Watch out for that space boulder!

Thomas D. Jones, PhD is a veteran NASA astronaut, scientist, speaker, author, and consultant. He holds a doctorate in planetary sciences, and in more than eleven years with NASA, flew on four space shuttle missions to Earth orbit. In 2001, Dr. Jones led three spacewalks to install the centerpiece of the International Space Station, the American Destiny Laboratory. He has been privileged to spend fifty-three days working and living in space.

Many of you may remember when Dr. Jones spoke here in May 2008 on spacewalking. He’ll be back on Tuesday, Nov. 17 with an all new lecture on near-Earth objects, potential impacts, the search for alien life, and the formation of planets.

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500-m-wide NEO Itokawa, imaged by the
Japanese Hayabusa probe in 2005 (JAXA)

On November 6, we had a close encounter with a near-Earth object, 2009 VA (a NEO is a near-Earth object, including both asteroids and dormant comets). The space boulder, a 7-meter-diameter asteroid, streaked by at a distance of only 14,000 km, well inside the orbits of our geosynchronous satellites. NASA’s Jet Propulsion Lab estimates that we have two such encounters each year, on average, with objects of this size. About every five years, Earth is struck by such a body, but objects this small burn up in the atmosphere, resulting in a fireball and the release of several kilotons of energy (TNT equivalent).

The close pass of 2009 VA surprised some news outlets, which speculated on why the small asteroid had not been detected sooner by astronomers (The University of Arizona’s Catalina Sky Survey picked up 2009 VA about 15 hours before the closest approach). The answer is that these small cosmic rocks are so numerous, and so difficult to observe, that we only discover them at random. NASA runs a search program, Spaceguard, to detect larger objects, 1 km and up, that may pose a civilization-ending threat to Earth. So far about 85% of those objects have been found; none pose an immediate threat to Earth, but may in future decades.

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Jet Propulsion Lab depiction
of recent close pass by 2009 VA

Impacts of small objects like 2009 VA create only sky-high fireworks, no harm to us here on the ground. But the Tunguska impact in Siberia a century ago devastated 2,000 square km of Siberian forest. That airburst of about 5 megatons (Mt) of TNT equivalent was caused by an object 30-40 m in diameter; large enough to level a city center. Such an object strikes us every few hundred years. The last one was a century ago; the next one to come along may hit us tomorrow. With current telescopes, we have only a small chance of seeing such an object before it strikes Earth.

Congress has asked NASA to look into what it would cost to search systematically for NEOs down to 140 m in diameter; if we found most of those objects, we would have greater confidence that no “city-buster” NEO is headed for an imminent collision with a populated area on Earth. A report to NASA on the prospects of detecting and even deflecting such potentially hazardous NEOs is due out by year’s end from the National Research Council.

Impact, or cosmic bombardment, is a process that has been altering the faces of the planets since the dawn of the solar system 4.6 billion years ago. Impacts by giant comets and asteroids have changed the course of life on Earth, possibly ending the reign of dinosaurs 65 million years ago, and possibly causing other mass extinctions through Earth’s long history. We now have the technology to both detect damaging NEOs heading for Earth, and with proper warning, to nudge them out of the way. What we lack is the international will to take action should a hazardous NEO be found on a collision course with Earth. The Association of Space Explorers is working with the United Nations to draft such a NEO decision-making agreement.

At the Houston Museum of Natural Science on Tuesday, Nov. 17, I will be speaking about impact and the other processes that shape the worlds of the solar system, in a talk called Planetology. My talk will discuss these processes — tectonics, volcanism, erosion, for example — and our search for life and “other Earths” across the galaxy. Please join me for the lecture that evening at 6:30 p.m., or turn the pages of Planetology, written by me and noted planetary geologist Ellen Stofan. After the talk, I’ll be answering questions and signing copies of the book.

See reviews and more info on Planetology at:

Meteorites and Meteor-wrongs

My role as Planetarium Astronomer includes answering astronomy questions from the public over the phone, by email, and in person.  Thus, it is up to me to examine meteorite samples brought in by the public.  Or, I should say, “meteor-wrongs,” as none of the samples brought in since 1996 (when I began doing this) have actually been meteorites. 

First, let’s define some terms.  A rock which is about to enter the Earth’s atmosphere is a meteoroid.  Someone who happens to see it as it is falling, and thus sees a streak of light in the sky, sees a meteor.  Once the rock has landed, it is a meteorite.  Most meteors that we see burn up completely in the atmosphere and therefore never land as meteorites.  A meteorite, then, is a rock which originated in outer space.

which is a meteorite?

Can you tell which of these is a meteorite?

If you have a sample you believe came from outer space, here are 4 simple tests you can do at home.  Note that passing these four tests will not guarantee that your sample is a meteorite; they serve primarily to eliminate ‘meteor-wrongs.’

1) Is the sample heavy for its size?  Meteorites are denser than Earth rocks; they have more mass per volume.  A meteorite will be heavier than an Earth rock of the same size.

2) Does the sample attract a magnet?  Most meteorites found and brought in are iron meteorites.  Even the stony meteorites, which are more common but rarely reported because they superficially resemble Earth rocks, have some iron in them.  A meteorite sample, then, should attract a magnet.  Any magnet, including the ones on your fridge, will suffice for this test.

3) Is there a dark fusion crust? Upon entry into our atmosphere, a meteorite acquires a thin ‘fusion crust’ because its surface melts under the heat of entry.   This crust is black when the meteorite is freshly fallen but may turn brownish due to weathering and rust.  Bright colored or silvery samples are not meteorites.

4) Does the sample have bubble holes?  Many volcanic rocks on Earth have these holes, which form when a bubble of gas or steam expands as the rock solidifies.  A meteorite, however, is never fully molten (only the surface melts on entry into the atmosphere).  Thus, a meteorite sample is a solid hunk, without tiny holes or perforations. 


The many large holes in this rock
are a big clue that it is not a meteorite.

So, which of the four is a meteorite? If you go back to the first photo in this post, you should be able to see holes in the top two samples – so those are out. And the bottom right is bright and silvery = not a meteorite. So, the winner is the smallest of all four, in the bottom left.  

For more information, surf to: , (the site is in English, too), or