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Publicado el 20 de diciembre 2007
The NASA Spaceguard survey is a 10 years program aimed at discovering 90% of the 1km sized Near Earth Objects, eventhough we don't know exactly how much this number is. This size threshold was chosen because it is estimated to be the smallest size of object able to produce global consequences on Earth (i.e. not only around the impact zone or continent).
The Spaceguard Survey started officially in 1998, so 2007 marks its first ten years. At the end of 1997, we had discovered 447 near earth objects. End of 2007, 5052.... The sample of known NEOs has grown by a factor of more than 11. We are I believe relatively far from the original goal of 90% of 1km diameter asteroids discovered, but it is not too relevant in a way. We can only hope that the coming 5 or 10 years will see this goal achieved, and also an increase in the telescope size used for this project, in order to multiply again the rate of discovery. This program has been made with what was available, as far as telescopes (all the spaceguard telescopes are old, eventually modified telescopes), and people (well, the people too are old and have been modified :) ).
The data : in the first weeks of 2008, I uploaded 3 files from the Minor Planet Center web site, i.e. the Amor, Apollo and Aten files, kept only the useful data (i.e. removed the headers and footers of the page, juste keeping the numerical data, removed the objects already discovered in 2008), then merged them in a large text file, which I later used in Excel and Kaleidagraph (another data processing software available for both PC and Macintoshes).
Some important definitions :
- Apparent magnitude is a measure of the apparent brightness of a given star, and in this context asteroid. It depends of the object size, its albedo (coefficient of reflection of the light), its distance to the sun and to the Earth and the angle between the 3.
- Absolute magnitude is the magnitude that an asteroid would have if located at the distance of one astronomical unit (AU) of the sun (i.e. the distance of the Earth to the Sun, i.e. roughly speaking 150 millions km) and one AU from the Earth. This measures allow to compare asteroids between each other, whereas the apparent magnitude of a given asteroid varies with its position on its orbit versus the position of the Earth.
- In all this data, I rounded the published absolute magnitudes by taking the integer part of the magnitude (i.e. 16 for any object between 16.00 and 16.99). Therefore, in this data, and this is an important point to understand, all objects brighter than absolute magnitude 18 are published here as objects of magnitude 17. This is important in order to be able to compare with other publications made in particular by NASA.I tried to point out also when the data was by individual magnitude class (i.e. all objects between magnitude 17 and 18, noted magnitude 17) and when it was refering to cumulative magnitude (i.e. all objects brighter than magnitude 18, i.e. from the brightest, to magnitude 17.99 )
- I will not explain what Apollo, Aten and Amor mean.
Statistics of discovery :
Number of asteroids.
At the end of 2007, there were 5052 individual objects
646 NEOs have been discovered this year.
These 5052 asteroids can be divided in 815 numbered asteroids (260 numbered NEOs this year), 1308 unnumbered asteroids with 2 to more oppositions, and 2929 objects with less than one opposition.
There are 2148 Amor asteroids, 2469 Apollo asteroids and 435 Aten asteroids.
Here is the evolution of discoveries for Aten (yellow), Apollo (pink) and Amor (blue) asteroids (cumulative number)
As far as the discoveries, most of them have been done by the LINEAR program, which started observing in 1998. LINEAR passed this year its 2000 NEO discovered !
Second comes the Catalina Sky Survey program then Spacewatch, NEAT and LONEOS. 342 discoveries were credited to all other discoverers, except a few individuals who have done a large contribution, like Carolyn and Gene Shoemakers and the many different helpers, Eleanor Helin and her helpers at the time of Palomar, and Robert Mc Naught et al at Siding Spring Observatory in the early 90s.
For 2007, the discoveries were distributed as follow:
One can see that Catalina is the largest program and by far with almost 72 % of the discoveries this year (466 asteroids), then LINEAR (111), then Spacewatch (44), LONEOS (12) NEAT (4) and other, mostly amateur sky surveys with 1 NEO discovered each (Batters, Lulin, Balam, Tholen, Ferrando, Maticic and Bickel).
One can see the evolution of the number of discoveries per year and per program. I summed all the discoveries made at Palomar (obs codes 675, then 566 then 608 then 644 ).
Evolution of discoveries with time.
This evolution has been mainly driven by the number of active search telescopes. It is apparently also affected by the meteorological conditions (good years, bad years...). It is very difficult to establish a model for the efficiency of search telescopes. The same telescope, used in a different manner at a different time in the survey (exposing more time to get deeper for example) would not discover the same number and the same brightness class.
One sees the efficiency of NASA's Spaceguard Survey in this diagram.
The randomness of early discoveries shows the fact that nobody was looking for these objects. Then came the Shoemaker and Helin times starting in the early seventies at Palomar with discoveries becoming more frequent. After 1989, while the Palomar programs were still going on, Spacewatch started to discover a few faint objects, then LINEAR and NEAT came in action and the discovery rate exploded. It will be interesting to see the same diagram in a few more years.
One can see that eventhough Spaceguard is finding more and more asteroids, it is finding less and less objects of less than 1km diameter. The average number of brighter than mag 18 asteroids is now at about 35 per year. If there are 1100 such asteroids, and if we want to discover 90% of them, i.e. 990, knowing we have already discovered 835 of them (i.e. missing another 155), at the current rate it will take 5 years, unfortunately this will not be the case ,the number will get lower and lower with time, and we can count that it will take another 10 years to finish the survey, verifying the 90% law engineering : 90% of a project take 90% of the time, and the remaining 10% take another 90%.
Of course this is not taking into account any larger telescope which could start surveying the sky. As we can see in this diagram, the number of NEO discovered per year has remained more or less constant in the last 3 years. I believe this is not related to the number of telescopes, but more to the diameter of the largest telescope available.
Here is in detail the evolution of discoveries since the beginning of the spaceguard survey per magnitude class:
There are only 3 NEOs brighter than absolute magnitude 13 (Ganymede, Eric and Eros), all discovered before the beggining of the Spaceguard Survey.
Only one object of magnitude 15, 2006AO4 was discovered last year, but this year, 3 were discovered (2007HA59, 2007 VA85 and 2007VQ4). Small number statistics. There should be less and less of these objects out there.
Discovery of magnitude 16 and 17 objects are decreasing but slowly, 18 and 19 more or less constant.
Here we see that we keep discovering more and more of the fainter asteroids.
Fainter than magnitude 26, the detection of fainter object is caused by their random approach to the Earth, with the small telescopes currently used, we are not detecting these objects very far from Earth.
Here is an histogram of the distribution of discovered asteroids per magnitude class
This concludes this brief update of last year's web page.
Keep up the good work Spaceguarders !
For any comments, send me an email