Indian Space Progress #22: How Chandrayaan 1, 2 and 3 leveraged their view from the Moon

Thanks to the great reader responses on all recent special editions of my Indian Space Progress blog+newsletter, such as the one on ISRO’s Venus orbiter, the merging of Chandrayaan and Gaganyaan, and challenges for rocket startups, I’m doubling down on this theme-based approach of capturing trajectories of Indian space. In this edition, I highlight an exciting aspect of lunar exploration that very few know or talk about, and top it up with the recent updates on Chandrayaan 4 and 5. Early next year, expect Indian Space Progress to do thematic, news-tied dives into space startups, ISRO’s satellite fleet, and more.

That said, here we go 🌏...........................................🛰️🌗

Going to the Moon isn’t only about lunar exploration. China has long repurposed their Mooncraft to enable deep space exploration in multiple ways. The US scientific community is keen to conduct radio astronomy from the Moon, starting with unique cosmological measurements to be made by an upcoming NASA-funded robotic lander. And, in the future space agencies and companies hope for the Moon to be a literal launchpad to other worlds in the Solar System. India has added three more things to this list that leverage the Moon’s vantage point. Its ISRO Chandrayaan craft have viewed a solar eclipse, studied the Sun’s flares, and observed Earth as an exoplanet, all from Luna.

Capturing a solar eclipse from the Moon

This is a story shared with me by the late Srinivasa Hegde, the Mission Director of India’s first lunar orbiter Chandrayaan 1.

When the spacecraft imaged the total solar eclipse of July 22, 2009, it was not in a nominal state. In April 2009, only about five months post-launch, one of the two star sensors on the orbiter had failed. Shortly after, exposure to extreme solar radiation combined with other factors took out the backup star sensor too. Having lost both its star sensors, Chandrayaan 1 lost the ability to precisely point to a desired attitude in space. A mission failure was looming.

Instead of calling quits, ISRO engineers came up with a solution. Chandrayaan 1 mission operators used the spacecraft’s Sun sensors to get knowledge of two spatial axes, and then fixed the third axis with aid from communicating ground stations. With this inferred information in hand, operators utilized the orbiter’s gyroscopes to then point the spacecraft again where needed with reasonable accuracy. The mission was back up and running.

However, this method came with its restrictions. For instance, it could not be used during a New Moon or Full Moon because the Earth and Sun wouldn’t be at different angles for the craft. On the other end, using the Earth-Sun angle was restricted to a maximum of 25° to allow reasonable accuracy in pointing, and thus in spacecraft operations.

While limited in scope, this method proved reliable enough to work for about six months after the failure of the critical star sensors, culminating in a good chunk of the mission’s many discoveries. It’s also is under such conditions that Chandrayaan 1 captured the century’s longest solar eclipse from the Moon. Being far away from Earth, and at the very planetary body causing the eclipse, the spacecraft was at a convenient vantage point to image the Moon’s totality shadow cast on Earth. The only problem? Capturing the shadow would require accurate spacecraft pointing, which the aforementioned method would fall short of in providing by itself.

Mission operators came up with yet another solution. To image Earth during the eclipse, ISRO would use Chandrayaan 1’s Terrain Mapping Camera, which had a Field of View of 11°. The spacecraft was thus made to pitch at a slow rate to scan the Earth between +/- 10°. To account for the error in the antenna pointing mechanism, operators also made the spacecraft roll by +/- 3°.

The resulting motion and pointing of the spacecraft would thus ensure that the totality shadow would be somewhere within the view of Chandrayaan 1.

Operators were ready to image Earth on the day of the solar eclipse. But they realized there was yet another problem. If the spacecraft downlinks to the ground station during the eclipse, executing real-time commands may not be reliable due to bandwidth and time uncertainties, which could mean missing capturing the short-lived eclipse shadow. This is why what mission operators did was send time-tagged commands to Chandrayaan 1 a-priori. The spacecraft then automatically executed all those commands sequentially during the eclipse period.

