Ten years of Curiosity on Mars

On August 6, 2012, the largest space probe ever sent by humanity to the neighboring planet successfully landed in the Gale Crater on Mars. With a mass of 899 kg, the Curiosity rover was also the heaviest wheeled rover launched on Mars, as well as being the first to be powered by nuclear power thanks to its plutonium-238-based radioisotope generator (RTG) (although it was not the first to carry plutonium, since all previous NASA rovers —Sojourner, Spirit and Opportunity— carried heaters with this isotope). The MSL mission (Mars Science Laboratory) was born in 2003 with the aim of creating a rover capable of studying the Martian environment in detail. Although the world had fallen in love with the MER, Spirit and Opportunity rovers, the truth is that their scientific capabilities were very limited. What the scientific community wanted was a true mobile laboratory that would not only be able to take beautiful photos, but could also deeply investigate the surrounding terrain. The result was the largest Martian space probe ever designed, equipped with two main instruments that were true miniature laboratories, SAM (Sample Analysis at Mars) y PATH (Chemistry and Mineralogy) capable of analyzing the samples provided by a complex robotic arm equipped with a drill and an even more complex distribution system for the Martian regolith.

Curiosity in front of Mount Mercou (6 meters high) in March 2021 in a selfie made from 60 images from the MAHLI camera (NASA/JPL-Caltech/MSSS).

But to send this technological marvel it was necessary to build the largest capsule used by a Mars probe, with a heat shield 4.5 meters in diameter – the Pathfinder and MER probes had used a shield of 2.65 meters, while the Viking had a 3.56-meter-diameter shield)—as well as an entirely new landing system called sky crane —celestial crane— that would allow a vehicle weighing almost one ton to be deposited on the Martian surface within the limits of mass and volume imposed by the size of the capsule and the launcher rocket. All of these challenges caused the cost of the mission to skyrocket from an initial $1.6 billion to more than $2.5 billion, as well as a delay in launch date from 2009 to 2011. In 2009, the MSL rover was named Curiosity according to the proposal of Clara Ma, a 12-year-old girl who participated in a national contest (however, the official name of the mission is still MSL, as well as that of the rest of the hardware apart from the rover itself). In July 2011, just months before launch, it was decided that the chosen target was Gale Crater, a 154-kilometer-diameter crater that was believed to have harbored liquid water eons ago. The MSL mission took off on November 26, 2011 at 15:02 UTC aboard an Atlas V 541. After 36 weeks of travel and four maneuvers to correct its trajectory, Curiosity entered the Martian atmosphere at 5.8 km/s . When the descent capsule had slowed down to 1,450 km/h, it deployed the largest parachute ever used on Mars, a large dome 15.61 meters in diameter and 49.9 meters long.

Curiosity’s instruments (NASA).
Curiosity descending into Gale Crater by parachute on August 6, 2022 as seen by NASA’s MRO probe.

At 1.67 km altitude the parachute with the rear heat shield separated and shortly thereafter the eight hydrazine MLE thrusters of the descent stage ignited while the vehicle was still falling at a speed of 284 km/h. When the speed was reduced to 2.7 km/h and the ship was at an altitude of 20 meters, the complex maneuver began sky crane and the descent stage began to lower the rover thanks to three cables of 7.5 meters in length made of nylon and Vectran. Curiosity unfolded its six wheels, and as soon as the descent stage felt the wheels were firmly on the ground, the cables were cut so the descent stage would roll away until it crashed in a safe location far enough away from the rover. Curiosity hit the ground at 2.16 km/h instead of the predicted 2.7 km/h while moving horizontally at 0.36 km/h. This small deviation originated from not taking into account the local variation of the Martian gravitational field, which deviated from the mean by 0.1%. Despite the smallness of the error, if the horizontal speed had been slightly higher, Curiosity’s wheels could have been seriously damaged. Be that as it may, Curiosity successfully landed in Gale Crater 431 seconds after beginning atmospheric entry at 05:17:57 UTC on August 6, 2012. Mission control erupted in applause when the long-awaited message was announced. Nominal Tango Delta”, the code to indicate a successful landing (tango and delta refer to the initials TD, from touchdown, ‘landing’). The ‘crazy’ maneuver sky crane It had been a resounding success.

Some of the lakes that formed in Gale Crater eons ago (NASA/JPL-Caltech).

