Chandrayaan’s ChaSTE scores a first after taking moon’s temperature

This image collage shows the location of the ChaSTE instrument onboard the Vikram lander. The lander was photographed by the Pragyan rover.
| Photo Credit: ISRO

As the Vikram lander of Chandrayaan-3 touched down on the moon on August 23, 2023, a thermal probe tucked snugly in its panels, slowly worked itself free and stretched its arms. Its motors started to whir, sending the little probe into the soil. Once the probe reached its intended depth, it clicked in place with a latch.

This is Chandra’s Surface Thermophysical Experiment (ChaSTE) — the first instrument to measure temperatures in situ near the moon’s south pole. Scientists used this data to report that water ice is more prevalent on the moon than expected.

ChaSTE also became the first mission to successfully penetrate the soil of a celestial body to deploy a thermal probe after two previous missions had fallen short.

The ChaSTE probe features 10 temperature sensors spaced about 1 cm apart along its length, near the nose-tip. It uses a rotation-based deployment mechanism.

When its motor rotates, ChaSTE’s probe needle pushes down until its tip touches the moon’s surface. By monitoring the temperature from the sensor at the end of the probe, scientists can identify if it has touched the surface. As the probe continues to pierce, the soil offers more and more resistance. This requires the motor to exert greater force. That is how scientists confirm how far the probe has descended.

ChaSTE tunnelled into the soil to a final depth of 10 cm, then collected measurements throughout the Chandrayaan-3 mission, until September 2, 2023.

In November 12, 2014, the European Space Agency’s Philae lander, hitchhiking on the Rosetta spacecraft, landed on comet 67P/Churyumov-Gerasimenko. But it bounced — twice. Its Multi-Purpose Sensors for Surface and Subsurface Science (MUPUS) instrument onboard was designed to measure temperature by digging into the terrain. However, scientists couldn’t deploy it due to the awkward landing position Philae found itself in on that desolate icy rock, 500 million km away.

The German-Polish team behind MUPUS got another chance when NASA’s InSight robotic spacecraft landed on Mars on November 26, 2018. It carried a temperature-sensing instrument called the Heat Flow and Physical Properties Package (HP³). It consisted of a self-hammering nail, nicknamed “The Mole”, designed to penetrate 5 m below Mars’s surface.

But the friction between the probe and the sand was too low for the mole to hammer itself down more than a few centimetres. After more than a year’s action-packed struggle, the 35-cm Mole had finally descended fully into the Martian sand. But scientists couldn’t get any temperature data. This was because HP³’s temperature sensors were not on the mole. They were attached to a tether that was supposed to trail The Mole as it burrowed through the sand.

“While both the instruments [MUPUS and HP³] used a hammering device, the ChaSTE probe was pushed into the soil by a rotating device,” Durga Prasad K., principal investigator of ChaSTE from the Physical Research Laboratory, Ahmedabad, said. It was the secret sauce that made all the difference.

Unnati Ashar is a freelance science journalist.