The Week in Space and Physics: Europe's Next Deep Space Explorers
On Europe's future deep space probes, neutron stars and black holes, the end of Ingenuity and the SLIM moon lander
Europe’s space agency last week approved missions to explore Venus and research gravitational waves. Both promise to build on Europe’s history of deep space exploration, and both should offer new insights into the solar system and galaxy around us.
The first of those missions is EnVision, a probe that aims to explore Venus from its innermost core to its outermost atmosphere. Key to that mission are two powerful radars to be carried onboard the spacecraft. One will peer through Venus’ thick, cloud-filled atmosphere to map out its terrain. The other will look under the surface of Venus and study its internal structure.
Although the surface of Venus is today inhospitable - it is choked by a haze of acid smog and is searing hot - it was probably not always like this. Indeed, there are signs Venus once had vast oceans and could even have hosted life. EnVision should shed light on whether that was really the case, and if so, how Venus later became so awful.
It may also help settle lingering questions about whether life still clings on around Venus. A few years ago researchers claimed to have found phosphine in Venus’ atmosphere, a gas that might be coming from living creatures. Later measurements found the gas probably wasn’t there, but to settle the question we need to send a probe to take a closer look.
Europe’s other mission is LISA, an ambitious project to pick up on subtle vibrations in space and time. These vibrations - known as gravitational waves - are constantly rippling through the solar system, bringing news of far away supernova and black hole collisions.
In the past decade, observatories on Earth have started to pick up these waves. Yet they are inherently limited in their capabilities. So far we can only detect the most powerful waves crashing over our planet. To pick up smaller ones - which could reveal details about the Big Bang and map out black holes across the galaxy - we need a more sophisticated detector.
This is what Europe is hoping to build with LISA. It will consist of three spacecraft flying in perfect formation far from Earth. Each will fire lasers towards the others, forming a triangle spanning more than a million miles across. By measuring slight shifts in the shape of that triangle, LISA will pick up on the subtle clues left by passing gravitational waves.
Both missions are planned for the next decade. EnVision should lift off in 2031, and enter orbit around Venus a few years later. LISA is currently planned to reach space no earlier than 2035, a date which reflects the extreme complexity of that mission.
Black Holes and Neutron Stars
A few years ago, astronomers were getting worried about missing black holes. Or, possibly, missing neutron stars. Whatever it was, something seemed to be missing between the biggest known neutron stars - which weigh in at about two and half times the mass of the Sun - and the smallest known black holes - five times the mass of the Sun.
This was a problem. When a big star dies, its core must collapse into either a neutron star or a black hole. The bigger the star, the more likely it is to end up as a black hole. And yet while we’ve found plenty of black holes resulting from truly giant stars, and plenty of neutron stars from those that are merely giant, there still seemed to be a gap in the middle.
The arrival of gravitational wave observatories helped calm things down. Since neutron stars and black holes are both dense objects, Einstein’s theories tell us they create ripples in space and time as they move. Of course, those ripples are very subtle, but modern instruments can just about spot those coming from especially violent events, such as when neutron stars and black holes smash into one another.
Armed with this data, researchers soon found objects lying between the largest neutron stars and smallest black holes. That was enough to close the gap, but questions still remained about how large a neutron star could get. Where, in other words, does the dividing line between neutron stars and black holes lie?
To answer that question, astronomers have been scouring the skies for the signs of heavy neutron stars. The record was set back in 2022, when a neutron star weighing 2.35 solar masses was spotted. At some point, presumably not much heavier than that, a star would be unable to resist the pressure of its own gravity, and so it would collapse into a black hole.
There has, then, been some excitement about a recent discovery of another possible neutron star. The object was found circling a pulsar some forty thousand light years away. The flashes of energy coming from that pulsar are slightly irregular, which hints at the nearby presence of a heavy object.
Intriguingly, that object seems to weigh between 2.1 and 2.7 solar masses. Almost certainly, its discoverers say, that means it is either a neutron star or black hole. Unfortunately they cannot yet say more - but given its probable mass, if it is a neutron star it is a big one, and perhaps the biggest ever found.
A Martian Crash Landing
When the Perseverance rover landed on Mars in 2021, it carried a small helicopter with it. Shortly after it arrived it took flight, whirring for a few seconds through the thin Martian air. It was a short flight, but enough to set a record as the first machine to fly on Mars. Back then the ambitions for Ingenuity were modest, and NASA only aimed to make four more test flights.
Over the past three years, however, Ingenuity has kept on flying. By January 18 it had flown seventy-two times, covered a total distance of eleven miles and in all spent more than two hours flying over Mars. It has helped scout out the terrain, directed the future course of Perseverance, and shown the viability of flight across Mars.
Sadly, Ingenuity’s mission has now come to an end. On January 6 the probe made an emergency landing, possibly, reports say, because it lost its bearings amidst an area of featureless terrain. On the 18th operators ordered it aloft once more, attempting a short flight to check its location.
Yet once more something went wrong, and Ingenuity seems to have struck the ground at an angle. Images returned from Ingenuity after the incident show the shadow of at least one broken blade, and signs of disturbed soil around the craft. It is likely, say NASA, that other blades have also snapped, leaving the helicopter stuck on the ground.
The success of Ingenuity did, at least, prove the concept of flight on Mars and other worlds. NASA now has plans to send two more helicopters to Mars, both of which could form part of a future sample return mission, and to Titan as a key part of the Dragonfly mission.
SLIM Awakens
Japan’s SLIM moon lander tipped over after landing, according to JAXA, the Japanese space agency. The mishap left the probe’s solar panels pointing west, away from the Sun, and so prevented them from properly recharging the spacecraft’s batteries. JAXA thus powered off the probe less than three hours after it touched down.
Telemetry from the probe shows a failure in one of SLIM’s two main engines as it was fifty meters above the surface. Although onboard algorithms were able to adapt and successfully bring SLIM to a landing, they were unable to stop a sideways drift in its motion. After touchdown that drift seems to have resulted in a roll, which then left the spacecraft pointing in the wrong direction.
Fortunately, JAXA yesterday reported that sunlight was finally falling on SLIM’s solar panels. With its batteries freshly charged, the probe has now awoken and started communicating with Earth. Its operators now have a brief window in which to complete its mission, before the start of a long lunar night on Thursday.