Spacecraft engineers at the Jet Propulsion Laboratory hope to cut the drive time for the “Curiosity” Mars Science Laboratory (MSL) rover with a smaller landing-ellipse target than originally anticipated, moving the planned “sky crane” touchdown about 4 mi. closer to the base of the mountain where scientists hope to explore layers of sedimentary rock for evidence that a wetter Mars could have supported life.

Pete Theisinger, MSL project manager at JPL, said June 11 that the navigation software already uploaded to the spacecraft for the Aug. 6 landing was refined to target a smaller landing zone, based on extensive Monte Carlo probability runs using flight software that indicated the margins originally programmed were overly conservative.

Before, Curiosity was aimed at an ellipse 12 mi. wide by 16 mi. long. With the changes, the nuclear-powered rover is targeting an ellipse 4 mi. wide by 12 mi. long, with the center closer to the foot of Mount Sharp on the floor of the equatorial Gale Crater on Mars.

The new landing zone still covers a crater-floor feature known as the High Thermal Inertial Unit, according to MSL Project Scientist John Grotziner. The lowest part of the “moat” around Mount Sharp was imaged from probes orbiting overhead, and the area may hold scientifically interesting deposits left by liquid water when Mars was warmer and wetter, he says.

Grotziner explained that the Gale Crater landing site was picked in part because it allows the possibility of scientific exploration even if hardware failure or terrain prevent exploration up a gully etched into the side of the mountain at the center of Gale Crater.

The third of six possible course-correction maneuvers for the MSL mission is coming up June 26, when controllers will adjust the trajectory to deliver the encapsulated rover to the top of the Martian atmosphere at a point calculated to bring it down inside Gale, between Mount Sharp and the crater wall, about 7 min. and 78 mi. of descent later. Unlike previous Mars landers, MSL is designed for thruster-guided flight through the atmosphere, which allows the tight landing ellipse.

Landing is scheduled for 1:31 a.m. EDT Aug. 6, a time that has the minimum 14-min. communications lag between Mars and Earth built in. However, Theisinger stressed that it could be as long as a day or two under adverse circumstances for controllers to learn if the rover has made it to the ground safely.

Three spacecraft orbiting Mars can act as communications relays, and two of them — Mars Odyssey and the Mars Reconnaissance Orbiter (MRO) — will be in position on landing day to get word back to JPL that the rover has landed on cables lowered from its hovering sky crane platform. But if Curiosity winds up with terrain blocking line of sight to the orbiters, it may take longer, Theisinger says.

Mars Odyssey, which can provide a “bent-pipe” relay to Earth for the fastest notification of a safe landing, went into safe mode last week when one of its reaction wheels froze for about 5 min. during a change in direction.

JPL Deputy Mars Office Director Roger Gibbs, who was project manager on Odyssey, said June 11 the problem is well understood, with more than one workaround possible and a decision on which one to use coming up in the next few days.

MRO could provide a store and forward relay, so confirmation of landing will take longer. Europe’s Mars Express orbiter could serve as a backup to the two NASA probes in a pinch.