The celestial objects with which we are at risk of collision are referred to a 'minor planets' and cover a range of classes including comets, asteroids, and the trash left over from earlier space flights. The types that get the most attention by the media are the ones typically referred to as 'Near Earth Objects' (NEO's).
When it comes to scenarios that could conceivably lead to the end of our species on Earth and mass extinctions on the scale of the disappearance of the dinosaurs, the group of NEO's consisting of rocks larger than one kilometer in diameter are referred to as 'Potentially Hazardous Asteroids' (PHA's) if they are also known to have a close approach to Earth in the foreseeable future.
Numerous objects smaller than one kilometer have been found, but most scientists currently feel that the damage one could do to the Earth in a collision is somewhat less than our extinction. The collapse of our civilization, death by starvation and disease of a very large percentage of the human population, and massive geopolitical upheaval are all quite possible, though, for some of the PHA's that are not quite large enough to be dinosaur killers.
Fortunately for us all, there are many who are concerned with knowing where these PHA's are and where they are going. Some funding is available to astronomers to hunt the night sky for these disasters waiting to happen. Anyone who knows the places on the internet where these scientists collect their findings can look into the future and know when the sky is going to fall on them.
Our Crystal Ball
One such site is known as the Minor Planet Center. It isn't a flashy site that will draw the typical web surfer to revisit its pages often. It is meant mostly for those researchers who are hunting for the rocks. Among the many links on its pages, though, are ones that an interested forecaster can use to know what is where and when it will come crashing by.
One of the second level pages is The NEO Page. This page is also unexciting, but it does sport links to a couple of active plotters that will draw gif images of the inner solar system with the locations of all the known asteroids including those not classified as NEO's. Any reader that wants to get an idea of just how cluttered our solar system is encouraged to have a look.
From the NEO Page is a more interesting page that may be used to predict a bit of your future. The first is the
Forthcoming close approaches page. This page includes a basic timeline with a naming of NEO's, how close they will approach us, and when the opposition will occur. Included below is a snippet of their page including events to come in our immediate future. (The table has been massaged a bit and an extra column added to show the approach distances in terms of the size of the Moons orbit around the Earth beside the distances in terms of the size of the Earth's orbit around the Sun. Know that the diameter of the Earth's orbit is about 384.6 times the diameter of the Moon's orbit.)
||2002 July 26.03
||2002 July 26.58
||2002 July 29.20
||2002 July 30.03
||2002 Aug. 05.68
||2002 Aug. 18.32
||2002 Aug. 22.87
||2002 Aug. 27.34
||2002 Aug. 30.07
Note that most of the entries show close approaches that are well outside our Moon's orbit. If we wish to focus only upon the more harrowing examples, we can visit a different page at PHA Close Approaches. This page displays the close approach events sorted by how close the shave is. Included below is a small snippet from the top of the page.
||2140 Dec. 1.75
||2039 June 26.96
||2027 Aug. 7.29
||2002 Aug. 18.32
||2028 June 25.74
||2080 Aug. 31.01
||2002 Jan. 7.32
||2166 Feb. 13.30
On both of these pages, a reader may find examples of potentially world killer events. That same reader may wonder why we aren't all running around scared to death about what we are learning. Some will tell that reader 'Trust in the scientists to figure out how to protect us.' Such trust makes for some interesting (and some bad) science fiction has Hollywood has proven over the last few decades. Some will tell that reader 'Trust in your faith to bring about the correct future.' That statement may work for some of the more faithful among us, but what of the rest?
For the rest of us, we must learn to read more on these pages than the closest approach columns and the related date. We must also learn to understand what the number of observed oppositions means and the amount of time covered by the orbit arc. These last two pieces of information tell us how confident the astronomers are in the results of their observations and how reliable the data is for predicting a future path for these rocks.
Our Crystal Ball is fuzzy
Any time a scientist makes a measurement of something, there is always a little bit of uncertainty involved with the accuracy of their numbers. Anyone who has looked through a telescope knows the astronomers are faced with measure the positions of fuzzy, faint dots of light. Modern astronomers have an advantage over their predecessors due to the fact that digital electronics has taken over the job of image recording from photographic plates. However, that advantage does not entirely eliminate observational uncertainties.
Imagine drawing two large dots on a page and connecting them with a line. The larger the dots are the more choices are available for the slope of that line. At the middle of the range is a line that connects the centers of the two dots, while the extreme slopes are represented by lines that run tangent to the upper (lower) edge of one dot and the lower (upper) edge of of the other. The range of possible slopes is greatly reduced, however, if the dots are moved far apart from each other.
The information available in the columns concerning orbit arc is the information we need to understand how far apart the observations are that lead to the predicted orbit for each NEO. If the number of oppositions is large, the observations are far apart. If the time span covered by the observations is large, the information deduced about the orbit of the NEO is reasonably accurate and has small uncertainties.. Therefore, the most trustable entries on the pages describing close approaches are those that have either a large number of observed oppositions or a large time span for the measurements.
The PHA page shows in its first two entries examples of close calls that each are predicted to pass within the Moon's orbit. The first one has the smallest approach distance, but it is also likely to be the more accurate one due to the larger number of observed oppositions. The third entry may seem a little strange. 1999 AN10 was discovered in 1999 yet the time span covered by observations is from 1955 to 1999. How can this be? Even without looking up the notices concerning this asteroid, we can know the most likely thing is some astronomer discovered someone else had taken a picture of the same rock during 1955 and then lost it or never knew it.
Our Crystal Ball is being upgraded
One bit of encouraging information to be found on other pages at the Minor Planet Center shows the number of discoveries by different teams over the years. While the number of NEO's and PHA's has been climbing at an ever-faster rate, our ability to know about them is transitioning from complete ignorance to an understanding that there is a lot of dangerous material out there. Many people have dedicated a lot of their work to discovering these hazards even if we don't know how to deal with the dangers when we find them.
Some of the groups engaged in this work can be found at the bottom of the NEO Page. Both NEAT and LINEAR are listed among them.
There is a project at the Space Frontier Foundation designed to get more non-governmental money to continue the research and get more astronomers the equipment they need at THE WATCH. Read up on it if you want to try to make a difference.
With these tools and a bit of practice, the readers can decide for themselves what level of alarm they wish to feel with each new announcement. The readers can also decide if they wish to do anything on a more personal level to change the future they read in the forecasts.