The largest group of Earth scientists has sent a letter to Texas Republican Senator Ted Cruz, making the case that NASA needs to continue studying our home planet.

Earth science has been part of NASA's mission since the agency's inception in 1958, but in a Senate hearing on Thursday, Cruz told NASA administrator Charles Bolden that NASA needs to move away from launching Earth observing satellites in favor of sending humans to explore deep space.

Cruz chairs the Senate Commerce Subcommittee on Space, Science and Competitiveness, which oversees NASA.

"Almost any American would agree that the core function of NASA is to explore space," Cruz said. "That's what inspires little boys and little girls across this country."

"I am concerned that NASA in the current environment has lost its full focus on that core mission."

Bolden disagreed with Cruz. And now he's got some backup.

The letter, sent Friday by the American Geophysical Union, an organization with more than 60,000 Earth and space scientists, pushes back against Cruz's comments that Earth sciences are not in the realm of "hard sciences" and don't inspire young Americans to seek NASA careers.

"Earth sciences are a fundamental part of science. They constitute hard sciences that help us understand the world we live in and provide a basis for knowledge and understanding of natural hazards, weather forecasting, air quality, and water availability, among other concerns," the letter states.

... Earth Science Division missions aid in flood prediction, earthquake response, and severe storm tracking across the Great Plains. Greater knowledge and prediction skill are urgent when we consider the effort, time and costs of protecting infrastructure along coasts, rebuilding fish populations, developing new water resources for manufacturing and agriculture, and restoring communities in the wake of hazards.

It is unusual for a scientific society to respond this way to a committee hearing, especially so soon afterward.

During the hearing on Thursday, Bolden defended spending more money on Earth science activities, saying he is "proud" of it since it's led to a greater understanding of the planet.

"We can't go anywhere if the Kennedy Space Center goes underwater and we don't know it — and that's understanding our environment," Bolden said, in a clear reference to global warming-related sea level rise.

"It is absolutely critical that we understand Earth's environment because this is the only place that we have to live."

Cruz, along with many of his Republican colleagues, denies that manmade emissions of greenhouse gases are causing global warming. NASA's own data shows that 2014 was the globe's warmest year on record, and that the polar ice caps are melting more rapidly, causing rising sea levels.