As the sun glides on a 3.9-billion-year-old, crater-pockmarked part of the Moon around 22 September and light falls on solar panels at the cold lunar dawn, India’s sleeping Pragyan rover and Vikram lander—both part of the successful Chandrayaan-3 mission—may awaken. When the two had fallen asleep, their batteries drained, their work had already been completed through the 14 earth days of the lunar day, with the data from the lunar experiments relayed back to Earth.
Whether they will awaken, having survived the lunar night when temperatures drop to immensely more cold than Antarctic winters, is unclear but the Indian Space Research Organisation (Isro) will certainly not have wasted the opportunity to test its options. Surviving the lunar night would both allow it capability for longer missions using only solar power, and open the way for private commercial landers, especially those waiting to take off through US space agency Nasa’s Artemis programme, with solar the most viable option.
Chandrayaan-1, launched in 2008, was India’s first planetary mission, with payloads from both India and international collaborators. Its scientific success brought a shift in what was thought to be a “bone dry", inert satellite. While scientists had considered the possibility of water on the Moon and there had been hints in previous missions such as Nasa’s Clementine in the 1990s, the broad presumption was that there wasn’t any water to squeeze.
All the samples from the US Apollo and Soviet Luna missions, which came from sites near the Moon’s equator, where it is easier to land, appeared to support this view. Chandrayaan-1 changed that. “Perhaps the most valuable result of these new observations is that they prompt a critical re-examination of the notion that the Moon is dry," an article in the journal Science, where papers related to instruments aboard Chandrayaan-1 were published in 2009, put it. It added pithily, “It is not."
Chandrayaan-2, launched in 2019, came at a time when Isro had already shown its technical prowess in planetary missions by successfully inserting an orbiter around Mars. But soft-landing on the Lunar South Pole had never been done before and a lander from an Israeli private company had, in fact, crashed a few months before the Isro lander’s planned descent. According to news reports, 13 minutes into the descent, during which the speed would have had to be brought down from over 6,000km per hour to 7km or lower, Isro lost contact with the lander. Had it reached the surface, there may have been significant behind-the-scenes tests apart from the planned scientific experiments.
Leading up to the Chandrayaan-2 mission, Isro was the first to publish papers on what it called passive survivability through the lunar night. This means that rather than trying to keep the batteries on the lander and rover warm, these are just allowed to get cold and freeze.
“The Isro work demonstrated that a cell type, common on many spacecraft, can tolerate freezing," writes Richard Oeftering at Nasa’s Glenn Research Center in Ohio, an engineer with over 40 years of experience who has been looking at power systems for space, on email. He adds, “and more remarkably, can thaw out and recover their original charge capacity."
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Nasa had done its own tests to validate these results and though it did not always work, it found that this could be a viable strategy. “The elegance of the hibernation approach is that the payload capacity to accommodate science instruments and experiments is largely unchanged, while the mission duration is dramatically extended," Oeftering adds.
It isn’t clear whether Pragyan and Vikram, aboard Chandrayaan-3, are using this particular approach, which would require a series of steps to allow powering up at dawn, including isolating the cells from the power bus till they return to normal temperature. But should the reawakening happen using this, or another, approach, it would cap a series of firsts for the space research organisation, and in a way truly herald a new era for lunar missions.
Racing to the Moon
Maybe it already is a new era. On the Moon, beside Cold War rovers like the US’ Apollo buggies and the earlier Soviet Lunokhods, crashed landers, like Beresheet from Israel as well as from other countries, roamVikram and the Yutu-2 or Jade Rabbit rovers from China.
There are several new entrants, too: Last year, South Korea launched its Danuri mission and the United Arab Emirates placed its Rashid rover on a Japanese craft that took off in December but lost contact. In the next month or so, a small satellite from Caltech, the Lunar Trailblazer, will launch. Meanwhile, if schedules stay on track, private companies Astrobotic and Intuitive Machines are planning to deliver Nasa payloads soon to the lunar surface.
India is planning its own astronaut mission, the Gaganyaan. This mission will take three astronauts to an Earth orbit of 400km before bringing them back. In August, Drogue parachute tests were carried out by Isro; these will be crucial in ensuring that the astronauts return safely to Earth.
In 2024, astronauts are expected to be back near the Moon after more than 50 years. This August, Nasa announced its Artemis II crew, which includes the first woman and first person of colour on a lunar mission. While this mission will fly by the Moon and not actually land, the eventual plan is to land: on the lunar South Pole.
For the moment, though, Pragyan and Vikram are virtually alone, having transmitted the most up-to-date data. Already, there have been tantalising discoveries, be it confirmation of the presence of sulphur, or a sharp temperature change centimetres below the lunar surface, not to mention reaching the South Pole itself, where no machine had landed on its feet till now.
