DNA Explainer: How ISRO’s lunar orbiter Chandrayaan-2 avoided collision with NASA’s LRO

According to the Indian Space Research Organization (ISRO), the Chandrayaan-2 Lunar Orbiter recently performed a Collision avoidance manoeuvre (CAM), to mitigate the risk of passing very close to NASA’s Lunar Reconnaissance Orbiter(LRO). As both lunar orbiters are studying the moon from a Polar orbit(flying in a manner that nearly covers both the north and south poles), they come fairly close to each other near the Lunar poles.

However, in the recent case, calculations by both ISRO and NASA showed that the radial operation between the spacecraft would be less than 100 meters and the closest approach distance would be 3km, at a certain time. This close approach was predicted to occur on 11:15 am IST, on October 20, near the Lunar North Pole. 

Thereafter, both agencies mutually agreed that the Indian orbiter would undergo the Collision avoidance manoeuvre. “The manoeuvre was scheduled on 18th October 2021, it was designed to ensure a sufficiently large radial separation at the next closest conjunction between the two spacecraft. The CAM was executed nominally at 14:52 UTC (8:22 pm IST), on October 18th” said ISRO. The Indian Space agency also added calculations based on the new orbit confirmed that there would be no further close passes in the near future. 

CAMs are performed to mitigate the collision risk of satellites in orbit with other spacecraft or with space debris. However, such manoeuvres are also considered quite resource-hungry, as the satellite will have to expend its onboard liquid fuel(stored in a limited quantity) to perform the engine firing(manoeuvre). Since satellites are in space for a specified mission time, such unplanned firings of their engines can reduce their mission life, owing to the loss of precious onboard fuel. 

ISRO regularly monitors such critical close approaches and execute CAMs for its operational satellites, whenever the collision risk is assessed to be critical. However, this is the first time such critically close conjunction was experienced for a space exploration mission of ISRO. Earlier, the Russian Space Agency had revealed that India’s Cartosat 2F weighing over 700kg dangerously approached the Kanopus-V spacecraft on November 27th, 2020. In a tweet, Roscosmos has said that the minimum distance between both Russian and foreign satellites was 224 meters. Both the satellites are meant for earth’s remote sensing. 

How significant and dangerous is a close-pass in space? 

A highly-placed source had earlier told Zee Media that, 1 Kilometer is an ideal distance between satellites in orbit, whereas anything less than 300 meters is scary and can be counted as a near-miss. Generally, when two satellites are predicted (based on calculations) to make a close pass, a decision is taken to manoeuvre one of them away in advance(usually days ahead). 

Given how space is getting increasingly crowded with satellites that are used for various purposes, it is said to be normal for top space agencies to manoeuvre one of their satellites every 3-4 weeks. It is notable that low Earth orbit (500-2000km) is the most congested, with satellites of various sizes overpopulating it - ranging from 10cm cubes to ones that are the size of a car or larger. 

However, the decision to manoeuvre is not very easy. Especially, when the satellite is performing a strategic role that requires it to be at a particular spot. Because the manoeuvre would affect the schedule of the pre-planned operation of surveillance (imaging or using other sensors). In the earlier case, Roscosmos had said that both Indian and Russian spacecraft were remote sensing satellites, which implies that they were in all likelihood being used for strategic purposes. 

The space community has been divided over the prediction models for tracking satellites in orbit. The existing ones are the European Model, American model, Russian model, and Indian model(fairly recent). The reason that the space community hotly debates the efficiency of the models is that there are differences between the calculations made using each model. 

Let’s assume, the Indian model said that the satellites would be 1km apart, whereas the Russian model would have predicted that they would be 500m apart. It is also highly likely that in reality, both models went wrong and the satellites get closer than the prediction showed. Hence, there is no single model that works accurately, as each one can provide a different result. 

The real danger posed by a possible collision of satellites would be the debris of varying sizes scattered across space in all directions, in high speeds with high potential of harming other satellites and even future missions to that respective region.

The last large collision of satellites (in earth orbit) happened in 2009, when a US commercial Iridium spacecraft hit a defunct Russian satellite over Siberia, producing thousands of pieces of debris. Presently, there are over 2,000 active satellites orbiting the earth. There are also more than 23,000 pieces of debris larger than 10cm (4inches) in orbit, according to NASA.

Space Situational Awareness (SSA)

Space Situational Awareness (SSA) is the science of tracking objects (man-made and natural) that are in orbit and also predicting whet they would be at a given point in time. So far, only America, Russia and Europe have similar established capabilities in place to track space objects and share collision warnings. 

In late 2020, the Indian Space Research Organization (ISRO) had set up a dedicated Control Centre to monitor, track and protect India’s space assets and coordinate between Indian agencies, their foreign counterparts and international bodies. The directorate of Space Situational Awareness and Management (DSSAM) has been established at ISRO to enhance Space Situational Awareness.

An entity focused on SSA is much-needed owing to the manner in which space is getting over-populated with government, private satellites, man-made space debris which pose a serious threat to satellites that are orbiting the earth. 

According to ISRO, NEtwork for space object TRacking and Analysis (NETRA) project is initiated as a first step towards meeting this goal. NETRA’s main elements would be a radar, an optical telescope facility, and a control centre. ISRO’s SSA Control Centre, “NETRA”, is now set up within the ISTRAC campus at Peenya, Bangalore.

Some core SSA activities will comprise close approach analysis between ISRO’s satellite and launch vehicles, timely dissemination of advance alerts on collision avoidance of space assets, prediction of atmospheric re-entry of derelict satellites and rocket bodies.