Look at this story, which has had a rather low presence on news sites over the last few days. An asteroid passed very close to the Earth - within 39,000 miles (compare to almost 250,000 miles from here to the Moon), which is tiny when you consider the scales we're talking about. One news story I saw said that an impact would cause an explosion comparable in size to the Tunguska event.
But not to worry, says the news article:
"If discovered in advance and with enough lead time, there is the possibility of pushing it off course, if you have decades of advance warning," McNaught said. "If you have only a few days, you can evacuate the area of impact, but there's not a great deal [else] you can do."
Well, guess what - this thing was discovered only a few days ago.
This is why you didn't hear about it until after it had passed. Look at the name - 2009 DD45. The '2009' stands for the year it was discovered, and a look at Wikipedia confirms this. It was discovered on February 27, 2009, only a few days before its closest approach to the Earth. Which means that had this thing been on a crash course with us, we would have been totally unprepared. And this was not mentioned in this news story, or any other I have read concerning this asteroid.
This potential huge natural disaster was discovered only a few days in advance, not decades as one might think. An asteroid of this size (25 - 50 yards across) is no little thing: read the descriptions on Wikipedia from the Tunguska event (caused by a similar size object):
"At breakfast time I was sitting by the house at Vanavara trading post (65 kilometres/40 miles south of the explosion), facing North. [...] I suddenly saw that directly to the North, over Onkoul's Tunguska road, the sky split in two and fire appeared high and wide over the forest (as Semenov showed, about 50 degrees up - expedition note). The split in the sky grew larger, and the entire Northern side was covered with fire. At that moment I became so hot that I couldn't bear it, as if my shirt was on fire; from the northern side, where the fire was, came strong heat. I wanted to tear off my shirt and throw it down, but then the sky shut closed, and a strong thump sounded, and I was thrown a few yards. I lost my senses for a moment, but then my wife ran out and led me to the house. After that such noise came, as if rocks were falling or cannons were firing, the earth shook, and when I was on the ground, I pressed my head down, fearing rocks would smash it. When the sky opened up, hot wind raced between the houses, like from cannons, which left traces in the ground like pathways, and it damaged some crops. Later we saw that many windows were shattered, and in the barn a part of the iron lock snapped."
That was 40 miles away. What if this was a populated area? What if it hits in the water, does it cause a tsunami? More descriptions:
The explosion registered on seismic stations across Eurasia. Although the Richter scale was not developed until 1935, in some places the shock wave would have been equivalent to an earthquake of 5.0 on the Richter scale.[9] It also produced fluctuations in atmospheric pressure strong enough to be detected in Great Britain. Over the next few weeks, night skies[where?] were aglow such that one could read in their light, caused by dust suspended in the stratosphere by the explosion. In the United States, the Smithsonian Astrophysical Observatory and the Mount Wilson Observatory observed a decrease in atmospheric transparency that lasted for several months, also from the suspended dust.
Also, for some background, read up on the Torino Scale which rates asteroids based on their size, and probability for impact with Earth. I don't think info on the latter is publicly available, but we know the size. Read the description for a level 3 event, which is something of this size with a 1 to 99 percent chance of hitting us:
A close encounter, meriting attention by astronomers. Current calculations give a 1% or greater chance of collision capable of localized destruction. Most likely, new telescopic observations will lead to re-assignment to Level 0. Attention by public and by public officials is merited if the encounter is less than a decade away.
Less than a decade away? What about 72 hours? Is it possible that they discovered this thing, and it had between a 1 and 99 percent chance of hitting us, but they didn't announce it for fear of causing mass hysteria?
Finally, read up on Shoemaker-Levy 9, a comet which hit Jupiter in 1993. It had fragments up to 5 km across, it actually hit a planet, in July after being discovered in March. That is, there was 4 months between discovery and impact. An asteroid of this size, according to the Torino scale, is "capable of causing global climatic catastrophe that may threaten the future of civilization as we know it, whether impacting land or ocean," and warns that "for such a threat in this century, international contingency planning is warranted, especially to determine urgently and conclusively whether a collision will occur."
I wonder if we will have a century worth of warning, or only 4 months. Or only 3 days even.
Now, I am not an astronomer, and I don't want to say that we should start spending gobs of money on asteroid monitoring. But, in this case, it is probable that the threat was played down and not made public until the asteroid had already passed by the Earth. Hopefully the extent of the threat, and our relative unpreparedness, would be made public after the threat had passed. However, no media story has even done any poking around - I found this out using Wikipedia, after being suspicious of the thing's name (specifically the '2009' part), but no news organization has pointed out this simple fact that this thing, capable of doing a lot of damage, was discovered with three days' notice.