The Gemini North observatory in Hawaii fires a laser into the sky as part of its adaptive optics system (Image: Gemini Observatory)

Could astronomers accidentally blind Earth-observing satellites? That seems to be the worry of the US air force, which restricts the use of lasers pointed at the sky to help focus telescopes. But some astronomers warn they will miss key observations under the rules, which have tightened in recent years.

Many of the world’s largest observatories, including Lick, Gemini North, Palomar and Keck in the US, shine lasers into the sky to measure atmospheric turbulence, which distorts images.

The laser causes a layer of sodium atoms at an altitude of about 90 kilometres to glow, producing an artificial star whose twinkles reveal the turbulence. Shape-shifting mirrors on the telescopes, called adaptive optics, then correct for the blurring by adjusting their shape many times per second.


If such a laser were to hit the optics of an Earth-observing satellite, it could cause damage. So the air force’s Space Command has for years restricted when and where US observatories can fire them, and the observatories have voluntarily complied, with little impact on astronomy.

Then about two years ago, just as kinks in the laser technology were being ironed out and interest in the lasers was growing, the rules were tightened. Now astronomers say the restrictions are beginning to chafe, according to a story first reported by the American Physical Society.

“Significant negative impacts of these new restrictions on scientific productivity are being felt,” says a 2008 report (pdf) by the US Association of Universities for Research in Astronomy, which is based in Washington, DC.

Off limits

The restricted zones are now so large that they can rule out observations even when a satellite is below the horizon, the report says.

About half to two-thirds of the objects astronomers seek to observe have off-limits periods, or closures, in a given night, the report adds. These periods last a few seconds to a few minutes each.

“Typically we’d get a couple closures a night [before the changes], and now we get hundreds – sort of a dozen per target we submit,” says Antonin Bouchez, who manages the laser adaptive optics system at the Palomar Observatory in California.

This means astronomers sometimes have to interrupt long exposures of faint objects. It is possible to work around this by combining several shorter exposures, but this adds noise to the image, degrading its quality, Bouchez says.

Long lead time

But the biggest impact of the restrictions is that they prevent laser-adaptive-optics observations of one-off events like supernovae, which appear and fade away in a matter of days, and gamma-ray bursts, which have afterglows in visible and infrared light that often fade away in a matter of hours.

It is impossible to use the lasers for these sudden, brief events because under current rules, US observatories have to submit their proposed targets to the air force three days in advance.

Bouchez says he is not arguing to completely eliminate the restrictions. Satellites with optics pointed at the ground could in principle be damaged by laser beams pointing upwards, he says.

Unknown risks

But it is not clear how sensitive the satellites’ optics are, or how likely a given laser is to directly hit a satellite’s optics, since details on some US Department of Defense satellites are not publicly released. The air force simply takes astronomers’ proposed laser-assisted observations and tells them when to turn the lasers off after crunching its own data on satellite orbits.

“None of us [astronomers] really knows what the risk is,” he says. “The air force people are presumably the only ones who do, because we don’t know what’s up there.”

Even if some restrictions are necessary, faster responses from the air force would make them less burdensome, he says. “If you could get an answer within minutes or seconds, then you could observe time-variable things like gamma-ray bursts” with laser-assisted adaptive optics, he says.

The air force did not immediately respond to New Scientist inquiries about the restrictions.