NEW YORK - US space agency's (Nasa) InSight mission has positioned the second of its surface instruments on Mars. Known as HP3, the heat-flow probe was picked up off the deck of the lander with a robot arm and placed next to the SEIS seismometer package, which was deployed in December. Together with an onboard radio experiment, these sensor systems will be used to investigate the interior of the planet, to understand its present-day activity and how the sub-surface rocks are layered.