Curiosity landed at coordinates 4.5895º south, 137.4417º east, just 2.39 kilometers from the predicted point. Never before has a landing on Mars been so precise. Over this decade, Curiosity has traveled 29 kilometers across the surface of Gale Crater and has analyzed a total of 41 Martian soil samples. He has also climbed 625 meters vertically while traveling on the slopes of Mount Aeolis (named Mount Sharp by NASA). Curiosity’s main discovery has been the confirmation that numerous lakes of liquid water formed in Gale Crater between 3.8 and 3.3 billion years ago, most with a pH and salinity compatible with life as we know it. Although it is not yet clear how many lakes existed, it seems that there were at least three episodes of significant lake formation, and all of them after Mount Aeolis more or less acquired its current morphology (before Curiosity’s arrival it was thought that the presence of water in Gale Crater could predate the formation of Mt.) The maximum depth of the lakes reached 700 meters. The key question is how long these lakes lasted, and although there is no firm answer yet, it is believed that each one existed for at least several million years. Another great debate is the age of these lakes, because it would indicate when Mars ceased to be habitable. Given the impossibility of directly dating the samples, researchers have to resort to indirect techniques that are sometimes controversial. In any case, it seems that the lakes existed until 3.5 to 3.3 billion years ago, considerably longer than some Martian climate models had predicted.

Curiosity image taken on November 16, 2021 (mission sol 32999) (NASA/JPL-Caltech/MSSS).
Otro selfie de Curiosity (NASA/JPL-Caltech/MSSS).

Curiosity has also detected organic molecules in Martian samples. Although it is by no means proof that life existed on Mars, it is a necessary condition for for this to arise. Unfortunately, despite its advanced instruments, Curiosity cannot accurately determine the exact nature of all these molecules, especially the more complex ones, which, of course, are the most interesting. What is striking is that these organic substances are found just 5 centimeters deep in the rocks, despite the fact that, according to some models, the radiation could have completely destroyed them. In addition, the rover has detected large amounts of the carbon-12 isotope relative to carbon-13 in half of the samples. This result conflicts with the proportion of these isotopes found in carbon dioxide in the atmosphere and could be the result of the action of ancient Martian life forms, although there are also many less attractive abiotic explanations. In this sense, in 2015 Curiosity confirmed that 3.5 billion years ago Mars had nitrogen reserves —in the form of nitrogen oxides— that could be used by hypothetical life forms for their biological processes. And, of course, we cannot forget the mystery of the methane that has been detected by Curiosity since 2014. To date, it is not clear what the mechanism that generates it is, although, more than bacteria or geological activity, everything points to the meteorites falling on the red planet.

Clouds of carbon dioxide crystals over Monte Mercou (NASA/JPL-Caltech/MSSS).
Path traveled by Curiosity until 2020 (NASA).

In another order of things, the RAD instrument (Radiation Assessment Detector) has measured the radiation that an astronaut would experience on the trip to Mars and on its surface. The radiation doses are worrying, although within what is expected. In a single trip to Mars, an astronaut without specific protection would exceed the cumulative dose allowed by NASA over a lifetime. The main radiation concern is cosmic rays, especially heavy nuclei, whose long-term impact on the human body is not known. The good news is that simply by being on the surface the dose of cosmic rays is reduced by less than half and, in the same way, the doses of this type of radiation for a trip to Mars during solar minimum are also almost half. than at the maximum activity of our star (0.65 sievert vs. 1.59 sievert).

Curiosity’s wheels have suffered significant wear (NASA/JPL-Caltech/MSSS).
Image from March 15, 2022 (mission sol 3415) showing the type of rock Curiosity must avoid to avoid damaging its wheels (NASA/JPL-Caltech/MSSS).

These ten years have not passed in vain and Curiosity already has some ailments, the most notorious being the state of the wheels, which have suffered much greater damage than expected. Mission engineers have been able to slow wheel deterioration by choosing paths with less sharp rocks and implementing algorithms for Curiosity to avoid skidding on rocks. In addition, the rover has experienced short circuits, restarts of its two computers and for a year and a half – between December 2016 and May 2018 – it was unable to use its drill due to a failure of its motor. And one of the REMS weather station’s sensors was knocked out shortly after landing by a pebble hit. However, the condition of the rover remains very good overall and it probably has many years ahead of it. Curiosity will continue to climb the slopes of Mount Aeolis, studying the area’s sulfate-rich minerals, to get a better picture of the changing climate and habitability of the neighboring planet. Hopefully in another ten years we can celebrate the 20th anniversary of Curiosity’s landing with the rover still active.

Past and future trajectory of Curiosity towards the top of Mount Aeolis (NASA).
Here’s to another ten years (NASA).

References:

  • https://www.nasa.gov/feature/goddard/2022/sam-top-5
  • https://mars.nasa.gov/news/9240/10-years-since-landing-nasas-curiosity-mars-rover-still-has-drive/
  • https://mars.nasa.gov/msl/mission/where-is-the-rover/

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