Go tiny or go home. That’s one suggestion for building telescopes to find a city-smashing asteroid before it finds us. Asteroids are space rocks, which can range from boulder sized up to chunks of rock nearly 1,000 kilometers (621 miles across). Most of them orbit between Mars and Jupiter. A fleet of pint-sized satellites orbiting the sun could track down the majority of asteroids that threaten Earth's safety. Or that’s what a team of scientists now proposes.

Others aren’t so sure. This plan has holes big enough to drive an asteroid through, some scientists charge.

The U.S. Congress directed NASA, the nation’s space agency, to identify 90 percent of all asteroids wider than 140 meters (roughly 460 feet). Congress set a deadline of 2020. The space agency could meet the first goal using five miniature observatories, say scientists on a team that includes Michael Shao. He’s an astrophysicist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. All it would take, his group says, is to space the satellites out evenly just inside the orbit of Venus.

Some astronomers have looked into building and launching a single space telescope, tasked with keeping its eye out for asteroids. Building it could take eight to 10 years and cost about half a billion dollars. But those five mini telescopes could do the same thing, Shao’s group proposed online March 29 on arXiv.org. Once launched, they could find all the high-risk asteroids in perhaps three years — and for just one-tenth a big telescope's cost.

They would be really tiny

Scott Higginbotham at NASA’s Kennedy Space Center, Florida holds a mini CubeSat. Used in recent years for many low-cost projects, they might one day scout for killer asteroids.
NASA (CC BY-NC 2.0)
Shao proposes mounting telescopes on miniature satellites. Called CubeSats, each is typically just 10 centimeters (4 inches) on a side. CubeSat technology has come a long way in the last few years, he says. Today, researchers can turn CubeSats into orbiting observatories with inexpensive, off-the-shelf parts. Onboard computers can use special algorithms to process what the telescopes spy. This should help them to identify quickly moving asteroids. And the computers on these little systems can find fainter rocks than larger ground-based telescopes would typically turn up, Shao says.

Placing the mini-satellites far out in space makes up for some of what’s lost by their telescopes’ tiny size. Unlike on Earth, there’s no moon or weather to worry about. There’s also no atmosphere, which tends to blur images of space.

And moving the mini armada to a Venus-like orbit lets the fleet whip around the sun faster. That lets the satellites see more of the sky in less time than if they were near to Earth.

What the critics say
Saying CubeSats can do the work of a traditional space telescope for a tenth of the cost sounds too good to be true, says Eric Christensen. He’s an astronomer at the University of Arizona in Tucson. The idea of launching a quintuplet of mini space telescopes is “very interesting,” he says. Still, it could be hard for them to achieve what Shao's group claims.

Here’s one reason. Radio links would not be able to keep up with the rate of data being collected by the CubeSats, Christensen says. So any satellite outside Earth's orbit must do all image processing onboard — without human help. Christensen is also concerned about how the CubeSats would map out precise orbits of the asteroids they see. After all, he notes, these satellites might have very limited observations of individual space rocks.

Timothy Spahr is similarly skeptical. This astronomer is the former director of the Minor Planet Center at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. (Minor planet is another name that astronomers have given asteroids.) “If somebody was going to come along and solve this problem for one-tenth the price, I would love it,” he says. But, he adds, a flotilla of teeny telescopes isn’t going to cut it. “There’s almost no scientific merit to what they’ve presented,” Spahr contends.

He also doesn’t see how minisatellites could detect all of the big asteroids in a mere three years. “There’s zero percent chance of that happening,” Spahr says. Many of the asteroids are on five- to six-year orbits that keep them far from the sun — and too faint to see — most of the time. He also doubts CubeSats could see objects as faint as Shao's team predicts.

It would take too much math, he argues. The algorithms those scientists talk about using are quite complicated. The analysis for each image, he says, could require billions of calculations. “You don't do billions of calculations on a spacecraft that’s the size of your chest,” says Spahr.

Shao disagrees. A recent trial run of the new algorithms at the ground-based Palomar Observatory near San Diego turned up a fainter asteroid — with more precise positions — than could be done using normal techniques, he reports. And computer analyses by his group show that, on average, CubeSats could get about 20 observations for each large asteroid. That should be enough to identify their path, and whether they’ll be heading toward Earth.

Everyone does agree on one thing: Current efforts can’t hit the 90 percent detection mark set by Congress for at least another 20 years. “We’re simply not going to do any better unless we throw a ton of money at the problem,” Spahr says. While Congress gave NASA engineers a tough challenge, it has not provided the agency all of the funds it needs to meet the 2020 deadline.