Two of the five Earth-science missions NASA plans to launch this year will end up on the International Space Station (ISS), which is growing in importance as a relatively low-cost spot to operate downward-looking sensors.

The station’s low altitude and relatively high inclination also can give scientists a new perspective for their observations, which typically are taken from sun-synchronous polar orbits.

“The ISS has become a tool that Earth sciences can use to increase accessibility to orbit to test new instruments, to cross-calibrate new instruments and to fill new data gaps when instruments fail,” says Julie Robinson, NASA’s chief ISS scientist.

SpaceX will use the unpressurized “trunk” on its Dragon commercial cargo carrier to deliver the two NASA instruments to the space station. On June 6 the SpaceX-4 mission will deliver RapidScat to the ISS, where it will continue the ongoing collection of ocean-wind data from around the world.

The SpaceX-5 mission, scheduled for Sept. 12, is scheduled to deliver the new Cloud-Aerosol Transport System (CATS), a technology-test mission that will evaluate the use of lasers to observe aerosols in the atmosphere originating from dust, smoke, air pollution and volcanoes. Both missions have the potential to advance climate science as well as applied work.

Using the space station as an all-purpose spacecraft bus allows instrument developers to avoid the sunk costs of doing it themselves. The CATS instrument costs on the order of $14 million, and RapidScat about $26 million to develop, according to Robinson.

By comparison, says Michael Freilich, director of the Earth Sciences Division at NASA headquarters, the two-year baseline Orbiting Carbon Observatory (OCO-2) mission, with its launch vehicle and the spacecraft, will cost about $372 million.

Robinson says NASA plans to fly more Earth-observation missions in 2015, including OCO-3 — a unit carrying spare OCO-2 flight instruments that will allow scientists to cross calibrate the instruments from its lower-altitude ISS perch. Altogether, there are 22 external sites on the station for scientific instruments, and because there is a crew on board they can be shifted and swapped out over time.

In addition to the two station missions, the U.S. space agency and its international partners plan three free-flying missions this year, beginning with the Global Precipitation Measurement (GPM) set for launch on a Japanese H-IIA rocket Feb. 27. The first element in an international constellation of water-measurement spacecraft, GPM will observe rain and snowfall over almost all of the planet for scientists studying the global water cycle, and for water-resource managers and meteorologists. Built at Goddard Space Flight Center, GPM is a joint project of NASA and the Japan Aerospace Exploration Agency.

Other free-flyer missions NASA plans this year are the replacement OCO-2 and the Soil Moisture Active Passive (SMAP) mission. Both will be launched from Vandenberg AFB, Calif., on Delta II rockets — OCO-2 in July and SMAP in November.

The carbon observatory will replace a spacecraft lost in 2009 to a fairing-separation failure on its Taurus launch vehicle. Its precise measurements of worldwide carbon dioxide concentrations will help scientists and politicians better understand how much of the greenhouse gas is generated by human activity, and how it is taken up by plants, the oceans and atmospheric phenomena.

SMAP is designed to measure the water content of soil, both for climate science and to help in weather and agriculture-yields forecasting.