NASA's Mars Curiosity rover has successfully completed three drillings so far and was scheduled for another this month. The space agency canceled its fourth drilling however after the potential target was determined to be unstable.

The fourth proposed target, the "Bonanza King" rock, was found last week after taking the rover through the northeastern end of Hidden Valley on its way to the landing site in Gale crater to Mount Sharp.

When the rover attempted to use the percussive drill on the rock, it was found to be too unstable, according to NASA.

A sample from Bonanza King could allow researchers to better understand how environments varied and evolved.

 Curiosity project manager Jim Erickson of NASA's Jet Propulsion Laboratory confirmed that initial tests unveiled that it is not a worthy candidate for drilling. The team will instead continue to move Curiosity towards its goal of reaching Mount Sharp.

The rover has traveled 5.5 miles since landing on Mars back in Aug. 2012. Curiosity would have to travel 2 more miles to enter the layered slopes of Mount Sharp. The rover has faced some difficulties along the way however.

The alternative route has sand that is too slippery. This has led controllers to ask Curiosity to come back to a safe location and another safe route is being determined.

Originally, it was believed that the Hidden Valley's sand would be too much for the rover's wheels, as they have already been damaged from sharp rocks on the surfaces of the planet.

"After further analysis of the sand, Hidden Valley does not appear to be navigable with the desired degree of confidence," said Ericksonm according to the space agency.

The rover has provided a lot of information, like the possibility of ancient river and lake environments. 

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