ISS-RapidScat's two-part payload is shown in the trunk of a SpaceX Dragon cargo craft at NASA's Kennedy Space Center in Florida. Credit: SpaceX
ISS-RapidScat’s two-part payload is shown in the trunk of a SpaceX Dragon cargo craft at NASA’s Kennedy Space Center in Florida. Credit: SpaceX
ISS-RapidScat’s two-part payload is shown in the trunk of a SpaceX Dragon cargo craft at NASA’s Kennedy Space Center in Florida. Credit: SpaceX

WASHINGTON — Despite an orbit that is out of the ordinary for NASA’s Earth Science Division, the international space station’s convenience as a host platform has convinced the agency to install five more Earth-facing instruments at the outpost during the next four years — and perhaps add as many as five times that number of instruments before the station’s planned end of life sometime next decade, NASA officials said in a Sept. 9 media briefing.

The five instruments slated to be installed by 2018 will make a total of seven aboard station, which already hosts two, Julie Robinson, chief scientist for the ISS program, said here during the briefing, which also featured NASA officials from other centers who participated via video conference.

The plan is to keep adding Earth-observing instrument to ISS — which is officially set to fly through 2020, although the White House is seeking an extension through 2024 — “until we have all 25 external [ISS] sites completely full,” Robinson said.

The space station’s orbit is inclined at 51.6 degrees, so an Earth-facing ISS instrument will never get a glimpse of latitudes further north or south than that. This rules station out as a platform for observing the Arctic or Antarctic poles. While ISS flies over the same spot every three days, it sees the same spot in the same lighting conditions only once every 62 days. An ISS-mounted instrument therefore “takes a lot longer to get a massive database” of comparable images than a similar instrument mounted over a sun-synchronous satellite, which sees every point in its orbit under the same lighting conditions each orbit, Steve Volz, the Earth Science Division’s associate director for flight programs, said during the briefing.

The space station also “wiggles and jiggles,” so unless an instrument team is “willing to spend millions on stability systems and star trackers,” it would not be possible to clearly observe objects smaller than about a meter in size from station’s orbit, Matthew McGill, principal investigator for the Cloud-Aerosol Transport System instrument headed to station in December aboard a Space Exploration Technologies Corp. Dragon capsule, said in an interview with SpaceNews.

On the upside, the station’s peculiar orbit offers Earth scientists a way to observe any point between Earth’s 51.6-degree parallels under different conditions — something a geographically limited geostationary satellite or a temporally constrained sun-synchronous satellite cannot, Volz said.

“It gives you a different perspective,” Volz said. For example, soil moisture conditions and wind speeds vary day by day and hour by hour, as does “the way the vegetation reflects light. Seeing [that] from different angles, you get different perspectives on how the vegetation health is as well,” Volz said.

“We’re really observing a maturity of the space station as an Earth science platform,” Robinson said.

Despite the maturation, ISS is still in an adolescent phase as an Earth observation platform. The station’s first Earth-science instrument, the Hyperspectral Imager for the Coastal Ocean, arrived in 2009. The instrument was developed by the Office of Naval Research, but was later taken over by NASA. The space agency is interested in hyperspectral imaging, which offers about a hundred bands of information. By comparison, the two imaging instruments on Landsat 8, the latest in the 40-year-plus Landsat series of Earth imaging satellites, combine for 11 bands of information.

After the Hyperspectral Imager for the Coastal Ocean came the internally mounted ISS Servir Environmental Research and Visualization System: a high-quality digital camera that launched in 2012 that captures three images of the Earth every second, each of which covers an area of about 19 kilometers by 11 kilometers.

The next five instruments to launch will be:

  • ISS-RapidScat, which will launch aboard a SpaceX Dragon spacecraft no earlier than Sept. 19 to monitor ocean winds for climate research, weather predictions and hurricane monitoring.
  • Cloud-Aerosol Transport System, a light-detection and ranging system that will measure atmospheric pollutants. SpaceX will also launch this sensor, which is slated to ride to ISS aboard a Dragon due to lift off in early December.
  • The Stratospheric Aerosol and Gas Experiment 3 on ISS, which will observe the protective layer of ozone that shields Earth from solar radiation. SpaceX is slated to deliver the sensor in March 2015.
  • The Lighting Imaging Sensor, which will measure the frequency and occurrence of lightning strikes. The sensor will launch in 2016, the same year a similar sensor aboard NASA’s Tropical Rainfall Measuring Mission will be destroyed along with the host satellite in a planned end-of-mission re-entry into Earth’s atmosphere.
  • The Ecosystem Spaceborne Thermal Radiometer Experiment on Space Station, a multispectral infrared imager launching in 2017.
  • The Global Ecosystem Dynamics Investigation Lidar, which will use a laser-based system to observe, among other things, forest canopy structures over the tropics, and the tundra in high northern latitudes.

Even if all of these instruments perform well, it will not mean an automatic ticket for a successor instrument to fly aboard its own satellite, Volz said.

However, Volz added, “successful demonstration by any or all of these instruments could certainly … influence our decisions on how to go about getting the longer-term measurements.”

SpaceNews correspondent Debra Werner contributed to this story from San Francisco.

Dan Leone is a SpaceNews staff writer, covering NASA, NOAA and a growing number of entrepreneurial space companies. He earned a bachelor’s degree in public communications from the American University in Washington.