ICESat-2: Tracking Earth's Ice with Unparalleled Detail

The Ice, Cloud and land Elevation Satellite-2, or ICESat-2, is NASA's most technologically advanced ice-monitoring spacecraft to date. Capable of measuring changes in ice thickness, forest growth and cloud height down to 0.02 inches (0.4 millimeters) every year — the thickness of a No. 2 pencil, according to NASA — ICESat-2 offers scientists an unprecedented view of Earth's changing systems, especially at its poles.

"Watching and understanding how it [ice] is changing helps us understand why it’s changing," said Waleed Abdalati, a geographer at the University of Colorado in Boulder and a concept designer of ICESat-2. In turn, the information will sharpen environmental-prediction models and help scientists better forecast rising sea levels and climate shifts because of melting ice.

ICESat-2 was manufactured by Northrop Grumman Innovation Systems in Dulles, Virginia, and constructed at NASA Goddard Space Center in Washington, D.C. At 12.5 feet (3.81 meters) tall and with a base measuring 8.2 feet by 6.2 feet (2.5 by 1.9 m — about the size of a small camper trailer), ICESat-2 is one of the largest satellites built at Goddard, according to Donya Douglas-Bradshaw, instrument manager at Goddard.

After 10 years and multiple re-scheduled launch dates, the satellite is scheduled to take-off aboard a Delta II rocket on Sept. 15, 2018, from the Vandenberg Air Force Base in California.

Science objectives and rationales

ICESat-2 will set off with four primary objectives:

• Measure changing mass on ice sheets and glaciers from around the world.
• Measure how much ice melt from Greenland and Antarctica contributes to rising sea levels. Ice melt in Antarctica, for example, has accelerated over the past five years. "We know that there's going to be some sea level rise," said Peter Neff, a glaciologist at the University of Washington in Seattle, but scientists don't know how much or how soon.
• Estimate the thickness of sea ice in the Arctic and monitor any changes.  Scientists know how much sea ice area in the Arctic has changed since 1980. But satellites like ICESat-2 help determine changes in sea ice thickness. A decrease in sea ice thickness could exacerbate ocean warming by allowing more light to reach the ocean's surface and increase water temperature.
• Measure forest height to calculate the amount of carbon stored in plants. Plants stash carbon as food and use it to grow. That stored carbon can't contribute to warming the climate or acidifying oceans. 

The data ICESat-2 will collect builds upon 15 years of data started by its predecessor, ICESat, which was in space from 2003 to 2010. ICESat-2 was first proposed in 2008, but construction didn’t begin until 2010, with an estimated launch date in 2015. Between ice satellites, NASA used its airborne Operation IceBridge to continue monitoring crucial areas of ice from 2009 until now.

ATLAS - ICESat-2's scientific powerhouse

ICESat-2 was a $1.056 billion project, Richard Slonaker, the ICESat-2 program executive in Washington, DC, said during a media teleconference. Much of that money went toward developing a single, high-tech instrument dubbed the Advanced Topographic Laser Altimeter System, or ATLAS. The instrument basically works like a stopwatch: It shoots a laser down to Earth and times how long it takes for the light to hit the surface and come back. The shorter the time, the higher the elevation of whatever the light hit.

But that simple explanation belies the instrument's sophistication. ATLAS fires a single green (532 nanometers) laser beam into a diffractor that breaks the beam into six rays, coupled as three pairs. At Earth's surface, these lasers form a line. Each laser in a pair sits 295 feet (90 meters) apart, and each pair of lasers lies 2.1 miles (3.3 kilometers) from one another.

This design allows ICESat-2 to calculate the slope of a surface, which, if not accounted for, could appear as changes in elevation — a problem NASA discovered with ICESat's single-laser system, GLAS. ATLAS is also equipped with a back-up laser in case of malfunctions.

ATLAS shoots 10,000 laser pulses every second, with each pulse containing about 300 trillion photons. Only about a dozen photons return to the satellite, where they bounce off a 2.6-foot (0.8 meter) reflective beryllium mirror into a sensor. The sensor stops the "stopwatch" and measures the photons' travel time to within 1-billionth of a second. This timing is matched with the satellite's location — determined by an onboard GPS system and a Star Tracker that observes nearby constellations to figure the spacecraft's orientation — and determines what object on Earth was measured

"To put this in perspective, in the half-second it takes a person to blink, ICESat-2 will collect 5,000 elevation measurements in each of its six beams," said Tom Wagner, ICESat-2's program scientist at NASA headquarters in Washington, D.C. And each of those measurements is 28 inches (71 centimeters) apart.

Moving in an orbit that goes from pole to pole, ICESat-2 traverses 1,387 different orbital paths every 91 days. Consequently, the spacecraft will provide data for every season of the year.

At launch, the satellite was scheduled to operate for three years, but much of its 3,482 lbs. (1,580 kilograms) is fuel that, if desired, can extend ICESat-2's mission to seven years.