It is a surreal, alien landscape. Debris from crashed landers lies strewn not too far away, craters cast long shadows, their rims, seen in images, inviting you to peer into the void, where sunlight does not hit the floor kilometres below, and has not for periods of geologic time.
To find elements like sulphur here, which could in a particular form indicate water, or regolith, which could be used as a thermal insulator, is to find valuable resources in a punishing environment that don’t have be sent from Earth.
Hydrogen from water can be used as a fuel, oxygen for breathing, and insulating bricks made of regolith to build. Resources are scarce on the Moon and we would rather use what is already out there than take it from Earth as an expensive payload. It is what proponents call in-situ resource utilisation, and it’s the way forward to human habitation.
Narendra Bhandari, former chair of the Planetary Sciences and Exploration Program at Isro, contends that the main aim of Chandrayaan-3 was to study the landing site for future missions. “The aim of this mission is not to look for water," he says. “It is to look for chemistry and minerals, soil characteristics and temperature profiles…"
Prospecting for water from a global perspective is best done by an orbiter. “We have a synthetic aperture radar with two bands on the Chandrayaan-2 orbiter," Bhandari says. “There was only one band in Chandrayaan-1." This allows the radar greater penetration into the lunar surface. With the lander, the immediate surroundings, a unique area geologically, can be studied. “This particular area where it has landed is very old," he says. “We call it the Nectarian Age, 3.9 billion years old."
Estimates of water abundance have already come from payloads aboard the Chandrayaan-1 and Nasa’s LCROSS (Lunar Crater Observation and Sensing Satellite) missions, more than a decade ago. There have been several orbiters since that have improved on the estimates. Planned crashes, such as that of Isro’s Moon Impact Probe, simultaneously brought confirmation from the surface. This drew from a suggestion made by the former president, the late A.P.J. Abdul Kalam.
LCROSS had its own dramatic planned crash in 2009. The spent upper stage of a rocket called Centaur, hurled into a crater at the South Pole, released tonnes of lunar material. LCROSS then flew into the debris plume, collecting data before it too crashed into the lunar surface. In turn, a third spacecraft, the LRO, recorded this data.
Nasa later announced that water crystals had been found inside the crater, and that the Moon has a water cycle. But going down these steep, forbidding craters, which have the permanently shadowed areas where water is thought to be most abundant, is a challenge for future missions.
The Moon’s spin axis, its axis of rotation, is oriented about perpendicular to the Sun. If you are standing at the pole, the Sun would be revolving around the horizon. “But if you are down inside a crater," says Paul Hayne, a geophysicist at the University of Colorado, Boulder, “then the sun is blocked on all sides by the walls of the crater."
Hayne has been studying whether there could be more accessible places called micro cold traps where water could be found. The idea was that maybe there were shadowed areas that were not being resolved from orbiters because of their smaller size. “The idea of the micro cold traps came about because we recognised that given these limitations in the spatial resolution of our existing orbital observations, we may be missing, you know, a big part of the story," he says.
As we go to smaller and smaller scales, the kind of scales seen if we stood on the surface, “little shadows" would be seen which could trap water. “Because there’s no atmosphere, those little shadows are just as good at trapping water as the big shadows," Hayne adds.
These variations in resolution are visible in the Vikram lander images, shadows that don’t get resolved from orbit. “Once you get below about a metre, then there’s a transition, you can see this in the Vikram lander images, where the surface texture becomes different," he says. “It doesn’t look like craters pockmarking the surface, it’s more of a general roughness, kind of like sandpaper."
Data from Vikram will help validate the models for potential micro cold traps at the particular latitude close to the South Pole. “It helps us to see the shadows to actually visually identify those shadows at different sun angles," Hayne says.
The presence of sulphur, as shown by Chandrayaan-3’s LIBS, or Laser Induced Breakdown Spectroscope experiment, would be very relevant too. Sulphur is found in the generic form of iron sulphide in rocks returned from the Moon. That would imply a mineralogical origin but it could also be present in a volatile form. “If it’s volatile sulphur, then it might be trapped in these micro cold traps," he says. “And so maybe the LIBS experiment is liberating some of those sulphur atoms that might be just stuck in these cold, micro cold traps."
Such micro cold traps, if they are confirmed, would be relevant to India’s future Moon missions, specifically Chandrayaan-4 or the LUPEX mission planned with the Japanese space agency JAXA. In a 2021 abstract from the annual Lunar and Planetary Science Conference, JAXA researchers state this directly.
