China claimed recently that they have successfully tested Anti-Ballestic Missile System that can intercept missile like India's present Agni-5 ICBM (Range : 5,500-8,000 KM) & future Agni-6 (Range : 6,500-10,500 KM), but Agni-5 Missile development team's Head Mr. Avinash Chander have already claimed with firm belief that nobody can intercept Agni-5 & Agni-6 ICBM including existing Interceptor Missiles of USA & the under-development interceptor missiles like China's....& US experts have also remarked that India's Agni-5 & Agni-6 ICBM will be the two most dangerous missiles in the world in future having a speed of 30+ Mach.......while India's under-development Anti-Ballestic Missile System will be capable of intercepting missiles having more than 25 Mach, missiles having more than this speed may also be intercepted, DRDO is busy to develop that capability to destroy missiles 5,000 Km away with Mach 30 speed.....so, we need to have faith on our scientists in DRDO, they are working fantastically, I'm proud of them
Missile interception is a broad term describing a class of rather varied tactics, techniques, and weapons, so I'll try to be brief here.
First off, a no ballistic missile is 100% intercept-able or 100% un-intercept-able. Even a 1950s era Scud can still strike if it is launched in a depressed trajectory at an extremely close target, and even the most complicated ballistic missile is easy to track through the boost phase (when the ballistic missile is still 'climbing'), if one can get an anti-aircraft system in range of the ballistic missile launcher. (This is why you will often hear arms salesmen saying so-and-so SAM missile can shoot down ballistic missiles - every SAM system can, even a 60s era SA-2, if it is within 40 km of the ballistic missile launcher itself.)
Now, for the specific situation of Agni5 and Agni6 versus China or the US:
First, neither country will be able to get anti-aircraft systems into range of them during the boost phase, and satellite-based lasers that constantly orbit over India are at least a decade away, so boost-phase intercept is out.
Mid-course intercept, or when the ballistic missile has finished gaining altitude and is gliding through the upper atmosphere / lower reaches of space, is the type of intercept that Avinash Chander is referring to. Mid-course intercept is very difficult. So far, the US Ground-Based Midcourse Defense (GMD) system only has a 53% success rate in its tests against less than a dozen dummy targets at a time, not the hundreds or even thousands of warheads a China/Russia/India would toss into the air - and the US GMD system is probably the most advanced one, since the Chinese equivalent has only been tested on slow-moving satellites without even any decoys around them, and Russia has no midcourse defense program at all.
Hence, Chander is correct when he says existing US and Chinese midcourse defense systems could not intercept the Agni 5 and Agni 6. However, he is incorrect when he attributes that fact to their speed, since speed is more a factor for the third phase of intercept - terminal defense. We will touch on that point later.
Trying to hit something in midcourse requires extremely powerful, phased array radars - often spaced hundreds or even thousands of km apart, each consuming as much electricity as a small town, and connected with ultra-low-latency fiber optics - as well as missiles as fast as (or even faster than) ballistic missiles themselves.
Countering midcourse defense involves chaff - tons of aluminum and mylar confetti, balloons, ball bearings, and even empty dummy warheads, often weighing as much as the warheads themselves - and "dancing" rather than speed, or being able to change the sub-orbital trajectory of the warhead "bus" or warhead itself in the lower part of space while ensuring the warhead will still strike the target after each new reorientation. Ideally, this dancing is done using a computer and guidance system on the warhead itself, since it is reasonable to assume the target nation will be jamming earth-to-space communications (and GPS signals across the globe) like crazy if it is under nuclear attack. This is why radiation- and acceleration-hardened microchips are so important, since most chips cannot stand unshielded solar radiation and rocket acceleration. This is also why Russia, China, and the US tracks the brightness and position change of all the stars visible to the naked eye, since matching the position of those millions of stars to an internal simulation enables you to figure out where you are above Earth without GPS signals, and it is practically impossible to fool a camera looking away from the earth at stars, since you'd need to spoof thousands of stars at a point higher than 200km above earth.
Now, onto terminal defense, or trying to shoot down the warheads when they are descending through the atmosphere towards the target. Here is where warheads pick up a ton of speed (and heat - they literally look like meteors falling through the sky) and can try various looping (but pre-programmed) maneuvers to hit the target - or glide, in the case of the Chinese WU-14 and US ISINGLASS, Prompt Global Strike, X-51 Waverider, etc. This is as difficult, or even more difficult, than midcourse defense. Most interceptors of this kind today - the S-300 and S-400, the SM-3 Block 3, the Patriot 4, etc - have a radar and missile range only 200km-500km in any direction, including up, and their missiles fly slower than a descending ballistic warhead. Hence, you need the "top" of your "engagement bubble" to be as close as possible to the intended target, which means your interceptors have to be very, very close to the intended target - sometimes literally parked on the target themselves, which is why US, Russian, and Chinese national command centers are all easy to spot, even though they are completely underground - just look for cluster of 24 S-400 or Patriot launchers and tracking radars in the middle of nowhere.
Overall, Chander is simplifying things when he says that the Agni-5 and -6 are un-interceptable. Most ballistic missiles under most circumstances are. However, he is wrong when he makes their speed something to be proud of. A slower warhead with a lot more microelectronics would be much more impressive.
