BENGALURU, India\u2014Todo el descenso lunar de Chandrayaan-3 ten\u00eda que ser completamente aut\u00f3nomo.\u00a0Durante esta etapa crucial de la misi\u00f3n, las se\u00f1ales tardan unos tres segundos en ir desde el m\u00f3dulo de aterrizaje hasta la Tierra y regresar, un retraso demasiado largo para que los ingenieros de ISRO en tierra gu\u00eden el aterrizaje de manera confiable.<\/p>\n
BENGALURU, India\u2014Quiet moments of nail-biting tension gave way to cheers of joy in the Indian Space Research Organization (ISRO) mission control center as the space agency sent its lunar lander\u2014and India\u2014into the annals of history. On August 23 at 12:33 P.M. UTC\u00a0India\u2019s\u00a0Chandrayaan-3<\/a>\u00a0mission\u2019s robotic lander, named Vikram, touched down on the moon near its south pole.\u00a0Launched<\/a>\u00a0on July 14, Chandrayaan-3 was the result of ISRO doubling down on its bet on lunar landing after the unfortunate\u00a0crash<\/a>\u00a0of its\u00a0Chandrayaan-2<\/a>\u00a0mission in 2019. With the spacecraft now safely on the moon, ISRO\u2019s efforts have paid off, and India has become the fourth country to achieve a soft lunar landing, following the former Soviet Union, the U.S. and China.<\/p>\n Chandrayaan-3\u2019s entire lunar descent had to be fully autonomous. During this crucial stage of the mission, signals take about three seconds to go from the lander to Earth and back again\u2014a delay too long for earthbound ISRO engineers to reliably guide the landing. So Vikram\u2019s task was to reduce its high orbital velocity to zero such that it would stay as close to its intended trajectory as possible, all the way until a safe touchdown. To do so, it needed to orchestrate the firing of its engines based on continuous measurements of distance, velocity and orientation.<\/p>\n To stick the landing this time around, ISRO built far more redundancies and safeguards into Chandrayaan-3 than it had for Chandrayaan-2. In an\u00a0August 5 talk<\/a>\u00a0detailing these changes, ISRO\u2019s chief S. Somanath emphasized how Chandrayaan-3 carried more fuel and a better guidance, navigation and control system to correct even major deviations from the intended paths. \u201cThere were improvements to 21 subsystems for Chandrayaan-3. These changes have been reinforced by numerous helicopter- and crane-based ground tests,\u201d says\u00a0Nilesh Desai<\/a>, director of ISRO\u2019s Space Applications Center (SAC) in Ahmedabad, India.<\/p>\n Evidently, these improvements have culminated in the triumphant touchdown of Chandrayaan-3. This success wasn\u2019t a given, especially when considering that four out of the previous six lunar landing attempts within the past five years have failed. The latest failure occurred on August 19, when Russia\u2019s Luna-25 spacecraft misfired its engines and\u00a0crashed into the moon<\/a>\u2014a brutal reminder that getting to the lunar surface in one piece remains risky. Luna-25 thus joins the ruins of the Israel-based company SpaceIL\u2019s\u00a0Beresheet<\/a>, India\u2019s\u00a0Chandrayaan-2<\/a>\u00a0and the private Japanese firm ispace\u2019s\u00a0Hakuto-R<\/a>\u00a0spacecraft. Thankfully, at least Chandrayaan-3\u2019s outcome has instead followed those of China\u2019s\u00a0Chang\u2019e 4<\/a>\u00a0and\u00a0Chang\u2019e 5<\/a>\u00a0landers, the only other recent successes.<\/p>\n \u201cWe now have a tremendous responsibility to inspire India and the world at levels no less than this landing,\u201d said Sankaran Muthusamy, director of the U. R. Rao Satellite Center (URSC), the ISRO center that led the construction and integration of the Chandrayaan-3 spacecraft and mission.<\/p>\n HOW CHANDRAYAAN-3 MADE IT TO THE MOON<\/strong><\/p>\n Chandrayaan-3\u2019s about\u00a019-minute-long lunar descent<\/a>\u00a0comprised four major phases. The first, the \u201crough braking\u201d phase,\u00a0began<\/a>\u00a0when the spacecraft was 30 kilometers above the moon in its orbit and about 750 km downrange from its landing site. By firing all of its four 800-newton main engines for about 12 minutes until it was at a 7-km\u00a0altitude, Chandrayaan-3 reduced its high horizontal velocity of about 1.7 kilometers per second by some 80 percent.<\/p>\n Next came a brief but crucial 10-second\u00a0\u201cattitude hold\u201d phase, wherein the lander stabilized itself using its eight smaller thrusters to gain a steady view of the looming lunar surface for its various\u00a0landing sensors<\/a>.