Those waiting for the asteroid 2012 DA14 to harmlessly pass by Earth Friday were shocked by another event in the sky as a 7,000-ton meteor exploded in the air over Chelyabinsk in Russia's Ural Mountains. The airburst explosion was equivalent to about 300 kilotons of TNT, though it may have been as large as 500 kilotons. The news of injuries and property damage from the shockwave overshadowed the close fly-by of the 150-foot diameter asteroid, which passed within just 17,150 miles of the Earth later that day without harming anything.

Experts chalked the two events up to cosmic coincidence. The Russia meteor, measuring about one-third the size of DA14, was not related to the asteroid. The meteor traveled in a different direction and speed than DA14 and it originated in the asteroid belt between Mars and Jupiter. DA14 is a "near Earth object" so its orbit around the sun is similar to the Earth's.

Something like DA14 passes near the Earth every 40 years on average and an impact happens roughly once every 1,200 years. DA14 has no chance of hitting Earth for at least the next century. Something like DA14 hit the plains of northern Arizona about 50,000 years ago to make the kilometer-wide crater now called Meteor Crater. However, the Meteor Crater asteroid was made of iron, so it packed even more of a punch than would a typical asteroid like DA14 that is made primarily of stone.

The largest known meteor in recent times caused the "Tunguska event" -- flattening thousands of square miles of forest in remote Siberia in 1908. An impact of something as small as DA14 would be equivalent to 2.4 megatons of TNT -- enough to wipe out a city the size of Bakersfield but not have a global effect. Something a kilometer or more in size hitting the Earth would have a global effect, threatening the survival of our civilization.

The Tunguska event has inspired all sorts of "creative" explanations about what could have caused the airburst from the boring (but very likely) asteroid impact or comet impact to fantastical stories like mini-black holes or wayward spaceships. I am teaching an astrobiology class this semester (as I have done in previous semesters) and one of the topics that comes up is UFOs -- space aliens in spaceships.

Of course, Hollywood and science fiction TV just love that stuff, but even supposedly factual-based networks like Discovery and the History Channel have run stories on present-day space aliens or "ancient astronauts."

When I found out the energy requirements it would take to travel among the stars, I became even more skeptical of the claims by space alien enthusiasts and the near-parity of humans vs. aliens battles depicted in popular shows. To travel at even a tenth of the speed of light from the nearest star system -- Alpha Centauri -- to Earth would require about 40 percent of the entire world's annual energy usage for a crew of 10. (The world's annual energy usage is approximately equivalent to 10,000 megatons of TNT.) Faster speeds or larger crew numbers would require even greater energy (the energy required increases as the square of the speed).

Space aliens that can harness that much energy in a craft the size of a large house are not going to be too worried about our weaponry. If the space aliens are warping space to travel through hyperspace, they would be working with energies on the order of what our sun will generate in its entire 10-billion-year lifetime. Now, in just one second, our sun generates a million times the entire annual world's energy production, and there are about 30 million seconds in a year. Multiplying these already huge numbers by 10 billion (for the sun's lifetime), you come up with energies about a trillion times more than what it would take to pulverize the entire Earth to powder.

Even if I'm off in my calculations by a factor of 10 (heck, even 100 -- maybe they have a Prius warp drive), I doubt that warp-travel space aliens would be using laser blasters to hurt us. They could just wipe out the Earth, blip! Well, how depressing -- and that wouldn't make a good ending to a movie. Sigh!

This column probably dooms my dream of being a science consultant for the Syfy channel. However, I, like most astronomers, don't watch science fiction movies or TV shows for the science but for the stories, so I promise I won't critique a movie's science (or lack thereof) while I watch it with you.

Mars and Curiosity

In tonight's sky, Mercury reaches its greatest distance from the sun on our sky ("greatest elongation" in astronomy talk). Look for it as a bright star low in the west shortly after sunset (see the attached star chart). Over the next week, Mercury will sink back toward the sun and will be lost in the sunlight glare by the end of the month. Half a fist-width at arm's length below Mercury will be the dimmer orange-red Mars. The two planets might just barely fit within the same field as your binoculars.

The Curiosity rover on Mars has made its first drillings into a rock, and we should have preliminary results back from the two main science instruments SAM and CheMin from the analysis of the rock powder. The results may be simply that the aspirin-mass sample of powder was deposited successfully into the two instruments because the Curiosity team are being very careful in testing out every step of the process to make sure everything on the rover is working as expected.

Some of the powder was used to clean the drill hardware of trace organic compounds that may have been deposited while the rover was still on Earth despite a very thorough cleaning before launch. Because the SAM instrument can detect very, very low quantities of organic materials, the cleanliness requirement for the hardware was 40 parts per billion. This is far, far better than the cleanest hospital surgery room in the world.

The rock powder is transferred to a collection area called CHIMRA where it is sifted so that only particles smaller than 150 microns across are transferred to the SAM and CheMin instruments. They don't want larger particles getting stuck in the instruments because there's no one there to clean out the instruments if they get clogged up.

All of this processing of the sample can take several days before it even gets to the SAM and CheMin instruments. As I said the Curiosity team is being very careful and meticulous in making sure everything is working correctly.

In the night sky

As shown in the attached star chart, a very fat waxing, crescent moon will be midway up in the southwestern sky at the western edge of Taurus. Sunday night it will be at first quarter phase and half a fist-width below-right of very bright Jupiter. Jupiter continues to crawl eastward getting closer to Taurus' head. By the end of next month, it will reached the area between the two horns. Orion is due south at about 7:45 p.m.

The pre-dawn sky belongs to Saturn now that Venus has disappeared into the sunlight glare, and Jupiter has set by 1 a.m. Saturn looks like a fairly bright star midway between the stars of Virgo and Libra in the east before sunrise.