Planetary Defense Conference: Interesting Tidbits
Posted by Bruce Betts
30-04-2009 16:03 CDT
by Bruce Betts
A lot gets covered during a multi-day science conference, especially one like this that schedules 12 hours of programming a day (scientists can be quite devoted). Since I can cover only a fraction of what's been happening here, I'm going to hit (in no particular order) some points that are either "basic" for current understanding, new or relatively new in the field, or that I just think are neat. Let's get started.
- In the realm of basic NEO concepts: there are far more small NEOs out there than bigger ones. Very few big ones, lots and lots of small ones. So, small impacts are much more likely at any given time than large impacts. To take the extremes, dust- and sand-sized meteors are visible every night, but dinosaurs only get wiped out once in awhile.
- About 6000 near Earth objects have been discovered. Of these, about 800 are larger than 1 km, and about 20% cross Earth's path enough to be considered potentially hazardous objects (PHOs). Scientists think about 90% of NEOs larger than 1 km have been discovered now. A NEO of this size would cause regional or global disaster if it impacted. Consistent with finding 90%, the discovery rate of these large NEOs has actually dropped in the last few years as there are fewer and fewer to discover. Of the 6000 NEOs, about 80 are comets.
- In the mid-1990s, the U.S. Congress mandated that NASA should find 90% of 1 km NEOs by 2008, and that pretty much happened, primarily due to a number of professional surveys, with additional discoveries and follow-up from other observers such as our Shoemaker NEO grant winners.
- Now, Congress has mandated finding 90% of asteroids larger than 140 meters in the next decade or so. Some of these are being discovered with existing surveys and other observations. But, to really find many of the smaller end of this spectrum will require new surveys. A prototype of this "next generation" survey called Pan-STARRS-1 will come on line in Hawaii in the next few weeks. To really get closer to doing the assigned job, though, the full Pan-STARRS will need more equivalents to Pan-STARRS-1 coming on line in the next few years. Further down the road (no earlier than 2015), there is the hope for LSST, a system using an 8.4 meter telescope with a very wide field of view that would be able to observe really dim objects, i.e., find small stuff.
- To get the diameter of a NEO, usually you have to do rough estimates except for the small number that come close enough for radar. Generally, one estimates the diameter based on the visible brightness: bigger things at the same distance reflect more light. But here's the rub: the observed visible brightness is highly dependent on the albedo of the asteroid, in other words the reflectivity of the surface: white stuff reflects more light than black stuff. So, if you don't know the albedo, it makes your size estimates very sloppy, particularly because it turns out that the albedo of NEOs vary over a pretty wide range, and they are bimodal -- most are either really dark or much brighter with not many in-between. So, when there are uncertainties in diameters, this is often the root cause. Turns out there are things that can be done to help determine the albedo if you can get the right kinds of observations -- these techniques range from using polarization observations to thermal infrared observations to using the spectral type of the NEO, which is often correlated to albedo.
- If and when humans have missions to NEOs, they will experience 10 to 30 second light time communication delays. This is far less than the minutes of communication light time for Mars, but plenty to remove communications from true real time, and thus change the operational modes the astronauts and ground will have to use.
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