Australia announced early Wednesday that it will work with the US and UK, through the AUKUS defence pact, to develop hypersonic missiles.

The AUKUS countries will also work together to develop missile defence systems and radars that can take out an enemy’s hypersonic missiles.

As the name suggests, hypersonic missiles will be weapons that travel fast over long distances, but it’s not necessarily the speed of the missiles that will be the main threat.

That could come from their manoeuvrability and the difficulty tracking them in flight.

What is a hypersonic weapon?

The US-based Missile Defense Advocacy Alliance described hypersonic missiles as “weapons that travel faster than Mach 5 (about 6100kmh) and have the capability to manoeuvre during the entire flight”.

The US Air Force's experimental X-51A Waverider combiustion ramjet-powered hypersonic flight test demonstsrator

An article published by the Russian International Affairs Council said hypersonic weapons had two major defining characteristics: “Speed exceeding Mach 5 [and the] capability to make manoeuvres (both vertical and horizontal) while travelling at this speed inside the atmosphere”.

An article published in February by the Stockholm International Peace Research Institute (SIPRI) said Mach 5 was used as the threshold because a range of physical effects started becoming a significant engineering challenge at that speed.

While hypersonic speed was often pointed to as one of the key factors – if not the key factor – that set hypersonic missiles apart from other missiles, in many cases the speed of ballistic missiles far exceeded that of today’s hypersonic missiles, SIPRI said.

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Hypersonic missiles were divided into two different types of systems:

• Hypersonic cruise missiles (HCMs) that kept a constant hypersonic speed (and usually altitude) and were powered over the entire course of their flight.
• Hypersonic glide vehicles (HGVs) that were usually launched on tops of ballistic missiles (often referred to as a boost-glide system) and then glided back through the atmosphere to their target at hypersonic speeds.

The longest range ballistic missiles are referred to as Intercontinental ballistic missiles (ICBMs). As explained by Live Science, they are mostly launched from the ground travel into outer space and finally re-enter Earth's atmosphere, plummeting rapidly until they hit their target. Once launched, ICBMs travel in a parabola, much like a baseball flying through the air.

When ICBMs were flying through space they could get to more than 27,000kmh, the article said.

Why are hypersonic weapons so hard to track?

In an article in January, Scientific American said that in contrast to ballistic missiles, hypersonic glide vehicles could manoeuvre on the way to a target, making it difficult to track them.

“Large rockets boost the weapons to an altitude near the edge of space and release them. Then the glide vehicles divert to a flatter trajectory – either exiting the atmosphere or staying just within it – and sail on unpowered.

“This near-space trajectory and the ability to shift course let hypersonic glide vehicles evade the combination of space and terrestrial sensors used to track ballistic missiles.

“The Pentagon can detect the launch, but the vehicle then slips out of view until late in the weapon’s flight because of ground radar’s line-of-sight limitations. Defensive systems have little, if any, time left to halt an incoming weapon,” Scientific American said.

It reported the US Department of Defense’s Missile Defense Agency (MDA) had given the go ahead for contractors to fabricate a prototype of a hypersonic and ballistic tracking space sensor system, intended to detect and track hypersonic glide vehicles.

Are hypersonic missiles a greater threat than existing weapons?

In a report updated in mid-March, the US Congressional Research Service said hypersonic weapons could challenge detection and defence due to their speed, manoeuvrability, and low altitude of flight. For example, terrestrial-based radar could not detect hypersonic weapons until late in the weapon’s flight.

“This delayed detection compresses the timeline for decision makers assessing their response options and for a defensive system to intercept the attacking weapon, potentially permitting only a single intercept attempt.”

US defence officials had stated that both terrestrial and current space-based sensors were insufficient to detect and track hypersonic weapons.

China’s pursuit of hypersonic weapons reflected a concern that US hypersonic weapons could enable the US to conduct a preemptive, decapitating strike on China’s nuclear arsenal and supporting infrastructure. US missile defence deployments could then limit China’s ability to conduct a retaliatory strike against the US, the research service said.

Which countries are working on hypersonic weapons?

