The Week in Space and Physics
Gravitational waves, Lucy hunts for asteroids, the origins of Oumuamua, and Boeing's problems on the launchpad
Gravitational waves are one of the stranger predictions of Einstein’s theory of gravity. They are, effectively, distortions in the very fabric of the universe, rippling out from some of the most intense events in the cosmos. Such waves are constantly washing over the Earth, though our ability to detect them has so far been limited.
The first confirmed sighting of a gravitational wave came just six years ago, at an advanced American detector known as LIGO. Researchers think it originated more than a billion light years away, from two black holes spiralling towards a collision. Since then another two dozen or so have been spotted, most coming from similarly extreme events.
That is a mere fraction of the gravitational waves that we could have seen, had we possessed more powerful detectors. Right now we only see the largest waves, equivalent to powerful tsunamis racing across an ocean. If we could see the smaller, everyday waves, then astronomers could trace distant black holes, watch neutron stars and even hear echoes of the big bang itself.
Developing detectors capable of doing that is hard. Gravitational waves, even from immense collisions, are tiny. That means detectors have to be both big and extraordinarily sensitive. Plans are afoot to build such a detector in space, with three satellites flying millions of miles apart. That will give us an unprecedented view of the gravitational landscape, but operations are unlikely to begin before the 2030s.
Until then astronomers need to dream up more inventive ways to spot gravitational waves. A paper published last week outlines one possible approach. Buried in the data gathered by LIGO are, the researchers estimate, more than a thousand possible sightings. Normally these measurements are dismissed, as they fall below the threshold scientists use to distinguish the signal from random noise.
The team of researchers acknowledge this, and believe that many of the sightings they identify are indeed just random noise. But they also think that by combining these gravitational measurements with other techniques, we could find some that are real.
The approach relies on something known as multi-messenger astronomy. That means, essentially, looking at the same event through different eyes. Astronomers could, for example, look to see if the gravitational wave matches up with other observations. A possible signal linked to a sudden burst of neutrinos or a bright flash of light could be evidence of something interesting happening.
That would raise the chances that the gravitational signal is real, and evidence of something powerful happening in space. If astronomers get lucky, that could already shed new light on events such as supernova or stellar collisions.
Lucy In the Sky With Diamonds
NASA last week started preparing a new interplanetary mission for launch. Named Lucy, the craft will spend the next few years crossing the solar system in hunt of asteroids. In particular, Lucy will focus on a group of asteroids known as the “Trojans”.
These are asteroids that share Jupiter’s orbit, forming two large clusters that travel just in front and just behind the giant planet. In those areas the gravitational fields of Jupiter and the Sun balance out, allowing asteroids to quietly orbit, free of disturbances.
Astronomers believe that up to a million asteroids inhabit these regions, though only a few thousand have so far been identified. Lucy’s mission, lasting twelve years, will see it repeatedly fly between the two clusters and Earth. As it does, mission planners will carefully control its path to bring it close to six asteroids.
Models of the early Solar System suggest that these asteroid clouds formed early on, and have since been untouched for billions of years. If true, that should mean the asteroids hold clues about the birth of the planets, and even about the early movements of the gas giants.
Studies of other solar systems show that ours is somewhat unusual. Most planetary systems show rather extreme orbits, with planets swinging in close to their star before moving far out into space. In our solar system, by contrast, the planets trace almost circular orbits – which means they experience stable temperatures and climates.
That probably helped life form on Earth, but the reason why our solar system is so stable is not yet clear. Some models of the early Solar System suggest that its youth was far more chaotic. The gas giants – Jupiter, Saturn, Uranus and Neptune – plus perhaps one more giant planet originally formed closer to the Sun. Over tens of millions of years they slowly moved outwards, until a sudden tipping point was reached.
At that moment the orbits of Saturn and Jupiter fell into a resonance – a gravitational effect that destabilised the whole solar system. The order of the planets may even have flipped – with Jupiter moving inwards and Saturn moving outwards. Uranus and Neptune were thrown outwards, scattering asteroids and comets towards the inner planets. A fifth planet may even have been expelled from the Solar System completely, doomed to wander the galaxy alone.
