Re: What would happen if a large (.5km diameter) cesium asteroid hit Earth?

David,
Cesium is a very reactive metal with atmospheric oxygen, water, and ice. I assume you are interested in how fast a 0.5 km cesium asteroid might react upon passing through the atmosphere at 40,000 km/hr and if its combination of 28 deg. C melting point and extreme reactivity might consume it before it reached the surface of the Earth.

Let's examine the initial conditions a little more first. A 0.5 km solid sphere of cesium would have a volume of 0.065 km^3 or about 65,000,000 cubic meters (65 x 10^6 m^3). Cesium has a specific gravity of 1.873 at 20 deg. C. The asteroid would have a mass of approximately 32,499,000 U.S. tons.

Assuming the entry velocity of 40,000 km/hr, or 11 km/s (the escape velocity from the Earth), if the asteroid approached the planet head-on, it would have to pass through a minimum thickness of atmosphere. It would take approximately 7.5 seconds to cross the 100 km of denser, lower air, and reach the surface of the Earth. The low melting point of cesium would cause the outside of the asteroid to begin melting, consequently carrying away the latent heat of fusion, acting somewhat like an ablation layer on a manned spacecraft upon reentry to the atmosphere. This would tend to somewhat slow the temperature rise of the large asteroid body, almost surely allowing the majority of the asteroid to reach the surface of the Earth. This melting layer would be the first to react with atmospheric oxygen and produce cesium oxides through violent "burning." Atmospheric water vapor would also react with this liquid/vaporized cesium metal. When the remaining ball of cesium hit the Earth, with a 75% chance of hitting ocean water, it would begin vigorously reacting, producing cesium hydroxide (CsOH), hydrogen gas and a great deal of energy (equivalent to a very large thermonuclear explosion). Cesium metal fragments of all sizes would continue to react with the water until they were all consumed and converted to the hydroxide. The liberated hydrogen gas would ignite and there would be a flame so long as the oxidation reaction continued with the water. This would be a very large flame, because 32,499,000 tons of cesium is 2.2 X 10^11 moles. The balanced reaction is

            2Cs + 2H2O ==> 2CsOH + H2

This means that almost 1.1 x 10^11 moles or approximately 242,290 tons of hydrogen would be liberated.

The exact details of this scenario would have to be worked out by a qualified Astronomer, but I believe the overall sequence of events to be close enough to give you a qualitative answer to your question.

For more information about cesium, see Encyclopedia Brittanica, vol. 3, "Cesium" (1990), The Book of Popular Science, vol. 10, "Cesium" (1967), or the webelements.com entry on cesium.

I hope this helps.

[Moderator: The key point of this answer is that most of the asteroid would survive intact to the Earth's surface. Indeed, even relatively small meteorites (say, something that is about the size of an adult's hand) have cold interiors because of their rapid passage through the atmosphere. Finally, it should be noted that a 0.5 km asteroid of pure cesium would not occur in nature.]