The US, China and Russia are the main players.

In a statement on Tuesday (local time), the US Defense Advanced Research Projects Agency (DARPA) said that, along with the US Air Force, it had recently completed a free flight test of a hypersonic air-breathing weapon concept (HAWC).

The vehicle was released from an aircraft and cruised at faster than Mach 5 for an “extended period”. It flew to a height above 19.8km, and flew for more than 550km.

Air-breathing vehicles used air from the atmosphere for sustained propulsion, Darpa said.

“The speed and manoeuvrability of such hypersonic cruise missiles allow both evasion of defences and quick strikes. Their kinetic energy can effectively destroy targets even without high explosives.”

The US Congressional Research Service said the US had been working on the development of manoeuvring weapons that fly at speeds of at least Mach 5 since the early 2000s.

In the past funding had been “relatively restrained” but the Pentagon and Congress had shown a growing interest in pursuing the development and near-term deployment of hypersonic systems.

That was partly due to the advances in those technologies in Russia and China, both of which have a number of hypersonic weapons programmes and had likely fielded operational hypersonic glide vehicles – potentially armed with nuclear warheads. The US was unlikely to field an operational system before 2023.

Russia was pursuing two hypersonic weapons programmes – the Avangard and the 3M22 Tsirkon (or Zircon), and had reportedly fielded the Kinzhal (“Dagger”), a manoeuvring air-launched ballistic missile, the research service said.

Avangard was a hypersonic glide vehicle launched from an ICBM, which was thought to effectively give it unlimited range. Avangard reportedly reached speeds of Mach 20 during testing, and Russian news sources claimed it entered combat duty in 2019.

Russia has claimed it used Kinzhal missiles in Ukraine, but CNN reported the Kinzhal was simply a version of the Iskander short-range ballistic missile, and so was a variation of an established technology, not a revolution in hypersonic weaponry.

Bloomberg on Tuesday (local time) reported the head of the US Strategic Command Admiral Charles Richard had said China’s first test of an ICBM-launched hypersonic glide vehicle last July was a “technological achievement with serious implications for strategic stability”.

The comments were in prepared testimony posted on the website of the House Defense Appropriations subcommittee. It also said the hypersonic vehicle flew 40,000km for more than 100 minutes.

It was “the greatest distance and longest flight time of any land attack weapon system of any nation to date,” Richard said.

Is it possible the threat is being exaggerated?

The US Congressional Research Service said critics considered hypersonic weapons lacked defined mission requirements, contributed little to US military capability, and were unnecessary for deterrence.

Some analysts argued such weapons wouldn’t change much in terms of strategic balance and military capability, the research service said. China and Russia could already strike the US with ICBMs that would overwhelm US missile defences when launched in salvos.

It was also suggested the deterrent threat of a US counterstrike would stop any attack against the US using hypersonic weapons.

An article in January in Air Force magazine, the official publication of the Air Force Association in the US, sought to dispel any ideas that hypersonic weapons could “zig and zag across the sky”.

But it did say that even a slight course adjustment could make the weapons unpredictable and nearly impossible to intercept.

“Hypersonic weapons can fly low, evading radar and can manoeuvre during the cruise phase. They are intended to quickly destroy high-value targets, such as aircraft carriers,” the article said.

Experts at MIT and the University of Colorado had said the US could detect hypersonic weapons, but was not yet able to strike a hypersonic weapon in mid-course.

MIT physicist Dr David Wright calculated that a hypersonic glide vehicle travelling at Mach 15 would take seven minutes to turn 30 degrees, and that if it were flying at an altitude of 40 km, it would have to drop 2.5 km to achieve that objective.

MIT Hypersonics Research Laboratory Director Dr Wesley L Harris said hypersonic vehicles would weigh from about 4.5 to nine tonnes.

”So, a slight deflection can generate a big, heavy load leading to shockwaves, even leading to separation of fins, control surfaces, on these vehicles,” Harris told Air Force.

The magazine also said Russia and China were racing to develop scramjet and ramjet hypersonic engines that could yield smaller, more manoeuvrable, and harder-to-detect weapons.