Crashing into an Asteroid - Australia’s Front Row Seats

At 9:14 am Tuesday morning (Australian eastern standard time), Australian radio telescopes are going to be the first in the world to receive data from a rather remarkable space mission. It’s hoped that the data streaming through into the Canberra Deep Space Communication Complex (CDSCC) will help humanity learn about how to protect itself against the very real threat of impacting objects, such as asteroids and comets. 

The mission is known as the Double Asteroid Redirection Test (DART) - a mission that is intentionally crashing a spacecraft into a small asteroid with the objective of assessing if we (humanity) have it within our power to change the course of potentially dangerous space rocks that are on a collisional trajectory with our home planet. The more summarised term for it is ‘planetary defence’. 

“It’s really an interesting mission and is important on many levels,” said Professor Jonti Horner from the University of Southern Queensland. “When you have movies like Deep Impact and Armageddon, and facts like the death of the dinosaurs - there is a huge cultural awareness, and many people are interested in this.”

“To also have the ability to say that we are taking matters into our own hands and learning to protect ourselves as a species, is a statement that we can do this – that we can collectively come together, and work and defend the lives of many”

“Over the last few years, we’ve also discovered a lot more asteroids and can recognise a threat and do something about it. So this mission gives us the opportunity to learn a bit more about our processes, our abilities, and about how we can take action. But then you have the more scientific side of things, with this mission demonstrating that our measurements are good enough that we can do such a thing,” he said. 

DART, a 600-kilogram space probe that features no payload, but rather a suite of sensors and imaging tools, was launched in November 2021 aboard a SpaceX Falcon9 rocket. The mission, being run by NASA and the John Hopkins University Applied Physics Laboratory (APL) is humanity’s test to try and use a kinetic impactor to cause a deflection in the orbit of an astrophysical body. 

The target is the binary asteroid system Didymos, which features the larger, main asteroid Didymos (with a diameter of 780 metres) and a smaller moonlet, Dimorphos - which has a diameter of 160 metres. Neither Didymos, nor its moonlet poses any risk to the Earth as the system is not Earth-crossing, nor will any change in its orbit generated by the DART mission create any risk. 

Didymos was selected not only because it is a nearby binary system, but also because it is eclipsing from Earth’s perspective. That means, the smaller moonlet passes in front of the larger asteroid from our view, allowing Earth-based telescopes (even space-based telescopes) to make detailed observations of the variation in the light curve. And by measuring this variation prior to the impact, and after the impact, astronomers will be able to determine the orbital period of the moonlet, and thus, and variations the spacecraft impact had.

“Generally speaking, we are often limited in how much we can learn about these bodies that wander around the Solar system because the motion of the asteroid alone is not enough to tell us about its mass,” said Professor Horner. 

“But here, we are targeting the moonlet of the larger asteroid, which we have studied in detail, thanks to its orbital motion around the main body. We’ve been able to understand its orbital period, and thus mass, very well from this. And the slight change in this well-measured orbital period this impactor will make is a really elegant way of quantifying the deflection our efforts induce. That is, this is very measurable.”

DART is going to slam face-on into the 4.8 billion kilogram rock, in the opposite direction of the moonlet’s trajectory. Whilst the notion of a feeble 600-kilogram object hitting a multi-billion kilogram object might seem absurd, what’s expected to happen is that the impactor will have a small, but measurable effect, causing Dimorphos to slow down slightly. This small reduction in velocity will shift Dimporphos into a new, smaller orbit. 

As the spacecraft collides with the moonlet, it will deliver an impulse equivalent to about three tonnes of TNT, and if all goes to plan, shift the orbit by only millimetres. At first, this might not seem significant, but planetary defence missions are designed to work over the long-term - a small change in millimetres eventually leading to larger shifts in time. 

“So most likely – a bit like the Deep Impact mission - it won’t be an incredible spectacle – we’re not going to see this thing suddenly become brighter than the Moon or anything like that. It’s a bit like a fly hitting the windscreen of an SUV – except the differences in masses is even more dramatic than that,” said Professor Horner. 

Beyond the modelling, there are of course a number of unknowns as to how this orbital variation might unfold. For example, the topology of the surface might create slight, unaccounted variables that were never accounted for in the modelling. The expectation is that debris from the surface will form a large ejecta, leaving behind a crater on the surface. 