It worked. As the spacecraft passed between the Moon and Earth in its lunar orbit on July 22, 2009, it imaged Luna’s shadow cast on our planet. It was a first for any lunar mission at the time, one executed with a spacecraft in a non-nominal state. Below is a GIF of the view.

A Sun watcher at the Moon

India’s Chandrayaan 2 orbiter doesn’t only study the Moon’s surface and aid in surface lunar exploration but also observes the Sun. Scientists use its high-resolution Solar X-ray Monitor (XSM) to study solar flares. In turn, it provides a reference for the orbiter’s Chandrayaan-2 Large Area Soft X-ray Spectrometer (CLASS) instrument to map elements on the Moon’s surface.

Scientists have published multiple results based on unique observations of the Sun’s surface and atmospheric activities by XSM, including measuring many micro-flares, nano-flares, and background X-ray emissions.

XSM’s detections of micro-flares and nano-flares in particular is important because scientists think they’re relevant to unlocking a fundamental mystery about our Sun: why is its extended atmosphere, the corona, much hotter than its surface? Scientists have been debating since the 1940s how the Sun’s atmosphere is heated to a million degrees Celsius while the surface remains barely around 6,000 degrees. Close-up observations of many tiny eruptions across the Sun’s surface by ESA’s Solar Orbiter mission coupled with coronal measurements made by NASA’s Parker Solar Probe have helped scientists almost zero in on solving the coronal heating mystery. Micro-flare and nano-flare observations by other spacecraft from different vantage points like that of the Chandrayaan 2 orbiter help scientists contextualized and refine these results to improve our understanding of the Sun.

Protecting future lunar explorers

In addition to XSM, the Chandrayaan 2 orbiter’s CLASS instrument can detect some solar events too. In January 2022, CLASS detected two highly energetic proton emission events in the solar wind, the incessant stream of charged particles emanating from the Sun. NASA’s GOES-16 satellite couldn’t detect one of these two events because Earth’s magnetic field shielded it from said particles. The Chandrayaan 2 orbiter being at the Moon could detect them, just as other solar observers outside Earth’s magnetic field could.

From August 4 through 7 in 1972, the Sun blurted several bursts of flares and associated energetic particles between the Apollo 16 and 17 missions to the Moon. Had the astronauts been in lunar orbit or on the surface, they could’ve faced damaging levels of radiation. This could, in turn, lead to increased cancer risk. As we prepare to send astronauts on much longer Moon missions and beyond, we’ll need to protect our explorers from such solar flaring and energetic particles that reach the Earth-Moon space in a matter of hours. The Chandrayaan 2 orbiter is aiding in the same by improving our understanding of solar flares as well as the local nature of solar energetic particle events at the Moon.

Many thanks to the Takshashila Institution, PierSight, Gurbir Singh and KaleidEO for sponsoring this month’s Indian Space Progress edition. If you too appreciate my efforts to capture true trajectories of Indian space, kindly join them and support my independent writing.

Sponsored job listings: Following up on its upcoming Varuna demonstrator mission, PierSight is hiring a Senior FPGA Engineer, an Embedded Hardware Engineer, and a Power Electronics Engineer to join their Ahmedabad-based team in building a constellation of SAR and AIS satellites to provide persistent monitoring of all human and industrial activities at sea.

Observing Earth as an exoplanet

The Chandrayaan 3 mission’s propulsion module, which ISRO pulled from lunar orbit to Earth orbit late last year for testing, carries the Earth-observing experiment called SHAPE. While it was in lunar orbit, it observed the full Earth disk to improve our understanding of what the spectro-polarimetric signatures of distant Earth-like habitable exoplanets might look like. With such data, scientists can infer (known) physical properties of Earth, and from it those of Earth-like exoplanets we discover such as the strength and direction of their magnetic fields, atmospheric composition including presence of water and methane, and so on.

