WASHINGTON, D.C. — On September 15, 2018, NASA launched the Ice, Cloud and land Elevation Satellite-2. Most will call it ICESat-2, for short. As its name suggests, this orbiting probe will be checking out Earth’s frozen water. But it will track more than that.
Orbiting Earth at an altitude of about 500 kilometers (310 miles), it zips through space at some 25,200 kilometers (15,660 miles) per hour. At this speed, the satellite completes one orbit every 95 minutes or so. ICESat-2 moves over almost 1,400 unique paths on the ground, collecting data all along the way. It takes 91 days to complete them all. At that point, the cycle starts again.
Thomas Neumann works at NASA Goddard Space Flight Center in Greenbelt, Md. As a cryospheric (Kry-oh-SFEER-ik) scientist, he studies ice sheets and glaciers. He described ICESat-2 and its mission, here, in December 2018. He spoke at the fall meeting of the American Geophysical Union in Washington, D.C.
Unlike many NASA probes, ICESat-2 carries only one instrument — a laser altimeter (Al-TIM-eh-tur). It shoots green laser beams down at Earth. The device’s telescope collects the light that bounces back and a computer processes those data.
The reflected light allows the system to measure its own altitude, or height, above a surface. ICESat-2’s orbit is designed to be consistent. So, if the altimeter detects a difference from one spot to the next, it means that there’s been a change in the height of Earth’s surface. And if there’s a change in altitude from one ICESat-2 visit to the next, that will mean the surface has moved up or down over the past 91 days.
How the altimeter works
ICESat-2 fires its laser 10,000 times each second. Before a pulse leaves the satellite, each blip of green light gets split into six separate beams. (That means ICESat-2 is making a whopping 60,000 measurements every second!) Each light pulse contains some 300 trillion photons. Yet only a dozen or so of these will make it back to the altimeter’s sensors. Even so, says Neumann, there is a lot of information that can be gleaned from those few photons that make it back.
By the time the beams reach the ground, they will have spread out. Each will now make a circle some 17 meters (56 feet) across. In the instant between two laser pulses, the craft will have moved about 70 centimeters (2.3 feet) along its path. This means there will be a large overlap between one laser measurement and the next.
At polar sites, these data can be used to estimate the thickness of sea ice. Here’s how: Some photons will bounce off of floating ice. Others, meanwhile, may bounce off open water nearby. The ones reflected off the surface of the ice travel a slightly shorter path. That’s because the surface of the ice stands a bit higher than the water’s surface. So these photons will return to ICESat-2 a bit more quickly than those bouncing off of the water. Because scientists know exactly how fast light travels, the time difference between the first echoes and the last ones lets researchers calculate the height of the ice above the water.
In recent years, scientists have noted that sea ice in the Arctic has been thinning. Other satellite studies, together with field work, have revealed that many glaciers and ice sheets are thinning, too. ICESat-2 will precisely track these trends.
The altimeter can’t track changes at every spot on Earth, Neumann notes. For instance, it won’t pass closer than about 450 kilometers (280 miles) to the North or South Pole. That means there are some blind spots in its measurements. And even those paths that the satellite does take aren’t wide enough to provide full coverage of lower-latitude sites.
But one big benefit ICESat-2 does offer: This probe will be looking at the same spots again and again. So it can document changes from season to season and from year to year, Neumann says.
What’s happening on land?
ICESat-2 will give scientists the chance to monitor all sorts of terrain, even spots not covered with ice. (If you remember, part of ICESat-2’s full name includes the phrase “land elevation.”) Indeed, photons from the satellite’s altimeter will bounce back to space from anything on Earth’s surface.
That opens up a lot of possibilities, says Lori Magruder. She’s an ICESat-2 team leader who works at the University of Texas in Austin. She outlined a few of these possibilities at the 2018 AGU meeting.
ICESat-2 can monitor Earth’s forests, for instance. Just as its altimeter can measure the height of sea ice, she notes, it can measure the heights of trees. It takes a bit more work than measuring the ice. Why? Some photons traveling back to ICESat-2’s sensors will have bounced back from branches at the top of the tree. Others will have reflected off lower branches. Yet others will have made it all the way back from the forest floor. The time difference between the first photons to return and the last ones let the researchers estimate the heights of the trees within each 17-meter-wide laser spot. Researchers can use that info, along with data gathered by scientists on the ground, to estimate how much carbon has been stored in those forests. The carbon comes from the carbon dioxide (CO2) taken up by the trees. Measuring the trees’ carbon helps climate scientists monitor how planet-warming CO2 forests are removed from the air and storing by those trees.
Here’s another neat thing ICESat-2 can do: The laser’s green (532-nanometer wavelength) light can penetrate water. If that water is not cloudy or filled with suspended sediment, that light can reach to depths of 30 meters (98 feet). So ICESat-2 can monitor the depth of lakes and reservoirs. That allows scientists to gauge how much water such reservoirs hold.
ICESat-2’s altimeter also could be used to measure the depths of coastal waters, Magruder believes. Those depths don’t tend to change quickly. Ships can measure them even more accurately. However, she notes, data from ICESat-2 will provide a seamless set of measurements that stretch from coastal waters onto land and be repeated every three months. And that, she suggests, could help in building computer models to better project the heights of future tsunamis or storm surges. Those, she notes, can strike coasts during a hurricane or after a big earthquake at sea.
altimeter An instrument used to determine altitude, or height above a surface.
