David Morrison (with inputs from Alan Harris and Clark Chapman)
One often sees references to the "NASA Spaceguard Goal" of detecting 90% of NEAs larger than 1 km diameter within a decade. Following are some clarifications and historical context for this goal.
* Why NEAs (near-Earth asteroids) and not NEOs (all near-Earth objects)? There are two reasons. First, only NEAs (including the short-period comets) come past the Earth frequently enough to be detected in a sky survey like Spaceguard, which aims to find any hazardous objects decades before they pose a direct threat to the Earth. Second, we have no way of knowing the number of comets that may eventually pass near the Earth (become NEOs). There may be billions of them out in the distant Oort Cloud, but with individual periods of millions of years, they actually pose a smaller threat than the nearby NEAs. What really matters is not how many there are, but how many cross the Earth's orbit every year. So NEAs are the only population we can effectively survey, and they also account for most of the impact risk.
* Why 1 km diameter? Because objects 1 km or larger represent the greatest hazard. Studies carried out at the time of the original NASA Spaceguard Survey Working Group in 1992 identified a threshold at energies near 1 million megatons where an impact had global, not just local or regional, effects. As first discussed in Chapman & Morrison ("Impacts on the Earth by asteroids and comets: Assessing the hazard" Nature 367:33-40, 1994), the individual risk from impacts (the numerical hazard) jumps by roughly an order of magnitude for energies at and just above this threshold. More recent work suggests that this threshold for civilization-threatening impacts is probably nearer 2 km rather than 1 km diameter, but the Spaceguard objective of detecting 1-km NEAs seems like a reasonable and rather conservative figure.
* Why don't we measure diameter directly? Instead we measure brightness, expressed by an absolute magnitude called H. The practical objective is to find the NEAs brighter than H=18, which is approximately equivalent to 1 km diameter for an average asteroid. Of course, not all asteroids will have this average reflectance, but it would not be cost-effective to try to measure diameters of most NEAs, since this would require substantial effort with very large telescopes. If we want to be sure to get nearly all the darker 1-km asteroids, it is easiest just to extend the survey to fainter magnitudes, perhaps to H=18.5.
* Why only 90%? Because that is a reasonable metric. We can't ever get 100% (or at least we can't be sure of having found them all). Also, 90% is enough to reduce the hazard from the undiscovered NEAs below the risk from long period comets.
* When is the ten year deadline reached? The specific 10-year timescale was mentioned in Congressional language in 1994, when the US House of Representatives asked NASA for a program plan to carry out the Spaceguard Survey. In Congressional hearings in 1998, NASA officials adopted the Spaceguard Goal. Measuring from that time, the 90% goal should be reached by 2008.
This is a brief summary of how the goal was developed. It came initially from the 1992 NASA Spaceguard Survey Report, the timescale was articulated by the US Congress in 1994, and the goal was formally accepted by NASA in 1998.
There are two additional important points to be made.
1) We should all understand that the Spaceguard Survey is not limited to NEAs larger than 1 km diameter. NASA has been accused from time to time of "ignoring smaller NEAs", but this is not the case. Unlike fishing, where you "throw the small ones back," we collect and follow up everything within the capabilities of the observing systems without regard to size (or brightness). Today we are finding about twice as many NEAs smaller than 1 km as we do those larger than this value. It is a fact of geometry that the big ones are easier to find than the smaller ones, so we can expect to reach completion (or 90% completion) of larger objects sooner than smaller ones. Adding more or bigger telescopes does not change the fact that we will find all the big objects sooner than we will find all the smaller ones, however long that time may be.
2) The "Spaceguard Goal" can be regarded as a metric for tracking progress of the survey, irrespective of arguments over the cost-benefit of other levels of surveying. It is not a goal in the sense that we should stop surveying when we reach 90% completion of NEAs of H brighter than 18. In order to make a quantitative statement of progress, we must define specific parameters, namely some brightness limit to count and some completeness level versus time for that size. We choose H = 18, 90% completeness, and a target of achieving that in ten years. We could as well choose H = 19.5, which for average asteroids corresponds to 500 m diameter, and measure progress with that metric. Assigning priority to "larger" or "smaller" objects is a moot point. Many would agree there are valid reasons that we should eventually take care of the horrific tsunami-makers, which means going down to 500 m or even smaller. But there is no practical way to discover only larger or only smaller NEAs. Larger telescopes do the job faster, and this is increasingly important as we go to smaller NEAs. But you get what you get, and various survey strategies make little difference in the makeup of the catch, just the total numbers.