The Week in Space and Physics #29
The Week in Space and Physics #29
On deflecting asteroids, finding the ancient heart of the Milky Way, Saturn's lost moon and the SLS
One day – perhaps a decade from now, perhaps a millennium – we’ll stumble across something terrifying. A chunk of rock, anywhere from a few tens of metres across to a kilometre or more, will be found heading towards Earth. Impact may not be certain; nor will we know exactly where it will strike, if it does. But the risk will be high; the spectre of human extinction suddenly terribly real; the threat raised, at best, of death and destruction on an extraordinary scale.
This is not just a possibility. Sooner or later a large asteroid will strike our planet again. Should it fall in the ocean, it would send waves racing across oceans, wrecking havoc on coastal cities across the planet. Should it strike land, or shallow seas – as one did sixty odd million years ago – earthquakes would shatter continents, cities would be obliterated, skies obscured by dust and smoke. Civilization would be in peril; life on Earth would be dealt a devastating blow.
Fortunately, we would likely spot an incoming asteroid years in advance. Space agencies now have dedicated programs scanning the skies for signs of these perilous rocks. Estimates suggest that we’ve found most of the largest - and therefore most deadly - already. None, thankfully, presents much danger.
Yet what if, one day, we stumble across one that does? In Hollywood the answer is easy: we send a team of heroic astronauts to bomb the asteroid, or fire a series of nuclear missiles into its stony heart. Sadly, in reality things are not so simple. We’d need an enormous amount of firepower to destroy an asteroid, and, worse, failure may simply break it apart: creating a series of smaller, but still devastating, impacts.
Those who think about such morbid things believe a better approach can be found. Instead of bombing the asteroid, we simply need to deflect it. Somehow we subtly shift its path, so that instead of hurtling towards Earth, it flies harmlessly by. Do this far enough in advance- say a decade or more - and even a minor change in its orbit might be enough.
The easiest way to do this is with an impact. Crash something – like a spacecraft – into an asteroid and the result is a change in momentum; an alteration of velocities that pushes the asteroid onto a safer path. This, at least, is the theory. In practice things are more complex: everything from the structure of the asteroid to the way sunlight bounces off its surface can affect its motions and response to a collision.
To learn more, a test mission is now underway. NASA has chosen Didymos, an asteroid that is itself orbited by an even smaller asteroid, Dimorphos, as the subject for this experiment. On Monday evening DART, a spacecraft, smashed into that smaller asteroid. The collision, NASA hopes, will have slightly altered the orbit of Dimorphos, sending it closer to Didymos.
This change will take time to play out, and will be hard to see clearly from Earth. A follow-up mission, HERA, is thus planned for 2027. That spacecraft, currently being built by ESA, will examine the site of the collision in detail and carefully measure any change in the orbits of Didymos and Dimorphos. That, the world’s space agencies believe, will demonstrate asteroid deflection can indeed work, and provide hope for the dark day when an asteroid impact becomes all too possible.
The Milky Way’s Ancient Heart
For close to a decade, the Gaia satellite has been mapping our galaxy. The result is the largest and most accurate map of our cosmos ever made: tracing the location of a billion planets, asteroids and galaxies. Its main target, however, is the stars. Gaia seeks to measure the distances, colours and makeup of roughly one percent of all the stars present in our galaxy.
This enormous catalogue of heavenly objects has started to reveal interesting details about the history of the Milky Way. Astronomers have already found the remnants of alien galaxies shredded and absorbed by our own. They have spotted ultra fast stars moving out – and intriguingly into – our galaxy. Now they seem to have stumbled across something even more fascinating: the original core of the Milky Way.
Astronomers believe most galaxies formed roughly twelve billion years ago from smaller groups of stars known as protogalaxies. Over time these merged and grew; absorbing more and more stars until they contained tens of billions each. This is a process that has not yet finished: eons from now the Earth may find itself in a galaxy even vaster than the Milky Way; one home to trillions of stars.
It ia such a protogalaxy that astronomers believe they have now found in Gaia’s data: a cluster of eighteen thousand pure and extremely old stars located close to the galactic centre.
Unlike most other stars in the galaxy these ancient stars do not rotate with the galaxy, but instead haphazardly move in and out of its core. That, researchers think, is because these stars formed at a time when the galaxy didn’t spin, and didn’t yet have its famous spiral arms. That only came later, once the Milky Way started to absorb other galaxies and the angular momentum that came with them.
There is much still to learn about the birth of galaxies. Astronomers have long theorised about protogalaxies merging and growing, but have never caught any actually doing so. That’s mostly because this happened so long ago. Only with the James Webb do astronomers finally have a telescope able to capture what light still lingers from those ancient times. Yet, as this discovery shows, it is not always necessary to peer at the edges of the universe to find answers. Some, indeed, may be sitting a lot closer to home.
Did Saturn Lose a Moon?
Signs of ancient catastrophe litter Saturn’s system of rings and moons. The planet itself has an odd spin, tilting away from the plane of its orbit. One of its moons, Iapetus, has a long and hard-to-explain chain of mountains stretching across its equator. Titan, its largest moon, is drifting outwards. And Saturn’s dramatic rings are young, no more than two hundred million years old.
All of this, a team of researchers recently proposed, could be explained if Saturn once had another large moon. They name this hypothetical object Chrysalis, and propose it once lay between Titan and Iapetus. Yet at some recent date, they suggest, an orbital resonance between the moons formed: creating a gravitational interaction that quickly spun into chaos.
Chrysalis, computer models suggest, may then have swung inwards; brushing close to Saturn and either getting ripped apart or thrown out of the system entirely. Such a catastrophe would have knocked Saturn sideways, explaining its current tilted spin, and perhaps – if the moon was ripped apart – feeding the planet’s large and bright rings.
Still, the idea is not without its critics. Saturn’s rings may be far older than we currently think, perhaps getting regularly replenished by some unknown source of debris and ice. Proving the existence of a lost moon is of course hard – its most notable feature being its absence. Yet more concrete traces of its destruction may linger. A closer examination of Saturn, its rings and its moons may one day reveal surer hints of the long lost Chrysalis.
A Rocket and a Hurricane
Facing an incoming hurricane, NASA has once again been forced to delay the launch of its new moon rocket. An initial decision was taken on Saturday morning, as forecasts showed Tropical Storm Ian on course to cross the Florida peninsula, threatening the launch site at the Kennedy Space Center. At the time, however, NASA chose not to roll the rocket back into its hanger, a step that would delay launch to November, at the earliest.
Yet by Monday, as forecasts showed little improvement, NASA was forced to take that more drastic step. The giant rocket is therefore now being moved back to the hanger, where it will be protected from storm damage.
Yet this also means a launch cannot happen any time soon. NASA will miss the current launch window and probably the next one in October as well. That, inevitably, will delay the rest of the Moon program, and certainly pushes the first lunar landing of the twenty-first century to 2026, at the earliest.