A recent report from the National Academy of Sciences points out that NASA has been tasked to locate 90 percent of the most deadly objects that could conceivably strike our planet. Yet only about a third of this assignment has been completed, and the money has yet to be found to complete the job. The agency calculates it needs about $800 million between now and 2020 to make the needed inventory, while $300 million would allow it to find most objects larger than 300 meters across.

The problem is that even the smaller sum is not available, and this AP story quotes space policy expert John Logsdon (George Washington University) as saying the money may never come through, calling the program “a bit of a lame duck.” In other words, there is not yet enough pressure on Congress to produce the needed funds. Meanwhile, asteroid detection remains a low priority for other governments as well, making this a problem we’re choosing to ignore in the absence of recent reminders of its potential.

Asteroid Numbers and Risk

The absence, at least, of recent reminders on Earth — we just saw what happened on Jupiter, with its admittedly larger gravitational well. The comet strike on that giant world reminds us of NASA’s current estimate that there are 20,000 objects — comets and asteroids — that are potential threats to our own world, each larger than 140 meters in diameter. We know the position of about a third of these. The AP story cites Lindley Johnson, manager of NASA’s Near Earth Object Program:

At the moment, NASA has identified about five near-Earth objects that pose better than a 1-in-a-million risk of hitting Earth and being big enough to cause serious damage, Johnson said. That number changes from time to time, as new asteroids are added and old ones are removed as information is gathered on their orbits.

The space rocks astronomers are keeping a closest eye on are a 430-foot (130-meter) diameter object that has a 1-in-3,000 chance of hitting Earth in 2048 and a much-talked about asteroid, Apophis, which is twice that size and has a one-in-43,000 chance of hitting in 2036, 2037 or 2069.

A New Asteroid Population Near Earth?

Meanwhile, an interesting paper by Takashi Ito (National Astronomical Observatory, Tokyo) and Renu Malhotra (University of Arizona) looks at the asymmetric distribution of craters on the lunar surface, questioning whether what we now know about near-Earth asteroids can account for what we see there. Various possibilities exist, including tidal forces breaking asteroids apart to create more numerous craters than we would expect, but there is also the possibility that there is an undetected population of objects co-orbiting with the Earth that has yet to be detected.

To study the issue, the authors simulated the orbital evolution of a large number of test particles representing near-Earth asteroids, working with one population made up of currently known NEAs, and one created as a synthetic group outside the known NEA orbital distribution. What emerged is interesting when weighed against lunar observations:

The NEA-like particles that we used in our numerical integrations, particularly the population A that does not include the particles with large random orbital velocity, have low relative velocity with respect to the Earth-Moon system. In other words, these particles are the “slowest” (relative to Earth) among all the known small body populations in the solar system.

Can the particles accurately model what we see on the Moon? The paper continues:

The fact that even these slow particles do not fully reproduce the asymmetric distribution of impacts as large as what is seen in the lunar crater record, suggests that there may exist a presently-unobserved population of small objects near the Earth’s orbit that have even lower average relative velocity than the currently known near-Earth asteroids do.

We end up with the possible existence of a population of slow objects orbiting the Sun close to the Earth-Moon system. Based on their simulations, the authors estimate the population of ‘slow NEAs’ as roughly fifty percent more than the fraction of known slow NEAs. The definition of ‘slow NEA’ is a near-Earth asteroid with a potential lunar impact velocity of less than 11 kilometers per second. These are objects, in other words, that are nearly co-orbiting with the Earth.

Observing Programs Not Optional

Do such objects exist? The authors note the need for more complete observational surveys of near-Earth asteroids to test their prediction. And they’re careful to run through the alternatives, which include (in addition to asteroid fragmentation) the possibility that future mapping missions will give us a better dataset regarding the crater asymmetry.

Whatever we learn about lunar cratering, this paper yanks our chain yet again — we need to learn much more about the objects that could do our planet harm. We’re finally approaching the technological stage when we could actually do something about a predicted impact. What remains to be seen is whether we’ll firm up our observing programs in time to put that technology to work in the event we spot something truly dangerous.

The paper is Ito and Malhotra, “Asymmetric impacts of near-Earth asteroids on the Moon,” submitted to Astronomy & Astrophysics (preprint).