Scientists — including lead guitarist Brian May of Queen (Ph.D., astrophysics) and Bill Nye, the science guy — called Wednesday for a hundredfold increase in the efficacy of our search for asteroids that could wipe out a city, or a civilization.

But asteroid scientists affiliated with Catalina Sky Survey, the most prolific asteroid finder in the world, and OSIRIS-REx, NASA’s mission to an asteroid, say the search is important but the danger is often overhyped.

Yes, more needs to be done to guard against civilization-shattering impacts, but we’ve found more than 90 percent of the largest rocks in the inner solar system and none are on a trajectory that will impact Earth, said Eric Christensen, who leads the NASA-financed Catalina Sky Survey.

Using two small telescopes in the Santa Catalina Mountains north of Tucson, astronomers with the survey lead in the discovery of “near-Earth objects,” including two small ones that later crashed to Earth.

The surveys are part of a congressionally ordered push by NASA to find 90 percent of the one-kilometer and larger asteroids on a near-Earth trajectory. NASA “retired more than 90 percent of that risk,” Christensen said. The goal currently defined by Congress is to find 90 percent of near-Earth objects that are 140 meters or larger in diameter by 2020.

“If you want to do it more quickly, you’re going to need new facilities that can survey in much different magnitudes and ways,” said Christensen.

“If you don’t care about solving this problem in 10 years, if it can wait 20 or 30 or 40 years, you can continue surveying at the level we’re doing it now. Pay less and wait longer.”

Wednesday’s call urged quicker detection of even smaller objects, like the 20- to 40-meter rocks that can explode in the atmosphere with the force of a nuclear explosion.

It also urged global adoption of June 30, 2015, as Asteroid Day. The date was chosen because it is the anniversary of the 1908 Tunguska event, when an asteroid estimated at 40 meters in diameter exploded over Siberia with the force of a 3- to 5-megaton bomb.

The “100x declaration” was signed by 100 scientists, space entrepreneurs, cosmonauts and astronauts, including former space shuttle commander Mark Kelly of Tucson.

Christensen said he would challenge the group’s statement that there are a “million asteroids in our solar system that have the potential to strike Earth and destroy a city.”

He said the vast majority “have essentially zero chance of impacting our planet within our lifetime, and if one were to impact, there is only about a 3 percent chance that it would impact over a populated area.”

Current surveys are finding more than a thousand near-Earth objects (NEOs) per year, he said. “The idea that we need to increase the rate of discovery by a hundred times significantly overstates the actual impact risk, and it is not clear that it is even technologically possible to do so without spending billions or tens of billions of dollars.”

Finding smaller objects is difficult, but a ground-based telescope that will begin operating in Chile in 2021 says it can do the job, and do it cheaply.

The Tucson-based Large Synoptic Survey Telescope promises the deepest, fastest and widest look at the night sky of any survey telescope ever built. It pairs an 8.4-meter mirror built at the Steward Observatory Mirror Lab at the University of Arizona with what will be the world’s largest CCD camera.

It will image the entire night sky every three or four nights, said Lynne Jones, an LSST performance scientist from the University of Washington.

The LSST has a variety of science goals, but identification of NEOs and their trajectories is definitely one, she said.

“With LSST you get a very cost-effective way to find a lot of NEOs. The bulk of the cost is the wide array of (other) science that it’s doing.”

Current ground telescopes don’t cover the whole sky and can’t detect very faint objects, she said.

Space telescopes, meanwhile, need ground telescopes for follow-up. The LSST, with its continuing look at the sky can, in essence, do its own follow-up, Jones said.

Follow-up is the biggest problem for surveys right now, said Robert McMillan, who runs the University of Arizona’s Spacewatch program on Kitt Peak.

McMillan is one of the founders of modern asteroid surveys. He and astronomer Tom Gehrels first proposed the practice to NASA in 1980 and found the first asteroids with CCD cameras in 1984.

McMillan said discovery rates have since outpaced the ability of observers to identify the orbit of the NEOs, a key factor in evaluating risk. Spacewatch currently devotes its time to follow-up on four Kitt Peak telescopes.

Asteroids are a potential threat to Earth, said Ed Beshore, but “the scientific community has to be very careful about characterizing the threat more accurately.”

Beshore left the Catalina Sky Survey last year to become deputy principal investigator for OSIRIS-REx. “There is a lot of hyperbole out there,” he said.

Saving the world from disaster also requires two other elements, Beshore said — characterizing the threat and learning how to mitigate it.

He said that’s part of the mission of OSIRIS-REx, a NASA spacecraft that is managed by scientists at the UA’s Lunar and Planetary Laboratory. It will rendezvous with an asteroid named Bennu in 2019 and return a sample to Earth.

Like the recent Rosetta comet experience, it will build knowledge of the forces that caused asteroids to move into the inner solar system, rather than staying in the asteroid belt where they pose no threat.

“When you spend millions and billions of years floating around in space, small forces have big consequences,” Beshore said.

Understanding those forces will help scientists devise ways for nudging any Earth-bound NEOs into a friendlier orbit.

Contact reporter Tom Beal at tbeal@tucson.com or 573-4158.