If a number of the practices that the photographs illustrate "were to become commonplace worldwide," he writes, "at least one of our major planetary environmental challenges would be much better managed".
Estimating the distance to any particular planetary nebula is challenging because of the variety of shapes and masses of the ionized gas.
The size and construction of radar sounding antennas make the usual methods of meeting planetary protection requirements challenging.
But modeling the formation of those cores from bits of dust and getting the right number of cores in the right orbits has long challenged planetary scientists.
The discoveries of these extrasolar planetary systems have challenged our theories of how our own solar system and individual planets formed and promise to help us understand that process and history better.
They perpetuate the idea that humanity poses no challenge to the planet and that planetary limits pose no challenge to humanity.
Even now, two months after the World Trade Center attack, decision makers are looking toward shopworn, parochial concepts to address challenges of planetary, millennial scope.
Planetary gearboxes exhibit unique challenges in bearing fault detection.
However, condition monitoring of planetary gearbox still faces challenges due to complexity and weak condition feature.
When all we hear in the news is about illnesses floating around, economic challenges and planetary catastrophes, it's a wonder we ever feel happy.