“What is going to be interesting is the kind of scar it will leave. For example, with the Deep Impact mission, everybody expected it to open up a crater, which was part of the experiment. But was it going to open up a crater and expose fresh material or trigger activity on the comet? What actually happened was that the Deep Impact probe simply punched a hole into the comet, about the size of what the impactor was itself.”

“What will result from this impact is in some degree dependent on how porous or rubble pile this moonlet is – and I suspect the team who is running the experiment have a good idea of this. It could create a smaller crater or a bigger crater depending on how solid, and rigid the body is.”

“We can also learn from the shape of the impact and improve our models – which is really useful for people who study craters. We’ll have the Italian CubeSat in orbit around for some time, which will relay data and images from the impact and give us a nice view of how things settle down over the next few weeks, and in a few years, we’ll also have another mission [HERA] also going to have a closer look at things. This will really help scientists who produce impact modelling refine their work”

“It will also be interesting on what effect it has on the materials elsewhere on the asteroid. The question will be will this impact create different features in the moonlet elsewhere? That is, will it send shocks and quakes throughout the moonlet, causing material elsewhere across its surface to reconfigure? It might be too small to do that, but it would be really interesting to see – and these Italian CubeSats are going to be in the perfect position to observe this. We’ll learn a lot about the physical body itself not just from the impact, but from its effects over longer timeframes”.

Riding along with the DART mission is the LICIACube (the Light Italian CubeSat for Imaging of Asteroids), which has two optical cameras to conduct in-situ photography of the event unfolding, sending back data to Earth. Astronomers expect to know about a week after the event if there has been a change in the orbital velocity of the moonlet both from data captured by LICIACube, as well as Earth-based observations which will watch the lightcurve of the main asteroid dip as the Moon eclipses it, from our perspective. 

In a few years from now, a secondary mission - run by the European Space Agency (ESA), and known as Hera will revisit the Didymos system to take more detailed in-situ measurements, after some time has passed to determine how much the impactor actually deflected the moonlet. 

Once again, Australia’s network of radio antennas is going to play a pivotal role in this mission, in particular receiving the first images and data from the DART mission. In fact, the CDSCC will receive the final signals from the spacecraft as it approaches and impacts with the moonlet. Additionally, the complex - which is managed by Australia’s national science agency, CSIRO and located just outside Canberra in Tidbinbilla, will also be receiving data from the LICIA Cubesat as it follows and records events unfolding pre and post the impact. 

“The planets or in this case, the asteroids have literally aligned so that the southern hemisphere has the best view of DART’s collision with the moonlet, Dimorphos,” said Glen Nagle, who managed outreach and the visitor centre of the CDSCC. 

“The Canberra Deep Space Communication Complex (CDSCC), with its giant antennas have the two-way communication capability needed for any last-minute commanding to the DART spacecraft as well as with the LICIA Cubesat. CDSCC’s antennas also have the level of sensitivity needed to receive the images and data from both spacecraft as they complete their historic really close encounter.”

And thanks to the CDSCC, the rest of us are going to be able to watch events unfolding in almost real-time, as data is processed on board both crafts, before being transmitted back to the CDSCC and then out to the world in under a minute. 

“At 11 million kilometres away, the spacecraft’s radio signals travelling at the speed of light, will take 38 seconds to reach Earth,” said Mr Nagle. “Add to that about 1-2 seconds of onboard processing before the image is sent and then receiving, processing on Earth, in total we’re seeing the image about 40-42 seconds after they were taken. From DART’s Draco camera, we receive images at a rate of 1 per second until shortly before impact.”

Data to and from the DART mission will be delivered through DSS-43 - a 70-metre diameter antenna, and DSS-35 - a 34-metre antenna, both of which are located at the CDSCC. Additionally, ESA’s New Norcia tracking station (located in Western Australia, and also operated by CSIRO) has provided regular support to NASA’s Deep Space Network, tracking DART’s position.

And whilst the eventfulness of the actual impact are exciting, it's the ongoing tracking that is going to really allow an analysis of the science to continue - to let us know if this mission has been a success or not. 

“CDSCC, along with the other two Deep Space Network stations in Madrid, Spain and Goldstone, California will continue to downlink data from the Italian Space Agencies LICIA CubeSat,” said Mr Nagle. “Images and data collected by LICIA will take several weeks to download due to the much lower data rate capability of this small spacecraft”

This of course is not the first time the CDSCC has contributed to missions relating to asteroids and comets. 