<\/p>\n For height measurements, Chandrayaan-3 relied on two altimeters, one using lasers and the other using microwaves. While laser altimeters are commonly employed by several lunar landers, they can report anomalous heights at times if, say, a lander passes over mountainous terrain or large craters. \u201cInstead the microwave altimeter\u2019s wider footprint allowed Chandrayaan-3 to better tolerate abrupt changes in altitude,\u201d explains\u00a0Priyanka Mehrotra<\/a>\u00a0of SAC, who is lead system designer of Chandrayaan-3\u2019s Ka-Band microwave altimeter.<\/p>\n WHERE PAST LANDINGS FALTERED<\/strong><\/p>\n Chandrayaan-3\u2019s redundant altimetry is especially pertinent because of the role laser altimetry played during the\u00a0failed April 25 touchdown<\/a>\u00a0of ispace\u2019s\u00a0first lunar lander<\/a>. As that lander passed over the rim of the Atlas Crater to approach the\u00a0target landing site<\/a>\u00a0that lay within, its laser altimeter correctly reported an increased elevation of roughly 3 km, corresponding to the crater\u2019s depth. But onboard software designed to filter out certain abrupt values to keep the ispace lander\u2019s motion stable rejected the measurement as erroneous. The Japanese lander, thinking it was closer to the surface than it really was, continued decelerating slowly until it ran out of fuel and fell to a ruinous crash landing.<\/p>\n It was during the attitude hold phase that Chandrayaan-2 faltered. Its engines provided a\u00a0slightly greater thrust<\/a>\u00a0than expected because of an inadequately functioning thrust control valve, which accumulated navigation errors over time. ISRO had designed the onboard computer to correct such \u201coff-nominal\u201d paths only after the attitude hold phase ended. But the deviation quickly grew to be so large that the lander couldn\u2019t correct it in time despite its ability to throttle its thrust.<\/p>\n In response, ISRO ensured that Chandrayaan-3 could determine and correct such deviations from its intended trajectory far faster than its failed predecessor. Chandrayaan-3\u2019s lander also used a new instrument called a laser doppler velocimeter (LDV) to navigate more precisely in the first place. \u201cWhile there are other ways for a lunar lander to measure its velocity, an LDV provides a direct measurement of velocity with respect to the ground, which allows a lander to greatly reduce accumulation of navigation errors,\u201d says\u00a0William Coogan<\/a>, lunar lander chief engineer at Firefly Aerospace, a private company that has partnered with NASA via the space agency\u2019s\u00a0Commercial Lunar Payload Services (CLPS) program<\/a>\u00a0to deliver science and technology payloads to the moon in\u00a02024<\/a>\u00a0and\u00a02026<\/a>.<\/p>\n A FINE HOVER OR TWO<\/strong><\/p>\n After its fraught attitude hold phase, Chandrayaan-3 entered a three-minute \u201cfine braking\u201d\u00a0<\/em>phase in which it used only two of its four main engines to descend up to roughly 850 meters above the moon\u2019s surface and briefly hover there. This pause gave the lander a chance to capture pictures of the surface and compare them to preloaded onboard satellite images to determine whether it was above its desired landing region.<\/p>\n \u201cChandrayaan-3\u2019s target landing zone spans four by 2.5 kilometers. ISRO scientists and engineers divided it into 3,900 equal-sized subsections, meticulously assessed the safety level of each\u00a0for a landing and loaded it into the lander as reference information,\u201d Desai says. At this point, Chandrayaan-3 must have taken one of these two decisions: If it found itself above this predetermined landing zone, the onboard computer would have identified the safest feasible subsection area, then accordingly proceeded toward touchdown. If Chandrayaan 3 found itself elsewhere, it would have proceeded with an autonomous landing based on self-identified hazards from its imagery instead of the preprogrammed subsection-based landing. Confirmation of which decision was taken will be known after ISRO determines the landing site.<\/p>\n In the final \u201cterminal descent\u201d phase, Chandrayaan-3 lowered itself to about 150 meters\u00a0above the surface and then hovered again for about half a minute to assess the area below for landing hazards. At this point, since the surface right below the lander didn\u2019t look safe, the lander sought a safer adjacent area and deviated to touchdown there.