How NASA Will Use a Space Laser to Measure Earth's Thinning Ice Sheets

This weekend, NASA is planning to launch a spacecraft that will use half a dozen bright-green laser beams to measure seasonal and annual changes to Earth's icy poles.

The mission, called the Ice, Cloud and land Elevation Satellite-2 (ICESat-2), will take incredibly precise measurements of ice sheet depth, allowing scientists to add the third dimension they lose when looking at aerial or space-based images of the spread of ice.

That's crucial information when it comes to building an accurate picture of changes to ice over time, since thinning ice is just as concerning as shrinking ice. But just because it's an important type of data doesn't mean it's easy to gather, which is where NASA's affection for space-based lasers comes into play. [How Satellites Watched Birth of a Giant Iceberg in Antarctica (Photos)]

ICESat-2 will carry just one instrument, which is called the Advanced Topographic Laser Altimeter System (ATLAS). ATLAS produces six finely tuned laser beams of bright-green light, which it beams down to bounce off Earth's surface. (Never fear, the laser isn't strong enough to melt the ice.)

Many of those photons are lost, scattering off in all directions. But a precious few bounce back perfectly to meet the satellite again. Scientists can then translate the spacecraft's incredibly precise measurement of the travel time of each photon — accurate to within a billionth of a second — into the distance it traveled, essentially measuring the height of Earth's surface at that point.

For sea ice, the top of the ice is compared to the surface of the ocean around it. Then, scientists can calculate how much additional ice must be hidden floating in the water. For land ice, the process is a little more complicated but works on similar principles.

The spacecraft produces 10,000 pulses of photons every single second, and for each pulse, the same process plays out. ICESat-2 will orbit from pole to pole, taking measurements all along the way but offering the densest height maps near the poles.

Every three months, the spacecraft will travel over a total of 1,387 orbital paths, then begin to retrace its steps, ensuring it revisits the same swath of ice in 91-day increments. And because the instrument creates six separate laser beams in three pairs, scientists can adapt the data if the satellite ends up straying a little from its planned path.

That's a major advancement from the original ICESat mission, which operated from 2003 to 2010 but only produced data using a single laser beam. Nevertheless, that mission provided crucial evidence that the Greenland Ice Sheet was thinning.

The ICESat-2 mission cost a little over $1 billion and the spacecraft is about the size of a Smart car. The initial mission is scheduled to last for three years, with that timeline beginning about two months after launch to allow the team to calibrate the instruments. That said, the spacecraft carries enough fuel for 10 years, so the mission could potentially be extended.

The spacecraft is scheduled to take off from Vandenberg Air Force Base in California on Saturday, Sept. 15, during a launch window that will open at 5:46 a.m. local time (8:46 a.m. EDT, 1246 GMT) and last for 2 hours and 34 minutes. You can watch the launch live beginning at 8:10 a.m. EDT (1210 GMT) on Space.com, courtesy of NASA TV. The launch will be the final journey of the Delta II rocket, which has been flying for 29 years.

Email Meghan Bartels at mbartels@space.com or follow her @meghanbartels. Follow us @Spacedotcom, Facebook and Google+. Original article on Space.com.