The Week in Space and Physics: Looking for Supernova Neutrinos

On the supernova neutrino background, the launch of Ariane 6, Europa Clipper's problems with radiation and the discovery of a black hole

Rafale pilots guard and watch the launch of the Ariane 6. Image credit: The French Ministère des Armées.

Over a period of ten years the upcoming Rubin Observatory expects to see somewhere between three and four million supernovae. That, needless to say, is a lot: the figure implies a discovery rate of roughly a thousand stellar explosions every single day. 

Each of those supernovae will appear as a flash of light suddenly brightening and then fading away over a matter of months. All are powerful events, bright enough to outshine a galaxy, and visible across millions of light years. But even more powerful is the wave of neutrinos that comes with a supernova blast. Each event creates trillions upon trillions of fleeting neutrinos, and emits them all over a few seconds.

This neutrino flash is the first sign we can detect of an impending supernova. Yet spotting it is hard: neutrinos rarely interact with the atoms that make up our world, and can pass right through the Earth as if it were not there at all. Our most sophisticated detectors can pick up a mere fraction of them, and so can only observe the flashes coming from the closest supernova.

So far, we’ve only found one certain set of neutrinos from a supernova, that of 1987. Back then a star exploded in a nearby galaxy, the closest to do so in almost four centuries. A wave of trillions upon trillions of neutrinos washed over the Earth, of which a detector in Japan spotted twelve. It was not many, but it was the first, and so far only, unambiguous detection of supernova neutrinos.

Nevertheless, the sheer number of supernovae popping off in the universe, as well as the long-lasting nature of neutrinos, implies the cosmos should be flooded with them. We call this flood the “diffuse supernova neutrino background”, and finding it is one of the key goals of upcoming detectors.

In theory this background should look like a constant hum of neutrinos, coming from all directions in the sky. Its strength is unknown. But since the background includes neutrinos from every supernova in history, measuring it would give us an idea of the historical rate of supernovae. It is, thus, an indicator of how the population of giant stars has evolved over time.

According to a recent presentation, described in Nature, the Super-Kamiokande detector in Japan may have picked up the first hints of this background. So far the signal is weak and hard to distinguish from stronger sources like solar neutrinos. Yet if it is there, more data over the coming years should help to confirm the finding.

A true detection, however, is unlikely to come before the 2030s, when a series of new neutrino detectors will have gathered enough evidence. Among them is the Hyper-Kamiokande, under construction in Japan, and JUNO, an observatory that should soon start work in China. Both should have the sensitivity to spot the background if it exists as predicted.

The real prize of neutrino astronomy, however, will be another nearby supernova. That, whenever it happens, will treat our observatories to a blast of energetic neutrinos. Hundreds, perhaps thousands, will be picked up. That, for the first time, would give us a chance to really study the death of a star with neutrinos rather than photons.

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A Bad Week for Rocket Upper Stages

Europe’s Ariane 6 rocket got off the ground for the first time last Tuesday, restoring the continent’s independent access to space. The initial stages of the launch went to plan, and the rocket successfully deployed several small satellites into orbit.

Yet something then went wrong with the final stage of the rocket. Although the stage fired its engines twice to enter an orbit some five hundred and eighty kilometres high, it proved unable to do so a third time. That third burn should have sent the stage plunging back to Earth, where it would have deployed two small re-entry capsules before burning up in the atmosphere.

At blame was a problem with the APU, a system that pressurises the fuel in the stage’s tanks. According to ArianeGroup the APU powered up successfully before suddenly stopping. That, in turn, meant engineers could not guarantee a steady flow of fuel to the engines. Rather than risk a failed burn or explosion, operators instead passivated the stage - essentially turning it into a piece of space junk.

The Ariane 6 upper stage is now likely to stay in orbit for several decades. That is an embarrassment for the European Space Agency, which recently committed itself to cutting the amount of debris left in orbit. Arianegroup, meanwhile, downplayed the incident. They instead stressed that Ariane 6 is now ready to begin launching commercial and governmental satellites.

Problems with spacecraft upper stages were not, however, limited to Ariane. SpaceX suffered a rare failure on their Falcon 9 workhorse. According to SpaceX, the upper stage of the rocket failed around a hundred kilometres above the Earth. Though details are so far vague, it appears an engine exploded. The twenty Starlink satellites onboard were thus left to burn up in the atmosphere.

The incident will likely delay upcoming missions on the Falcon 9. That includes the flight of Polaris Dawn, a mission that should take its crew higher than any flight since the Apollo era and include the first private spacewalk. Also delayed could be NASA’s next flight to the International Space Station. Ironically, if the delay ends up being extended, America’s only option to reach the space station may now be Boeing’s troubled Starliner capsule.

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Radiation Woes for Europa Clipper

NASA’s upcoming Europa Clipper mission has run into a problem with its transistors. The spacecraft is scheduled to launch towards Jupiter in October but may, if this issue proves hard to solve, now be delayed by several years. 

The issue was first revealed at the end of May. Back then NASA said it was assessing the ability of the spacecraft’s transistors to survive the radiation around Jupiter. Testing data suggested some of them might fail at levels far below that expected around the giant planet. That could lead to failures in Europa Clipper’s onboard computers.

The radiation around Jupiter is known to be severe. Back in 1973, when Pioneer 10 flew past the planet, the radiation sparked false commands on the spacecraft. Later missions, after this incident, have designed probes to survive much higher radiation levels.

Europa Clipper, engineers had thought, met that need. However a manufacturing flaw in the transistors means it has a much lower radiation tolerance than expected. Engineers are now trying to assess how severe the problem is, and how much work will be needed to fix it.

In a worst case, the vulnerable transistors would need to be replaced. However that will take time, since many of them have already been sealed inside a vault in the spacecraft. Opening that up and switching out the transistors will take months, and that would mean delaying lift-off until a future launch window. NASA engineers are thus hoping to find ways to mitigate the vulnerability, and so launch on time without needing to replace the transistors.

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A Black Hole Revealed

Researchers have reported finding an intermediate mass black hole in our galaxy. The object appears to have a mass eight thousand times that of the Sun, and lies roughly eighteen thousand light years from Earth.

We cannot directly see the black hole, of course, but astronomers can see the effect it has on nearby stars. In particular, they saw seven stars moving through a cluster. Those stars are moving so fast that they should - based on the visible mass of the cluster - escape into space. Since they don’t, there must be a large unseen mass binding them to the cluster.

From their movements, the astronomers deduced the presence of a large compact object at the heart of the cluster. This invisible object can only be a black hole, and one far larger than most found previously. That could mean this cluster is actually the remnant of a galaxy that long ago collided with our own.