The scars of that tumultuous time can still be seen on the Moon. Evidence from Lunar craters suggests that large numbers of asteroids hit the Earth and Moon early in their histories. That may also have been when Jupiter captured its Trojan asteroids. Lucy’s measurements should allow researchers to work out if this model is correct, and so help explain how our Solar System came to be.
Where Did Oumuamua Come From?
When Oumuamua arrived in our Solar System four years ago, it attracted interest as the first known object from interstellar space. As the world’s telescopes turned towards it, several mysteries emerged. Its shape was unlike anything researchers had expected, seeming to be long and flat. It didn’t behave as they had expected either, neither melting like a comet nor following the orbit of an asteroid.
That sparked speculation about its properties, and where it came from. Some, including Harvard astronomer Avi Loeb, claimed it was a piece of alien technology. Others wondered if it was a bizarre object made from frozen hydrogen, or a fragment of a long destroyed planet.
Astronomers did note, however, that its orbit displayed several unusual features. Objects moving through our galaxy tend to wobble up and down against the plane of the galaxy, a motion caused by the tug of stars passing close by. The older an object is the more pronounced this oscillation becomes, which means astronomers can use it to estimate the age of things like Oumuamua.
They found that Oumuamua is young, as these things go, probably forming less than fifty million years ago. That would support the idea it was made of frozen hydrogen - such objects are not expected to last more than a few tens of millions of years.
Its youth also means we have a realistic possibility of tracing its orbit back to find its origins. Two nearby clouds of interstellar gas - known as Carina and Columba - have been posed as possibilities. Now a team of researchers at Yale have, by running detailed orbital calculations, concluded that Carina is the most likely birthplace. That gives Oumuamua an age of roughly thirty million years.
The researchers also conclude that such objects must be widespread in the galaxy. Clouds like Carina and Columba could be producing millions of Oumuamuas, scattering them among the heavens. If true, that would be exciting news. If another such object enters our solar system soon - which the numbers suggest might happen - astronomers would get a second chance for a close up look.
That could allow researchers to probe objects forming in the heart of molecular clouds like Carina, a region where stars are sometimes born. Oumuamua, or its successor, may one day give us a glimpse of our own ultimate origins.
Starliner and Starship
Boeing’s ill-fated Starliner suffered another delay on its voyage to the International Space Station. Originally scheduled to fly last week, its trip was postponed after Nauka’s troubled arrival. When engineers tried again on Tuesday, they were forced to scrap the effort after discovering technical problems with the capsule.
Starliner is intended to carry astronauts to the space station. But before NASA clears it to do so, it must pass a test flight that replicates all the steps involved. Starliner’s last attempt, at the end of 2019, ended in disaster after an onboard clock malfunctioned. That placed the capsule in the wrong orbit, and forced controllers to abort the mission.
Now Boeing wants to try again, but pre-launch checks found several misbehaving valves in the propulsion system. That could threaten Starliner’s ability to manoeuvre in space. Rather than risk a launch, engineers instead moved the rocket back to the hanger for further investigation.
If repairs are needed, they could take several weeks to complete. Even worse, the required launch pad site is likely to be reassigned to the Lucy mission, which may result in an even longer delay to the test flight. That would leave NASA relying on SpaceX’s Dragon capsule to ferry astronauts to the space station for many months more than planned.
SpaceX, meanwhile, moved ahead with preparing their mammoth Starship for an orbital launch. The company briefly fully assembled Starship and its associated Falcon Super Heavy rocket for tests in Boca Chica, Texas. Though the structure was later taken apart for futher work, SpaceX briefly possessed one of the most powerful rockets ever constructed.
SpaceX are unlikely to launch anytime soon. They still have significant work to complete both the rocket and Starship for an orbital flight, and must also get approval from the FAA. That could take many more months - though Musk is hoping to have everything ready by the end of the year.
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