The mission plan is to land a 350kg rover which will drill into the regolith. “The biggest challenge in a technology’s viewpoint is how to explore the large permanent shadowed region which was previously thought to be the major host of water ice," the researchers write, adding that “...recent computer simulations suggested that water ice is possibly present at the subsurface or in micro cold traps on the Moon, and majority of these traps are located at latitudes > 80°, which is the target of the LUPEX mission. And the rover may be able to explore one of the micro cold traps."
Shine on, you crazy rover
Vikram takes a nap and hops. Pragyan may wake up from its several-days sleep if it doesn’t catch frostbite. Anthropomorphising the landers and rovers seems natural in an unexpected way. To that image, add rovers chasing the Sun, or at least spots of sunlight, beginning with Nasa’s VIPER, or Volatiles Investigating Polar Exploration Rover, in late 2024.
The next missions by Nasa and Isro-JAXA plan rovers that will spend several lunar days on the Moon’s South Pole while still using solar arrays. Since space agencies are conservative, in that they plan missions only with tested technology, this longer mission life has to be enabled in a different way.
The Moon’s tilt gives it a seasonal variation that is extreme at the poles. The strategy for the forthcoming missions is to get more sun for their arrays. In doing this, the rovers will have to manoeuvre rough terrain such as crater ridges that cast long shadows. But traversing the circuit is worth it.
“By carefully planning the rover path," writes Oeftering, “the longest night may be as short as 90 hours instead of the 354 hours of a typical night." With batteries, the power requirements can be met. At the same time, the launch needs to be on time and landing, in a very specific location. These conditions have a seasonal aspect, too, for there is a lunar polar winter.
While the mission planning does not assume new battery technology, or approach, that again doesn’t mean it may not test it. The VIPER is to soft-land using Astrobotic’s lander. Astrobotic has been testing ways of wirelessly transferring power. Last year, it said in a press release that “during testing, the lightweight, ultra-fast wireless charging system proved it can transmit power in extreme hot and cold lunar temperatures".
Such a wireless system would deliver power from an Astrobotic lunar lander or solar arrays to power rovers, habitats, in-situ resource utilisation plants, and other large surface infrastructure to survive the lunar night, it added. JAXA, meanwhile, is working on a heat insulation system using a wireless power transfer system, as papers from the space agency show.
At the Nasa Glenn Research Center, which researches, designs, develops and tests innovative technology for aeronautics and spaceflight, researchers are studying the properties of a relatively new transistor material called Gallium nitride. This material works well at extremely low temperatures, when the lunar environment is at its coldest, writes Oeftering. It is able to manage the wild swings of temperature on the Moon, from an equatorial high noon to a polar winter dawn. “For restoring power after hibernating through the night," he writes, “we anticipate Gallium nitride circuits will successfully ‘cold start’ at extremely low temperatures at dawn and still be able to operate at high noon."
Once on the surface, the rovers will have to show extreme manoeuvrability. The VIPER has specially designed wheels for this. According to the Nasa website, it “will even be able to walk its wheels by moving each wheel independently to free itself" should it land in fluffy Moon terrain, another anthropomorphic image, like a person gingerly lifting their feet.
A key science aim of the VIPER is to drill into the lunar regolith. Hayne is looking forward to data from this experiment. He is also working on an experiment with the European Space Agency (ESA) called PROSPECT, to launch no earlier than 2025. There is evidence that water is buried more than a metre down, and PROSPECT hopes to reach that level.
Among the payloads on the same Nasa mission as PROSPECT will be a heat-sensing camera, the Lunar Compact Infrared Imaging System Hayne is involved in, that will take panoramic images of the landing site near the South Pole. “Our instrument is built. The hardware is sitting on a shelf here," he says. “So we are ready to go."
When the instrument launches, it would be through the Commercial Lunar Payload Program of Nasa, which aims to send a raft of lunar landers in the next couple of years. China, too, is planning a mission, Chang’e 6, in 2024; this will be its second sample return mission.
Bhandari, however, voices concern about the pace of Isro’s Moon exploration plans. “They are really going slow, that is my feeling," he says. “Two missions in 15 years is not the way to do it."
At the same time, while the next Moon mission for India may be LUPEX, likely in 2026, there are key technologies that its space exploration can build on. Within two weeks of the Vikram landing, the Aditya L-1 mission to study the Sun was launched. It is now into its four-month journey to park itself at the Lagrange 1 point. After the Mars Orbiter Mission (MOM), India’s first interplanetary mission to planet Mars, this is the farthest object Isro will slingshot into deep space.
By landing on the South Pole, meanwhile, Isro has already shown it has asteroid landing capabilities. “It is easier to land on an asteroid than on the Moon," says Bhandari, who has studied asteroid samples from other agencies.
