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Publicado el 20 de diciembre 2006
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 data : in the first weeks of 2007, 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 2007), 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, nor basic orbital elements signification.
Statistics of discovery :
Number of asteroids.
At the end of 2006, there were 4405 individual objects, compared to 3760 last years.
A record 645 new NEOs have been discovered this year, compared to 627 in 2005 and 533 in 2004.
These 4405 asteroids can be divided in 555 numbered asteroids (133 numbered NEOs this year), 1860 asteroids with 2 to more oppositions, and 2546 objects with less than one opposition.
There are 1842 Amor asteroids, 2183 Apollo asteroids and 380 Aten asteroids.
As far as the discoveries, most of them have been done by the LINEAR program, which started observing in 1998. 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 2006, the discoveries were distributed as follow:
One can see that Catalina is the largest program and by far with almost 62 % of the discoveries, having discovered 156 with the Catalina Schmidt, 178 with the 60" on Mt Lemmon and 62 at Siding Spring; then Spacewatch (99), LINEAR (96), NEAT (21) LONEOS (19). Amateur astronomers contributed 1 NEO discoveries in 2006. Herrenberg Sterwarte (IAU site 240), Klet (IAU site 246) and OAM (IAU site J75) each contributed one NEO this year.
Here is the tabular data :
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 544 ).
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.
Here the same diagram, but on a log scale.
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.
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 this year, showing that clearly we are nearing completion at these magnitudes.
Discovery of magnitude 16 and 17 objects are decreasing, 18 constant and 19 still increasing, because the survey is apparently going to deeper magnitudes.
Here we see the increase of activity, and the number of discovered objects keeps increasing.
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
and here a cumulative distribution (all asteroids larger than x magnitude)
This concludes this brief update of last year's web page.
Keep up the good work Spaceguarders !
For any comments, send me an email