The US space agency's robotic rover Opportunity has been sending back images as it approaches the edge of an 800m-wide crater on Mars.
Opportunity has been making its way to Victoria Crater for the past 21 Earth months - about half its mission.
The rover is moving towards a recess on the crater rim to get a prime view.
The depression has high walls with layers of exposed rock that should reveal significant new information about the Red Planet's geological past.
The researchers hope it will serve up a treasure trove of information about Martian history, particularly the role of water on the planet.
Between 27 and 28 September, Opportunity edged 3.7m (12ft) closer to the top of recess dubbed Duck Bay.
Victoria Crater is about five times wider than Endurance Crater, which Opportunity spent six months exploring in 2004, and about 40 times wider than Eagle Crater, where the rover first landed.
The rover has been travelling to Victoria Crater for 21 months
The US space agency's (Nasa) second rover, Spirit, is on the other side of the Red Planet.
It has been holed up at one northward-tilted position through the southern Mars winter in order to collect the maximum energy supply for its solar panels.
Spirit is conducting studies that benefit from staying in one place, such as monitoring effects of wind on dust. It will begin driving again when the Martian spring increases the amount of solar power available.
Both rovers will be on a reduced workload through October as Mars passes behind the Sun as viewed from Earth. This makes communication with the robots more difficult than usual.