Landing on the South Pole rather than the Equator was particularly intricate. As you go towards the poles, the ground is rotating very fast, he says. At the Equator, one degree covers an adequate area. “This is not so at poles. If you want to land at a pre-destined place, it is difficult at the poles compared to the equator."
To complete the process, Isro would in the end want to bring back samples from the Moon. This, too, is more difficult at the poles than near the equator, where there is additional rotational velocity due to the rotation of the Moon.
“That is the next big step," says Bhandari. “Right. So they had a hop also just to try to see whether they can, you know, jump." The jump could bring these samples back to Earth directly, or to a spacecraft in orbit, as with the Chinese Chang’e 5 mission, which then brings them to Earth.
Later this month, OSIRIS-ReX, the first Nasa asteroid sample return mission, will return a sample of the asteroid Bennu to Earth. Asteroids (like Bennu) are the “left-out" pieces out of which the planets were formed, as Bhandari puts it. They are time capsules from a period 4.5 billion years ago and can be studied to understand that period of solar system formation.
There is another instrument on the Chandrayaan-3 mission, now in lunar orbit, that hints at where Isro may go next. Stowed in the propulsion module, the Spectro-polarimetry of Habitable Planet Earth, or SHAPE, is like a video game Easter egg. Its antecedents go to an instrument aboard Venus Express, launched by the ESA in 2005, and it is pointed not towards the Moon but towards Earth.
To know the backstory behind this instrument would require standing back, almost literally.
The blue dot
Exoplanets are planets around stars other than the Sun, and now we know that most stars have planets around them, which implies there are billions and billions of planets. One of two main methods to detect these exoplanets is through dips in starlight as a planet comes around. Called the transit method, this is the method Nasa’s Kepler mission used to detect a few thousand exoplanets.
To find signatures of life, however, we need to study the atmospheres of these planets. With transit methods, the information coming through as the starlight hits the atmosphere is sparse. Moreover, if it has an orbital period like Earth, that is like having one data point in a year.
Precise instruments like the James Webb Space Telescope have been able to improve transit measurements, though in the menagerie of planets seen thus far, Earth analogues are far fewer. Now, there are plans being drawn to send telescopes, such as Nasa’s Habex, into space to directly image planets.
Daphne Stam at the Leiden Observatory in The Netherlands, who has worked on LOUPE, a spectro-polarimeter to observe Earth, with aims similar to SHAPE’s, says that for the near future these images will be like a pixel, and we will have to derive information about the planet. “For a very long time, it will not be more than one pixel," she says. On the other hand, the planets will likely rotate, offering a much faster time series.
For polarimetry, the angle of the incident light and the reflected light is crucial. But you cannot derive the polarisation signal over the whole Earth from very local measurements, she adds. Turns out, though, the Moon is well located for this. In a paper posted on the preprint Arxiv server, Isro researchers say, “The experiment is designed for observations from the Lunar orbit which allows capturing all the phase angles of Earth, mimicking the future observations of directly imaged exoplanets which could also be sampled for a large range of phase angles."
In computer simulations, Stam and her colleagues have derived the glint of oceans as sunlight hits them, and it is a strong signal. Clouds have also been accounted for in these models. But real data is required to validate them. Which is what the SHAPE instrument, a part of the Chandrayaan-3 propulsion module orbiting the Moon, can provide.
Work using polarimetry allows us to get information from underneath the clouds in a planet’s atmosphere. There is a historical precedent: Polarimetry allowed us to detect Venus’ sulphuric acid clouds in the 1970s. “If you only measure the brightness of Venus, you don’t know what the clouds are made of," says Stam. “They can be water clouds, they can be ammonia clouds, they can be anything. But with polarimetry, you know it is sulfuric acid clouds."
Measurements with polarimetry already exist for Venus. With data for Earth on its way, Stam says we will be able to look for similar signatures in the future. “This is Earth and this is Venus, and in polarisation you can see the difference very clearly."
Observing Earth from the Moon brings its own shift in perspective. On the Moon, you will see an almost full Earth, you will see a crescent Earth, and you will see half an Earth. While Stam and her colleagues looked at signals from oceans, it may be possible to detect other changes in the polarimetry signal as a planet rotates. “The leaves of the trees change colour or lose their leaves and so the polarisation changes…," she says, or “it may be the snow cover that changes".
In her presentations, Stam often ends with an image of the Pale Blue Dot, a 1990 photograph of Earth taken from the Voyager 1 probe, launched in 1977, then more than six billion kilometres away. A rotating Earth, captured in vibrating photons to find signatures of other similar planets in a pixel, is not the same. Nevertheless, it carries a haunting quality.
Virat Markandeya is a science journalist based in Delhi.
- FIRST PUBLISHED15.09.2023 | 05:00 PM IST