Academic journal article Science Scope

Grace, Grail, and Gravity

Academic journal article Science Scope

Grace, Grail, and Gravity

Article excerpt

To get a better understanding of what lies beneath the surface of the Moon and the Earth, NASA launched two missions to study their gravitational fields. The missions, which each consist of a pair of orbital satellites, have given us a better understanding of differentiation, the process responsible for the distribution of materials within a rocky planet, moons, and some asteroids. As planets cool. the materials in them begin to separate from one another. The heaviest materials, such as metallic iron, sink to form cores. Low-density magmas rise, forming crusts. However, the distribution of materials is not uniform, thanks in part to meteor strikes and plate tectonics that have resulted in mountain ranges and other dense surface features. By analyzing the gravitational fields of the Moon and Earth, NASA has developed a detailed gravitational map of both bodies, which in turn has yielded crucial information about the distribution and flow of mass within moons and planets. But how did a pair of satellites high in orbit around the Earth and Moon provide us with a look at what was happening below their surfaces?

A look below from above

The GRACE (Gravity Recovery and Climate Experiment) mission was launched in March of 2002. GRACE consists of two identical spacecraft that fly about 220 kilometers (137 miles) apart in a polar orbit 500 kilometers (310 miles) above Earth. GRACE maps Earth's gravity field by making accurate measurements of the distance between the two satellites, using a GPS and microwave ranging system. When the lead spacecraft passes over a more dense area of the Earth--for example, a mountain range--the increased gravity below subtly affects the speed of the spacecraft above. The trailing spacecraft measures the change in speed of the first, and these data are then used to map changes in the planet's gravitational field (Figure 1a). In addition to providing us with a glimpse of our planet's interior, the GRACE mission also provides us with valuable information about the mass of the water in oceans, the distribution of polar ice, and changes in the size of glaciers.


Above the Moon, a similar pair of satellites orbited as part of the GRAIL (Gravity Recovery and Interior Laboratory) mission. The satellites, nicknamed Ebb and Flow, were launched in 2011 and crashed into the Moon's North Pole after a 9-month mission. GRAIL s primary mission was to create a gravitational map of the Moon (Figure 1b), using the same technique employed by GRACE. The mission also helped pinpoint the location of mascons (mass concentrations), large, dense regions with greater gravitational pull that are hidden below the Moon's surface. The GRAIL data confirmed that lunar mascons were generated when large asteroids or comets impacted the ancient moon, when its interior was much hotter than it is now. The impacts created a distinctive pattern of density anomalies that we recognize as mascons. Knowing the location of the mascons will be helpful when NASA selects landings sites for future NASA missions.

ExoMars Mission

The European Space Agency (ESA) is planning a launch to Mars from the Baikonur Cosmodrome in Kazakhstan. Russia's Roscosmos will launch the spacecraft this month, with a planned arrival at Mars during October of this year. The mission, which includes an orbiter and a lander, is the first in the ESA's multi-mission program for the exploration of Mars. The Trace Gas Orbiter's primary mission is to analyze the atmosphere for gases such as methane and search for any possible sources of these gases. The Schiaparelli lander is essentially a technology test of the entry, descent, and landing phases of this mission and will provide information for future ExoMars missions to the Martian surface. The Trace Gas Orbiter has a planned mission length of five years, whereas the lander will only last as long as its onboard batteries have power. The lander won't be equiped to last on the surface for an extended length of time because it is only testing landing technologies, and space and weight limitations prevent sending more equipment than absolutely necessary for the mission. …

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