Earth is entertaining two transient visitors on Wednesday – a pair of small asteroids, 2010 RX30 and 2010 RF12, whose track takes them inside the moon's orbit around Earth.
Such close passes have appeared with increased frequency as telescopes and detectors dedicated to asteroid hunting have improved.
That capability holds the promise of improved warnings for potential collisions with relatively small objects that might otherwise seem to come out of the blue. By the time they come close enough to brighten sufficiently to spot, they've almost arrived.
The effects of these small objects on populated areas might be local – they would be too small to be "civilization busters" but too large to ignore, some researchers say – but even a few days' warning can allow evacuations to take place.
Wednesday's objects were much smaller than even those asteroids, and yet they were picked up three days before their fly-by. Although the telescopes and detectors were built to spot much larger objects, they also are proving adept at picking out smaller asteroids.
These survey telescopes "are likely to catch a much larger fraction of objects that could hit Earth than people originally thought," says Clark Chapman, an asteroid specialist at the Southwest Research Institute in Boulder, Colo.
Wednesday's objects posed no threat of impact. 2010 RX30 zipped past Earth some 154,000 miles away shortly before 6:00 Eastern Daylight Time this morning. 2010 RF12 is slated to hurtle past at a distance of about 50,000 miles at 5:12 p.m.
Both were first detected Sept. 5 by the Catalina Sky Survey. The survey is run by The University of Arizona using telescopes in Australia and on Mt. Lemmon in the Santa Catalina Mountains that border Tucson.
The asteroids' arrival comes as the US government is weighing fresh approaches to detecting and tracking near-Earth objects that cross Earth's orbit.
Earlier this year, the National Research Council (NRC) noted in a major report on planetary-defense efforts that the National Aeronautics and Space Administration (NASA) was likely to meet within the next few years a goal of finding 90 percent of near-Earth objects (NEOs) whose diameters are larger than about 2,000 feet across.
Scientists have estimated that an impact from one of these objects would trigger a global disaster.
But the goal was set in 1998, even as researchers were developing an increasing respect for the damage much smaller objects can inflict. The shockwave from an airburst over Siberia in 1908, for instance, flattened 830 square miles of forest. The incoming object, by one estimate as small as 130 feet across, exploded some three to six miles overhead.
In 2005, NASA was charged with finding 90 percent of all NEOs 500 feet across or larger by 2020. The space agency will not be able to meet that goal unless Congress and the White House provide more money for the effort, the NRC concluded.
The White House is seeking $16 million per year for NEO detection and data-analysis efforts in its FY 2011 budget. But Congress has yet to act on the proposal, which is part of a larger, controversial overhaul of NASA the administration has planned. NASA is currently spending about $4 million on these efforts.
In the meantime, a NASA task force is in the final stages of preparing recommendations regarding the future of the agency's NEO efforts.
Dr. Chapman notes that groups of astronomers have proposed setting up networks of small telescopes designed to quickly and repeatedly image large patches of the night sky to pick up small NEOs.
In addition, the NRC recently placed a top priority on a new ground-based telescope, the Large Synoptic Survey Telescope, designed to image the entire night sky once every three days. This instrument, Chapman explains, could dramatically improve the detection rate of even smaller objects on final approach.
In the end, however, improvements in technology aren't the only drivers behind hoped-for improvements in detection and warning, he says.
With increased observations, asteroid hunters have become more aware "that when something actually is going to hit the Earth, it gets a whole lot brighter than things that pass by at the distance of the moon."
That increasing brightness can turn even small telescopes into useful early-warning devices, he says.