By Dr. Ethan Siegel
When you think about gravitation here on Earth, you very likely think about how constant it is, at 9.8 m/s2 (32 ft/s2). Only, that’s not quite right. Depending on how thick the Earth’s crust is, whether you’re slightly closer to or farther from the Earth’s center, or what the density of the material beneath you is, you’ll experience slight variations in Earth’s gravity as large as 0.2%, something you’d need to account for if you were a pendulum-clock-maker.
But surprisingly, the amount of water content stored on land in the Earth actually changes the gravity field of where you are by a significant, measurable amount. Over land, water is stored in lakes, rivers, aquifers, soil moisture, snow and glaciers. Even a change of just a few centimeters in the water table of an area can be clearly discerned by our best space-borne mission: NASA’s twin Gravity Recovery and Climate Experiment (GRACE) satellites.
Since its 2002 launch, GRACE has seen the water-table-equivalent of the United States (and the rest of the world) change significantly over that time. Groundwater supplies are vital for agriculture and provide half of the world’s drinking water. Yet GRACE has seen California’s central valley and the southern high plains rapidly deplete their groundwater reserves, endangering a significant portion of the nation’s food supply. Meanwhile, the upper Missouri River Basin—recently home to severe flooding—continues to see its water table rise.
NASA’s GRACE satellites are the only pieces of equipment currently capable of making these global, precision measurements, providing our best knowledge for mitigating these terrestrial changes. Thanks to GRACE, we’ve been able to quantify the water loss of the Colorado River Basin (65 cubic kilometers), add months to the lead-time water managers have for flood prediction, and better predict the impacts of droughts worldwide. As NASA scientist Matthew Rodell says, “[W]ithout GRACE we would have no routine, global measurements of changes in groundwater availability. Other satellites can’t do it, and ground-based monitoring is inadequate.” Even though the GRACE satellites are nearing the end of their lives, the GRACE Follow-On satellites will be launched in 2017, providing us with this valuable data far into the future. Although the climate is surely changing, it’s water availability, not sea level rise, that’s the largest near-term danger, and the most important aspect we can work to understand!
Learn more about NASA’s GRACE mission here: http://www.nasa.gov/mission_pages/Grace/
Kids can learn al about launching objects into Earth’s orbit by shooting a (digital) cannonball on NASA’s Space Place website. Check it out at: http://spaceplace.nasa.gov/how-orbits-work/
After a decade in space and 4 billion miles, Europe’s Rosetta spacecraft has made history: For the first time ever, a robotic probe from Earth is flying with a comet and will soon enter orbit.
The European Space Agency’s Rosetta spacecraft arrived at its target, Comet 67P/Churyumov-Gerasimenko, today (Aug. 6) to end a 10-year journey across the solar system. The spacecraft performed an engine burn that brought it about 62 miles (100 kilometers) from the comet’s surface.
Comet 67P/C-G and Rosetta are now flying about 251 million miles (450 million kilometers) from Earth. Engineers on the ground had to program the probe to go through a series of complicated burns and maneuvers to make the spacecraft’s rendezvous with the comet a possibility. [Photos: Europe's Rosetta Comet Mission in Pictures]
“This is the end of 10 years of interplanetary flight,” Rosetta Flight Director Andrea Accomazzo said during ESA’s live comet rendezvous webcast Wednesday.
Applause broke out in Rosetta’s mission control center in Darmstadt, Germany, where a crowd of ESA dignitaries and officials had gathered to watch the historic event.
How would it feel like to hold an asteroid or spacecraft in your hands? NASA is giving you that chance through a special website that includes 3-D printable models of various things, ranging from the asteroid Eros to the Rosetta spacecraft, which is going to make an epic rendezvous with a comet in just a couple of weeks.
NASA’s 3D resources website now includes nearly two dozen models, including several released in the past few weeks. You can print out Curiosity’s landing site (Gale Crater), or perhaps the Voyager spacecraft that is further away than anything else humanity has sent out into the universe, or any other number of locations or hardware.
So if you learn best by using your hands, here’s your big chance to have some fun. Or to entertain the kids during summer vacation, if you can get access to a community or personal printer!
The River Bend Astronomy Club serves amateur astronomers from Southern Illinois and the St. Louis Metropolitan area, and beyond, fostering observation, education and a spirit of camaraderie.