Caption: The international space station is seen from Space Shuttle Discovery as the two spacecraft begin their relative separation in 2008. Credit: NASA photo

WASHINGTON — After heavily promoting the International Space Station’s exterior as an Earth observation platform, NASA told aspiring principal investigators to steer clear of the outside of the orbital outpost in the second-ever Earth Venture mission competition that began in early September.

“International space station payloads are not solicited,” reads the Earth Venture Mission-2 announcement of opportunity NASA released Sept.  3.

Earth Venture missions feature standalone spacecraft managed by a single principal investigator. The latest competition caps mission costs at $166 million in 2018 dollars, excluding launch, and sets a June 30, 2022, deadline for liftoff.

Those who want to propose must notify NASA of their intentions by October 2. Finished proposals are due Dec. 4. NASA will select one proposal for an award in August 2016, according to the solicitation.

While payloads mounted to the ISS exterior are banned in the Earth Venture Mission-2 competition, they were allowed in a NASA Earth Venture Instrument solicitation that closed in June, and which is pending selection, according to NASA spokesman Steven Cole.

Despite banning exterior payloads, the latest Earth Venture mission solicitation does not totally slam the door on station. Cubesat proposals are permitted, and launching them from the privately owned cubesat dispenser aboard ISS is not prohibited, according to the Earth Venture Mission-2 solicitation.

NASA is aiming for one Earth Venture mission solicitation every four years.

The first Earth Venture mission solicitation appeared in 2011 and was won in 2012 by the Cyclone Global Navigation Satellite System (CYGNSS), which is set to launch in October 2016 aboard Orbital ATK’s air-launched Pegasus XL rocket. The eight-spacecraft, hurricane-forecasting constellation began development at the Southwest Research Institute in San Antonio in August.

Around this time last year, NASA began a big publicity push for using instruments mounted to the exterior of the International Space Station for Earth science observations. NASA said then it planned to launch five such instruments to the outpost by the end of the decade — and perhaps as many as 20 more through the station’s expected end of life some time next decade.

In September 2014, ISS had two Earth science payloads on its exterior. It has since added two more: the ISS-Rapid Scatterometer in late 2014, and the Cloud-Aerosol Transport System in January. Both rode to space on SpaceX’s Dragon vehicle, which remains grounded after June’s Falcon 9 launch failure. Orbital ATK’s Cygnus, NASA’s other domestic option for getting its instruments to station, was grounded after the October 2014 launch failure of its Antares rocket but is scheduled to return to flight in December atop an Atlas 5 rocket.

Earth Venture missions have a mandate to improve understanding of global changes from the atmosphere down, but that does not necessarily equate to a mandate to observe Earth itself.

For example, Constantine Lukashin, a scientist at NASA’s Langley Research Center in Hampton, Virginia, was considering an Earth Venture Mission-2 proposal that would have observed the moon for three years. Langley was seeking partners for the project earlier this year, but has decided not to propose the mission under NASA’s latest Earth Science competition, Lukashin wrote in a Sept. 18 email.

Called ARCSTONE, the Langley concept seeks to measure lunar irradiance using a compact spectrometer installed on a small low-Earth-orbiting satellite weighing about 50 kilograms. The resulting data set could make it easier for Earth-observing instruments to calibrate their multispectral sensors using the moon as a reference point. The space-based ARCSTONE could measure lunar irradiance more accurately than ground-based telescopes.

Earth Venture missions are part of a larger program called Earth System Science Pathfinder, which also solicits suborbital missions and instrument proposals. Instrument solicitations allow principal investigators to propose hosting their payloads on nondedicated spacecraft. An award for the Earth Venture Instrument payload that closed in June, and which allowed for ISS-mounted payloads, is expected by March, Cole said.

Earth Venture missions are among the smallest principal investigator-led missions flown by NASA’s Science Mission Directorate. Only the Astrophysics Small Explorer program operates on a budget comparable to an Earth Venture-class mission; the latest Astrophysics Small Explorer solicitation, released in 2014, capped costs at $175 million.

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.