The announcement by the United States that it has deployed a space weapon in Earth's orbit has raised questions about what weaponry is in space, and what role it could play in efforts by great powers to dominate that arena.
Washington has given away little in its announcement. Secretary of the Air Force Troy Meink said the "on orbit" weapon was necessary to safeguard US forces against hostile enemy action, but gave no details about the weapon's capabilities or when it was put into orbit.
So what could this weapon be, and what is the wider picture like for military developments in space? Who else could have similar weaponry?
Space has been a militarised domain since the start of the space age - in the late 1960s, the Soviet Union deployed weapons up there. The Istrebitel Sputnikov (IS; Satellite Destroyer) programme was made up of satellites that were designed to release projectiles that detonated, releasing shrapnel into the path of another (usually American) satellite.
Nowadays, advanced militaries deploy satellites in space for a range of purposes, including reconnaissance, secure communications, detecting missile launches and providing precise location information for targeted strikes.
As warfare becomes more technologically advanced, analysts expect the use of military hardware in space to increase.
Dr Bleddyn Bowen, an associate fellow in military sciences at the Royal United Services Institute and associate professor in astropolitics at Durham University, said he was only able to hazard a guess about the weapon announced by the US.
"I would expect there to be some kind of electronic warfare or radio-jamming platform, possibly, this would be a non-destructive kind of anti-satellite weapon," he told BBC Radio 4's Today programme.
Such weaponry already exists on Earth and can disrupt satellite communications, he said. "If you take out the radio links then your satellite is very useless."
Bowen also would not rule out a riskier and more disruptive type of weaponry, which could create debris and destruction.
"Less likely would be a kinetic kill vehicle of some sort, so a vehicle that would release a projectile that could ram into a satellite and destroy it," he said.
A type of "close inspection and grappling satellite would be another possibility as well."
As to whether rivals such as Russia and China have similar weaponry already in orbit, is difficult to tell.
"If we're looking towards China and Russia, they have both displayed activity," Juliana Suess, a former research fellow at RUSI and researcher at the Stiftung Wissenschaft und Politik (German Institute for International and Security Affairs), told the BBC.
"For example, with Russia, we've seen some activity that hints at being a co-orbital weapons test. So we saw something that might indicate that they shot a projectile as a test in orbit.
"There are others out there, but without knowing the details, what the American [weapon] is, it's difficult to draw comparisons."
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But while the weapon placed in orbit may be unusual, the US - and indeed its Russian and Chinese rivals - have had weapons that can be pointed and shot into space for some time.
In a later BBC interview, Bowen was dismissive of China's reaction to the US announcement which warned against an arms race in space, describing it as "the pot calling the kettle black".
"If you're already an expert... You know how hollow those words are coming from the Russian and Chinese governments," he said.
"The United States, Russia and China and also India have all developed different kinds of anti-satellite or counter-space weapons, and Russia and China have put a lot of technologies in space which have a lot of weapons potential," he added.
In fact in the past 10 years, Russia and China have been doing a lot more manoeuvres in space, known as rendezvous and proximity operations (RPOs) - "buzzing" Western satellites.
"Theoretically, these RPO [satellites]can become hunter-killer satellites by physically colliding with them or by nudging them out of orbit," Dr Rod Thornton, reader in Russian Defence and Security Studies at King's College, London, told the BBC.
Bowen added that the Chinese have demonstrated "space tug capability", saying: "A vehicle can go up to a satellite, can grapple it and move it into a different orbit."
This, he said, could be a positive development - a way of removing space junk as well as knocking out working satellites.
However, some types of space weaponry also have the potential to create large amounts of space junk.
Dr Mark Hilborne, a specialist in space security and weaponisation at King's College, said direct ascent anti-satellite missile tests, such as one carried out by China in 2007, leave a great deal of debris in orbit for a long time.
Suess said debris from that test was still in orbit and still dangerous.
"It doesn't have to be a sort of huge piece of metal to do any damage, it can be a tiny piece of plastic, given the velocity that we're talking about in orbit," she said.
Thornton said that what Russia lacks in space weaponry and satellites, it makes up for in missile capability.
"The Russians... Have ground-based missiles which can be launched into space to strike [satellites] in low Earth orbit. They also have ASAT [air-launched antisatellite] missiles that can be launched from a great height by aircraft," he said.
"From the Russian point of view, they see Nato as having much better and more satellites than the Russian military can call up, therefore they have invested heavily in ASATs in order to 'level the battlefield' somewhat."
Thornton added a sobering thought: "As a last resort, the Russians can also launch nuclear weapons into space and the EMP [nuclear electromagnetic pulse] blast would 'fry' any unhardened satellite over a wide expanse."
So what is the likelihood of a nuclear conflict in space? Only if it was happening on the ground as well.
"What could happen in space is not something that would happen in isolation, really," Bowen said. "Nothing could happen, or something could."
Given that weapons for use in space have been around for so long, why did the US announcement come when it did? Experts agree that it's not a development as such, more a political signal.
Bowen said that it could be a response to Chinese developments.
"China has modernised its military forces over the past 30 years," he said. "It has put a lot more military satellites in space and has presented a lot more targets that are worth the American military going after and hitting."
"There's much more incentive for American space weapons now because China has a lot of vulnerability to space in the same way that the Americans have."
This situation seems to have created an imbalance.
Thornton said that, Russia would be the "big winner" in any space war, because it has so few satellites, unlike China and the US. It would involve "missiles fired up into space, manoeuvring satellites designed to nudge/destroy them and ground-based lasers," he said.