The strength of GRACE is that it accurately measures mass, providing hard numbers for the amount of ice lost, but only on geographic scales large enough to produce a gravity tug that can be sensed.
ICESat works nothing like GRACE, which is why glaciologists are curious to see whether the findings from the two projects corroborate each other.
When Zwally started advocating for ICESat among his colleagues, he knew the potential of using altimeters to measure ice, but also saw the limits of flying them on airplanes, which was the common method. Only a satellite orbiting from pole to pole could provide the coverage necessary to monitor content across an entire continent or subcontinent. Radar satellites were on the horizon, but Zwally knew those readings would be much less precise because radar wavelengths are longer than those of laser light. “Satellite radar can’t get all the way to the edges of ice,” Zwally says. “There’s penetration too. The radar penetrates some distance into the snow. And that has some variability depending on the properties of the snow. The laser measures directly to the surface.”
ICESat measures the height of Earth’s surface every 660 feet, so it can examine relatively small features. “What ICESat can do is provide very high resolution on a particular track: What’s happening on this glacier?” says Jim Abshire, a soft-spoken native of Tennessee whose job is to ensure the accuracy of ICESat’s laser
NASA had planned to run ICESat’s three lasers in sequence until they burned out—ideally after at least five years of continuous measurements. But ICESat managers were shocked when one of the lasers fizzled after just 36 days. NASA rushed to assemble a review panel headed by veteran NASA engineer Bob Kichak, an expert in spacecraft power systems, to determine what went wrong and whether anything could be done to save the remaining two lasers.
Inside each ICESat laser, dozens of half-inch-long light-emitting diodes project light onto a metallic crystal. The electrons in the crystal become so energized that they release pulses of laser light. The diodes are linked to a power supply by numerous hair-like gold wires. NASA officials disassembled a spare unit purchased from the manufacturer, Spectra Laser Diodes of San Jose, California, and examined the diodes under high magnification. Kichak’s board concluded that “excessive” amounts of metallic indium solder, used to hold the diode assembly together, had most likely contacted the gold wires and eroded them, causing a catastrophic loss of power. The most likely cause for the presence of the extra solder was a manufacturing error.
Based on Kichak’s recommendations, NASA ordered a complete overhaul of ICESat’s operating plan. To conserve the lasers, operators can turn them on only three times a year for 33-day-long “campaigns.” ICESat’s cooling system would slow any chemical erosion. Engineers eventually decided to run the lasers at a temperature of 14 degrees Fahrenheit, says Zwally.
The decision dimmed the laser’s visible, green light, so ICESat’s secondary measurements of aerosols and clouds can be gathered only at night. The ice and land elevation readings aren’t compromised because they require only the lasers’ infrared channel, Zwally says .
NASA officials breathed cautious sighs of relief as the second and third lasers were turned on and ICESat gathered data successfully. The second lasted about 100 days before officials decided to turn on the third.