January 25 and 29 may show the result of a collision between two asteroids. The image was captured with the new Wide Field Camera 3, which was installed during the May 2009 space shuttle servicing trip. The camera can spot house-sized fragments at the distance of the asteroid belt. The Hubble images show a solid nucleus that lies outside its own halo of dust. This pattern has never been seen before in a comet-like object. Scientists think this nucleus is the surviving remnant of the collision, and the tail is the rubble left over from the crash.

Collisions between asteroids should be common, but we have never seen direct evidence of it. We have however seen comets colliding with planets.

Way back in July 1994 the comet Shoemaker-Levy 9 collided with the planet Jupiter. On an earlier pass by the planetary giant in 1992, the comet fragmented into dozens of pieces. Each of these pieces hit Jupiter over period of six days. The larger comet fragments caused Earth sized dark clouds to form in the planet’s atmosphere. These dark spots lasted for a few months before dissipating.

A comet impacting a planet was thought to be a very rare occurrence in the solar system today. Even so, last July an amateur astronomer took photos of Jupiter that showed a new dark spot very similar to the spots formed during the Shoemaker-Levy event. So the rate of comet-planet collision may be much higher than thought. Of course Jupiter is a very big planet. Things are bound to hit it.

Conversely, if two asteroids collide at slow relative velocities, they can collide and stick. This is the basic mechanism that formed the objects of the solar system. We believe that the solar system formed from a cloud of gas and dust that contracted under the influence of gravity. The Sun, the planets, the moons of the planets, the asteroids, and the comets were all formed from collisions between small pieces of matter that stuck together to form larger objects.

Well most of them. We believe that the Earth’s moon was formed early in Earth’s history when an object the size of the planet Mars struck a glancing blow that destroyed the incoming object and knocked a large bit of the outer surface of the Earth. Most of this material eventually fell to become part of the Earth, but a good portion of the material remained in orbit around the Earth. This material collided with itself until it coalesced into the object we know today as the Moon.

We were led to this theory of the Moon’s formation by studying rock samples returned from the Moon’s surface by the Apollo astronauts in the early 1970s. The rocks were amazingly similar to rocks on the surface of the Earth. The percentages of elements in the moon rocks matched the percentages of elements in the Earth rocks. The only way they could be so similar was if they formed form the same original material in the same part of the early solar system. A great example of human exploration of the solar system expanding our understanding of the universe.

Prior to the Apollo missions there were several competing theories of Lunar formation. One theory involved the Moon forming in different part of the solar system and then getting captured into orbit by Earth’ gravitational field. If this were the case, Moon rocks would have different elemental proportions than Earth rocks, which is not the case.

A second competing theory was that the Moon formed out of the same material that was forming the Early Earth. The Earth was spinning so quickly while forming that a bit broke off to become the Moon. Both objects then cooled off and solidified. If this had occurred, Moon rocks would have a higher percentage of heavy elements, like iron, in them. They do not. The collision that formed the Moon occurred after the heavy elements had sunk to the center of the Earth. This is known as differentiation. When the Moon formed, the incoming object only dislodged material from the upper part of the Earth which has very low percentages of heavy elements.

So with one small step, two lunar formation theories were rendered invalid.

The next time you are outside enjoying the night sky and see a meteor flash across the sky, remember that you are observing the process that formed the planets in action. Of course the same process also caused the extinction of the dinosaurs. So if that meteor is really big and really bright, you might want to run. But you really can’t hide.

End of podcast:

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