Weather remained the main hurdle Sunday for an on-time launch of NASA’s Mars Atmosphere and Volatile Evolution (Maven) mission, an orbiter designed to answer the question “where did the water go” on the Red Planet.

A two-hour launch widow opens at 1:28 p.m. EST Monday, raising hope that expected cloud cover will clear enough during that time for the mission to get underway atop its Atlas V401 launch vehicle. Forecasters predicted a 40% change of a weather scrub on Monday, increasing to 60% on Tuesday and Wednesday when upper level winds were expected to complicate the picture.

The $671 million mission is designed to deliver nine atmospheric sensors into an elliptical orbit around Mars next September. They were chosen to measure the processes scientists believe may have caused the transformation from a wet planet with a relatively thick atmosphere to the cold dry planet we see today, where atmospheric pressure at the surface is comparable to the thin atmosphere at 100,000 ft. above Earth, according to Bruce Jakosky of the University of Colorado, the mission’s principal investigator.

Data generated by Maven may also one day help spacecraft engineers design a way to land the estimated 20-ton vehicles needed for human missions to the surface through that thin atmosphere.

In general, planetary scientists theorize Mars started losing its atmosphere about 1 billion years ago, which its “internal dynamo” essentially shut down as the planet’s core solidified, depriving its upper atmosphere of the protection provided on Earth by the global magnetic field. Exposed to the full force of the solar wind, the atmosphere gradually leaked into space, taking most of the water that once ran on the planet’s surface with it.

Maven’s sensors will study the processes that researchers believe enable that leakage, allowing them to backtrack to an estimate of what the primordial atmosphere was like. That information will advance understanding of whether life could have once existed on Mars, and perhaps if it still does, according to Janet Luhmann of the University of California at Berkeley, the mission’s deputy principal investigator.

The mission profile will also take the spacecraft on “deep dives” into the upper and middle Martian atmosphere for in situ measurements that can be correlated with broader data it generates. Those dives, which will remain high enough that the spacecraft or its wing-like solar arrays won’t be damaged, will provide information that may one day aid a human landing, according to Michael Gazarik, associate administrator for space technology at NASA headquarters.

The agency’s advanced-technology work includes a number of approaches to improve on the “Sky Crane” method used to land the 1-ton Curiosity rover on the surface, to the point that payloads 20-times larger can safely touch down. Gazarik’s organization already has tested an inflatable hypersonic decelerator at Mach 10 and 20 gs in a sounding rocket test, while the Jet Propulsion Laboratory is experimenting with supersonic Mars parachutes able to handle larger loads than Curiosity, which represents the state of the art today. Also in the works is supersonic retropropulsion, which would use retro-rockets to slow the lander.

“We have limited data,” Gazarik said. “Curisoity was the first time we measured comprehensively how do we fly through that atmosphere, what was the heat rate through the atmosphere, did we fly that vehicle like we predicted? Maven will also add to our ability to understand the Martian atmosphere.”