<\/p>\n \u201cThe processing system for hazard avoidance was sped up for Chandrayaan-3 to make the lander\u2019s decision-making during the critical final phases significantly faster than Chandrayaan-2,\u201d says\u00a0Rinku Agrawal<\/a>\u00a0of SAC, who led the team that developed the processing unit of the hazard detection and avoidance system.<\/p>\n \u201cHazard detection and avoidance allows for a critical divert maneuver if needed during the final moments to ensure a safe touchdown,\u201d says\u00a0Ander Solorzano<\/a>, flight director of aerospace company Astrobotic Technology\u2019s first moon landing mission, which will carry NASA\u00a0CLPS<\/a>\u00a0and\u00a0international<\/a>\u00a0payloads.<\/p>\n Finally, on touchdown, sensors on the lander\u2019s legs triggered the shutdown of its main engines. Chandrayaan-3 now stands tall on the moon.<\/p>\n ISRO designed the lander\u2019s legs to absorb most of the mechanical shock from the touchdown. The agency tested the legs on lunar simulant test beds on Earth to ensure that the lander could tolerate a high vertical velocity of three meters per second\u2014and even a horizontal velocity of one meter per second if it were to touch down askew.<\/p>\n \u201cThe touchdown was smooth; the vertical velocity was notably less than even the nominal upper bound of 2 meters per second,\u201d said ISRO chief S. Somanath in a post-landing press event.<\/p>\n Chandrayaan-3 landed near the lunar south pole shortly after local sunrise. Doing so maximizes the mission\u2019s surface operations lifetime to an entire period of lunar daylight (14 Earth days) because the lander and the rover it will deploy are both solar-powered. To begin Chandrayaan-3\u2019s\u00a0surface science mission<\/a>, Vikram will activate its four onboard instruments and deploy the rover via a ramp to start exploring the\u00a0geologically rich landing region<\/a>.<\/p>\n INDIA\u2019S NEXT MOONSHOT<\/strong><\/p>\n Chandrayaan-3\u00a0feeds<\/a>\u00a0into the\u00a0global frenzy<\/a>\u00a0of sending hardware to the moon, particularly to its south pole. The U.S.\u2019s upcoming Artemis crewed missions, China\u2019s Chang\u2019e robotic craft and the majority of other governmental as well as private endeavors (such as those under NASA\u2019s CLPS program) plan to explore this valuable lunar region. They\u00a0eventually aim<\/a>\u00a0to extract its\u00a0water ice<\/a>\u00a0and other resources to sustain long-duration missions and perhaps even to commercialize aspects of such operations.<\/p>\n It was thus quite the timing when, on June 21, India\u00a0signed<\/a>\u00a0the\u00a0Artemis Accords<\/a>, a U.S.-led framework for cooperative lunar exploration. As a signatory, India can now accelerate its lunar endeavors by better collaborating with the U.S. and other signatory nations. Astrobotic CEO\u00a0John Thornton<\/a>\u00a0says, \u201cI\u2019m encouraged by India\u2019s signing of the accords. It\u2019s certainly a signal for extended partnerships and co-developments between the two countries. The more we can do that as a species, the better chance we have of succeeding together.\u201d<\/p>\n For its next moon mission\u2014targeting launch before the end of this decade\u2014India may partner with Japan, another Artemis Accords participant. The pair\u2019s planned\u00a0LUPEX rover<\/a>\u00a0would directly study the nature, abundance and accessibility of\u00a0water ice<\/a>\u00a0on the moon\u2019s south pole and could provide vital data for future crewed missions launched there as part of NASA\u2019s\u00a0Artemis<\/a>\u00a0program. \u201cLUPEX requires a more precise touchdown with a much bigger lander. Chandrayaan-3\u2019s success will act as a stepping stone toward India building LUPEX\u2019s lander and thus playing a key role in the future exploration of our moon,\u201d says S. Megala, deputy director of ISRO\u2019s lunar science and exploration program.<\/p>\n First, however, India\u2019s government must formally approve the nation\u2019s involvement. (Japan has already given the green light for its own contribution.) And in the meantime, Japan will launch another lunar mission of its own: the nation\u2019s Smart Lander for Investigating Moon (SLIM) is slated for liftoff on August 26, with a goal of lunar touchdown later this year to demonstrate new technologies for precise and affordable moon landings amid complex terrain.<\/p>\n