Arctic A region that falls within the Arctic Circle. The edge of that circle is defined as the northernmost point at which the sun is visible on the northern winter solstice and the southernmost point at which the midnight sun can be seen on the northern summer solstice. The high Arctic is that most northerly third of this region. It’s a region dominated by snow cover much of the year.
carbon The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.
carbon dioxide (or CO2) A colorless, odorless gas produced by all animals when the oxygen they inhale reacts with the carbon-rich foods that they’ve eaten. Carbon dioxide also is released when organic matter burns (including fossil fuels like oil or gas). Carbon dioxide acts as a greenhouse gas, trapping heat in Earth’s atmosphere. Plants convert carbon dioxide into oxygen during photosynthesis, the process they use to make their own food.
climate The weather conditions that typically exist in one area, in general, or over a long period.
computer model A program that runs on a computer that creates a model, or simulation, of a real-world feature, phenomenon or event.
cryosphere (adj. cryospheric) A term for those parts of Earth’s surface that are so cold that surface water will exist almost entirely in frozen form.
elevation The height or altitude at which something exists.
field An area of study, as in: Her field of research was biology. Also a term to describe a real-world environment in which some research is conducted, such as at sea, in a forest, on a mountaintop or on a city street. It is the opposite of an artificial setting, such as a research laboratory.
glacier A slow-moving river of ice hundreds or thousands of meters deep. Glaciers are found in mountain valleys and also form parts of ice sheets.
ice sheet A broad blanket of ice, often kilometers deep. Ice sheets currently cover most of Antarctica. An ice sheet also blankets most of Greenland. During the last glaciation, ice sheets also covered much of North America and Europe.
laser A device that generates an intense beam of coherent light of a single color. Lasers are used in drilling and cutting, alignment and guidance, in data storage and in surgery.
latitude The distance from the equator measured in degrees (up to 90). Low latitudes are closer to the equator; high latitudes are closer to the poles.
monitor To test, sample or watch something, especially on a regular or ongoing basis.
NASA Short for the National Aeronautics and Space Administration. Created in 1958, this U.S. agency has become a leader in space research and in stimulating public interest in space exploration. It was through NASA that the United States sent people into orbit and ultimately to the moon. It also has sent research craft to study planets and other celestial objects in our solar system.
orbit The curved path of a celestial object or spacecraft around a star, planet or moon. One complete circuit around a celestial body.
photon A particle representing the smallest possible amount of light or other type of electromagnetic radiation.
planet A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and has cleared other objects out of the way in its orbital neighborhood.
reservoir A large store of something. Lakes are reservoirs that hold water. People who study infections refer to the environment in which germs can survive safely (such as the bodies of birds or pigs) as living reservoirs.
satellite A moon orbiting a planet or a vehicle or other manufactured object that orbits some celestial body in space.
sea An ocean (or region that is part of an ocean). Unlike lakes and streams, seawater — or ocean water — is salty.
sediment Material (such as stones and sand) deposited by water, wind or glaciers.
sensor A device that picks up information on physical or chemical conditions — such as temperature, barometric pressure, salinity, humidity, pH, light intensity or radiation — and stores or broadcasts that information. Scientists and engineers often rely on sensors to inform them of conditions that may change over time or that exist far from where a researcher can measure them directly.
storm surge A storm-generated rise in water above normal tidal level. In most cases, the largest cause of storm surge is strong onshore winds in a hurricane or tropical storm.
telescope Usually a light-collecting instrument that makes distant objects appear nearer through the use of lenses or a combination of curved mirrors and lenses. Some, however, collect radio emissions (energy from a different portion of the electromagnetic spectrum) through a network of antennas.
terrain The land in a particular area and whatever covers it. The term might refer to anything from a smooth, flat and dry landscape to a mountainous region covered with boulders, bogs and forest cover.
trillion A number representing a million million — or 1,000,000,000,000 — of something.
tsunami One or many long, high sea waves caused by an earthquake, submarine landslide or other disturbance.
unique Something that is unlike anything else; the only one of its kind.
wavelength The distance between one peak and the next in a series of waves, or the distance between one trough and the next. It’s also one of the “yardsticks” used to measure radiation. Visible light — which, like all electromagnetic radiation, travels in waves — includes wavelengths between about 380 nanometers (violet) and about 740 nanometers (red). Radiation with wavelengths shorter than visible light includes gamma rays, X-rays and ultraviolet light. Longer-wavelength radiation includes infrared light, microwaves and radio waves.
Meeting: T.A. Neumann et al. First results from NASA’s new ice-measuring space laser. Press conference at American Geophysical Union Fall Meeting 2018. December 11, 2018. Washington, D.C.
Meeting: T.A. Neumann et al. NASA Ice, Cloud and Land Elevation Satellite-2 Mission Status. Town hall presentation at American Geophysical Union Fall Meeting 2018. December 10, 2018. Washington, D.C.
Meeting: L.A. Magruder. Ice, Cloud and Land Elevation Satellite-2 Mission Status for Land and Vegetation applications. E-poster presentation at American Geophysical Union Fall Meeting 2018. December 13, 2018. Washington, D.C.