While more distant spacecraft than Chandrayaan 3 such as NASA’s Galileo Jupiter orbiter have conducted full disk Earth studies before, their observations had limited phase angles. On the other hand, Earth-based observations get affected by depolarization whereas Earthbound satellites cover limited spectral range. The Moon thus offers a unique vantage point from which to study Earth as an exoplanet. SHAPE’s observations allowed scientists to capture Earth-reflected sunlight over a broad spectral range at all phase angles and so in two polarization directions, thus feeding uniquely into the worldwide hunt for another Earth.

ISRO Chief S. Somanath has said that the agency has completed all of SHAPE’s baseline observations. However, unlike other scientific measurements from the Chandrayaan 3 mission, ISRO hasn’t yet publicly shared any data samples or results from SHAPE. With papers from the mission coming in at a rather slow pace, hopefully SHAPE shows up soon and in good shape.

More on Chandrayaan 4 and 5

Some more details on the first two of ISRO’s increasingly complex Chandrayaan missions have surfaced thanks to the talk by Chandrayaan 3’s Project Director Palanivel Veeramuthuvel at the 2024 International Astronautical Congress (IAC) in October:

• The landing site for the Chandrayaan 4 sample return mission will be somewhere between 85–90° on the Moon’s south pole, putting it squarely in the water-hosting polar region as opposed to the 70°S for Chandrayaan 3. Relatedly, Chethan Kumar recently reported that Chandrayaan 4’s preliminary design review is not yet complete.
• To buy down risk for Chandrayaan 4, which involves robotically docking many modules, there will be not one but three SPADEX satellite docking missions in Earth orbit—first in circular orbit, then elliptical. Chethan Kumar reports that first of these is targeted for launch as early as December 20. As a tangent, SPADEX will also reduce risk for the upcoming Gaganyaan human spaceflight missions, particularly for the end-of-decade cargo flight to the International Space Station and one to India’s first space station module.
• The landing site for the ISRO-JAXA LUPEX / Chandrayaan 5 mission could be 89.45°S, 222.85°E, which lies on an elevated ridge connecting the Shackleton and de Gerlache craters. The site has several permanently shadowed regions in its vicinity so that the LUPEX rover can directly study the nature, accessibility, and abundance of potential water ice deposits there as intended.

Also see: The actual status of LUPEX / Chandrayaan 5

In preparation for Chandrayaan 4, the ISRO-affiliated PRL institute conducted an in-person inaugural workshop for students mid-November to teach them via lab visits and hands-on sessions how to handle and analyze space and planetary samples. PRL’s Director Anil Bhardwaj stressed at the accompanying conference, which was also held in person and one I attended, that more such workshops are planned not just for students but for professional scientists across the country since realizing Chandrayaan 4 necessitates building national capacity to thoroughly prepare, store, curate, characterize, and analyze the first set of space samples to be fetched by India.

Even more Chandrayaan

• I was a guest on Carnegie India’s podcast Interpreting India. We discuss where India’s Moon exploration plans are heading, and what are the enablers and constraints on the increasingly complex road for ISRO to send an Indian to the Moon. Give it a listen on Spotify, Apple Podcasts, YouTube, or below:

• The seismometer on the Chandrayaan 3 lander, called Instrument for Lunar Seismic Activity (ILSA), was the first since the Apollo era decades ago to measure moonquakes. Here is the paper and an archived PDF. In the 12 days of its operations from August 24, 2023, ILSA measured 50 seismic events, each lasting several seconds. While ILSA was not meant to detect prolonged deep and shallow moonquakes—which would reveal insights about the Moon’s interior—due to the mission’s design life being just one lunar day, it’s helping scientists better understand micrometeorite impacts on the surface at high latitudes. The amount and rate of such impacts is not well constrained at the moment but is necessary to ensure safety of future human lunar polar explorers by designing protective suits and habitats accordingly. Furthermore, ILSA detected 200 signals correlated to known activities of either the lander, its instruments, or traverses of the Chandrayaan 3 rover. Such data will help engineers design safe lunar polar infrastructure.

• ISRO, in partnership with organizations like Protoplanet, has formally started terrestrial testing of what living on the Moon and Mars could be like for astronauts via baseline analog missions at dry, mountainous Ladakh.