Unaware of our dangerous neighbourhood
For millennia people have lived in ignorance of the space around our planet. Comets were thought to be bad omens and portenders of catastrophe. Yet, asteroids and meteoroids whizzed by us unnoticed. What we do notice are the frequent meteor showers caused by small particles burning up in the atmosphere and occasional fireballs and small explosions caused by larger rocks.
Very occasionally one of these larger rocks would get too close and explode low in the atmosphere or impact the planet leaving devastation and craters. And on very rare occasions there are devastating explosions. The best known of these was about 65 million years ago and is credited with killing off the dinosaurs.
It has only been very recently that we have recognized impact sites for what they are. The earth, weather, and time are very good at obliterating the evidence. Even the origin of the famous Barringer Crater in Arizona was controversial up until 1960! And the Barringer crater looks like it was moved from the moon to the dessert.
Tunguska didn't leave a crater and wasn't investigated until years later. It certainly isn't as obvious as Barringer or other sites. While the meteor explosion is the leading theory, there is an alternate one based on the idea of a explosive volcanic gases (a micro-Verneshot). There are also a large number of bizarre and wild ideas as to its' origin.
The legacy of Tunguska and other impacts has been to raise awareness amongst scientists, policy makers, and the public that impacts from meteors, comets, and asteroids do happen and that action can and should be taken to protect people and the environment.
What can we Do?
If we are unlucky enough to have a really large rock bearing down on us and about to strike then there is not much we can do. But these are exceptionally rare events and we should have lots of time to act. It's is certainly in our best interest to be prepared to do something to prevent this kind of event if we can.
1. Don't Panic!
While the thought of multi-megaton impacts sounds pretty scary, its important not to over react. How does this all stack up?
- Small impacts and near misses happen all the time. 
- Big impacts, like Tunguska, are uncommon. When they do happen, they are likely to be in remote areas or over oceans.
- Really massive impacts, like the one the killed off the dinosaurs, are incredibly rare. So rare, it's almost impossible to get a real feeling for the time-scale even though we can measure it.
2. Size the Risk
Consider, if Tunguska sized events happen once every hundred years, then there will have been more that 1/2 million of them since the dinosaurs died out. If they happened every 1000 years, that's still over 50,000 strikes. There is no historical evidence indicating similar strikes in written history which covers the last 6000 years or so.
How do we do this? By studying both the sky and the Earth.
- Geologists have been getting very good at finding impact sites on Earth. There is a online impact database, here (a Canadian contribution). This can tell us a lot about what reached the ground.
- There are all sky camera networks to detect fireballs from meteors, here and here.
- Recent work has detected Explosions on the Moon which can tell us a bit about how dangerous our neighbourhood is by watching the impact on a body unprotected by an atmosphere.
- The US Congress has mandated and funded an effort to detect, catalogue, and track any really catastrophically dangerous asteroids. This effort is also detecting a lot of smaller rocks including Tunguska sized meteoroids, as well as comets.
- Canada is launching a micro-satellite called Neossat to detect and help track Near Earth Objects. See Don't Blame Canada.
3. Prepare for Action
Advance warning will let us focus on the dangerous rocks and give us the years needed to prepare. It will also give us techniques that will be more effective and far less dramatic than throwing nuclear weapons around.
Research is already underway around the world on things that can be done. And a broad set of tools is being proposed to deal with the risk. They range from
- radio tagging an asteroid to better understand it's orbit
- painting one so that years of Sunlight alters its orbit for us.
- nudging one using a space tug
- 99942 Apophis is about 270 m and will come very close in 2029 with a possible impact in 2036. As measurements have improved the likelihood of an impact has been reduced and is now considered unlikely
- 1950DA is about 1.4 km and is a risk to impact in the year 2880
- 2007 VK184 is a 130 m rock of most concern right now with a chance of impact in 2048
- The Planetary Society's Apophis competition, here.
- B612 who what to alter an asteroids orbit by 2015, here.*
- US Bill supporting a mission to radio tag Apophis, here.*
- A Russian project to tag Apophis, here.*
Some other Tunguska anniversary posts
- Tunguska, One Century Later - Astroprof
- 100th Anniversary of a Cosmic Warning - Centauri Dreams
- 100 years ago today: KABLAM!!!!! - The Bad Astronomer
Oh, and back at Tunguska
An Italian research team may have found evidence of an impact body that survived the air burst, here.
 The explosion occurred at 7:14am local time on June 17th, 1908 in the Julian Calendar in use there at the time. This was June 30th on western calendars. Adjusting for time zones the blast was at 7:14PM EST June 29th. For a versatile date converter capable of converting between many calendars see here. My original notes for this were based on an old Wikipedia entry with an incorrect date and was going to publish this on June 30th on the Julian calendar - so I'm now late beaten to the punch :(
 The location, given as 60°55' N, 101°57'E, is pretty remote as you can see here.
 I've written about asteroid near misses before, here.
 I've also written about fear mongers before, here.
 There is some speculation that the Biblical tale of Lot's wife may relate to such a strike. But it seems designed to sell a book. See here, and here. A very strong critique (with some mild bad language) here. See  above.