A basic law of physics is that due to gravity and momentum there is an inverse relationship between the weight of a warhead and the range of a missile. If the same rocket boosters (better with a slower burning propellant) for the heavy load were used for a light load, amounting to one 500kg warhead, then the range of that warhead may be 10,000 km.
Ranges involve capabilities even if India has no intentions about friendly countries. The 10,000 km range would bring the capitals of three of the other major nuclear powers into range. Such a long range increases flexibility, important for deterrence. For political reasons India probably does not wish to talk about longer range ICBMs - with 13,000 km capable of reaching all nuclear powers.
India has a right to defend itself. Having nuclear missiles with equal capabilities to the missiles of other great powers is important.
India wishes the 10,000 km range missile, known as the Agni 6 (Agni VI), to have characteristics equal to (parity with) the latest ICBMs of India's main nuclear opponent, China. China's latest ICBM under development is the DF-41 (Dongfeng-41) which will have the range to hit any capital of its nuclear opponents, including London and Washington DC. A December 2014 report indicated that China conducted the full test of the DF-41 involving MIRVs The DF-41 has an estimated range of 12,000km and “can carry up to 10 warheads, which separate from the rocket body during the final, third stage of flight and target individual cities. The military has previously carried out tests of the DF-41 but these probably involved only a single warhead”.
The Agni 6 will be an evolutionary development of the Agni series of long range Indian ballistic missiles developed following the test of India's first nuclear device (1974).
Carrying multiple warheads (10 is the usual upper limit) on one missile is the most economical way to deploy warheads and such a deployment is more difficult to defeat with anti-missile defences. These multiple warheads are known as Multiple Independently Targetable Re-entry Vehicles (MIRVs). Along with live warheads light decoys can be carried (to draw off some anti-missile missiles) and various types of "chaff" (to confuse radar defences).
Agni 6 may be first tested in 2017 . Testing may last 4 years to 2021. Then in-service, operational around 2023 or later.
If India has developed fusion boosted fission weapons (like Joe-4) the yield of a single warhead missile may be up to 400 kT). If India has developed two-stage thermonuclear weapons - then each MIRV warhead may well have a yield between 100 to 250kt.
Cross reference this article with many concerning the Agni series including:
- The Second Agni 5 Test, Any MIRV? September 16, 2013
- China's, India's and Pakistan's Future Nuclear Rivalry August 12, 2013
- Indian Strategic Weapons Programs - Gradual Progress, July 3, 2013
- Agni 5's First Test in April 2012, April 27, 2012