“CDSCC has supported many missions to visit asteroids and comets across our solar system – JAXA’s two Hayabusa sample-return missions; NASA’s OSIRIS-Rex sample-return mission; and ESA’s Rosetta and Philae mission to comet 67P Churyumov-Gerasimenko.”

Along with this, the facility has continually worked to track, monitor and analyse other asteroids and bodies, as part of NASA’s Solar System Radar program. 

“By using the powerful transmitters on our antennas, we can ‘bounce’ a radio signal off the surface of a near-Earth asteroid, effectively scanning it like sonar. Return signals are then received by another station, which in recent years has been the CSIRO’s Australia Telescope Compact Array dishes located in Narrabri, NSW,” said Glen. 

“The radio images help scientists and astronomers to better characterise the size, rotation, and trajectory of these objects, as well as look for small companion asteroids such as Dimorphos.”

Whilst the apocalyptic threat of meteorite impacts has been over-glorified in Hollywood blockbusters like Armageddon, Deep Impact and more recently, Don’t Look Up and Greenland, the very real threat of Earth colliding with astrophysical bodies varies on a typical risk matrix scale, where the likelihood and effects are taken into consideration. A number of key factors dictate how these scenarios will unfold, such as the body's mass, composition and entry velocity when it does collide with the Earth. 

“We get hit all the time – it depends on how big we are talking. From our own backyard, we see tiny meteors streaking across the sky. These meteors don’t cause any damage,  as they ablate harmlessly around 80km above the ground, with nothing making it to Earth's surface,” said Professor Horner.

“On any night of the year, you should be able to see several such meteors every hour - but at some times of the year, you can see far more. The Earth is also bombarded all the time by smaller dust - so small that it slows down in the outer atmosphere before it can ablate (burn up). This produces some of the dust that falls around us – so some of the dust in your house is actually space dust.”

“It’s when we scale the mass of the projectiles up, that we start to see risk significantly increasing (though the likelihood is less often),” he said.

“In 1908, a 100-metre object came in over Tunguska and Russia, which we think was a fragment of a comet, and created an airburst that flattened trees in a 2,200 square-km radius. That’s about the size of greater Sydney, so objects of this size are known as city killers.”

“At the highest end, people often hold up the 1-kilometre diameter of an impacting object as being the boundary between the regionally and globally devastating effects, which is the kind of object where the numbers suggest that impacts on this scale could kill a quarter of the world’s population. Though, these happen much less often – on average about every 300,000 years.”

It’s also something that is never far from our minds - be that due to the Hollywood blockbusters, the constant reminder of the fall of the dinosaurs, astrophysical events that we witness (Jupiter has now taken several beatings in our lifetime), or when we have moderately large events here on Earth - like that experienced in Tunguska in 1908, or Chelyabinsk in 2013. 

“It’s one of those things when there is a threat to all of us where we can quantify and understand, and something we can fix as well,” he said. 

 “This is the sort of thing that the public is also conscious off - for example, the death of dinosaurs showed that asteroids could wipe out entire, successful species from the planet – so if it could happen to them it could happen to us.”

“These types of events also keep happening, and that keep it fresh in our memories – for example, we have all the scars we see on Earth from historical impacts, the incident when Shoemaker-Levy crashed into Jupiter in 1994, and in 2013, the fireball that exploded over Chelyabinsk, in Russia. It’s the stuff of disaster movies, but we can try to prevent it, and missions like this, help us understand how to do just this.”

Thankfully, the DART mission is not going to nudge this tiny moonlet into any new orbit, and in particular, a new orbit that causes it to intersect with Earth’s trajectory around the Sun. It’s also of the size that, with enough warning (and hopefully as this test will prove), we could potentially deflect it away from us, should the need ever arise for a similar-sized object. 

“This object is a bit larger than the 100-metre Tunguska object. We think that object, in particular, was more cometary rather than an asteroid, so its density was much less than what this moonlet’s is – and this of course will change the impact damage factor.”

“But this is a kinetic energy question, so it also strongly depends on the impact velocity”

“For a smaller scale version, the rock that came in over Arizona desert that created Meteor Crater about 50,000 years ago, was only 50-metres in diameter, and yet it created a scar that is more than 1-kilometre wide, and a couple of hundred metres deep. And that’s because that was pretty much solid iron, which made it through the atmosphere intact. So you get a different outcome depending on both the impact velocity and the composition – which is why it is so important that we work out what these things are made off,” concluded Professor Horner.