Adam Hibberd
September 2022
Of the existential threats posed by celestial objects to our Earth, the cradle of humanity and currently our only life-support, there are myriad such candidates. In fact we are in a veritable astronomical mine-field, the key question is not if but when. Take comets for instance. These objects composed primarily of dust and ice, originate in a spherical shell encompassing the Solar System at sun distances of between around 2000 au and 200,000 au.
They have inconspicuously lived in this domain, the Oort Cloud, for billions of years, and they will continue to inhabit it indefinitely, unless a gravitational nudge, say from galactic tides or maybe some passing object such as a rogue planet, send them hurtling towards our sun on a highly eccentric, low perihelion orbit, and on a potential collision-course with Earth. With collision speeds up to 70km/s, an impact would be devastating. But what other potentially hazardous objects (PHO) should concern us?
Interstellar objects (ISO), visitors to our Solar System after travelling vast distances through interstellar space, are also possible doomsday machines. Because of their high speeds their damage potential is even greater than comets, and what’s more we have direct evidence of just such an impact through work by Siraj and Loeb indicating a bolide observed in 2014 was interstellar, but that’s another story.
In addition near Earth asteroids (NEA) are generally connected with the asteroid belt, a diffuse collection of bodies which circle the sun between the orbits of Mars and Jupiter. The news here is good in that NASA is pretty certain that they have mapped most of these asteroids and are confident that there will be no Earth strike in at least the next 100 years.
All this doom-and-gloom is pivotal to the argument for humanity to seek pastures new and move into space – perhaps as a means to an end, human habitation of space, or perhaps in order to actually go somewhere and colonise other planets, possibly terraforming as we go. In my view this ambition tends to ignore one very basic premise and that is we are already on a spaceship, a massive ecosystem built not entirely by ourselves, but by the laws of nature, an abode in which billions can live unaided, without space suits. Earth has fabricated our collective support system by a combination of location, atmosphere and magnetic field. In this context surely we should be seeking to extend our existence here, we need a long term plan both to halt and reverse climate change and environmental pollution, and to address the concern I mentioned above – an effective planetary defence system. Well some US colleagues and I have noble ambitions in this regard.
Have you heard of Apophis? The erudite reader will know of this Egyptian deity as the god of Chaos. Apophis is in fact a very appropriate name for an NEA which when discovered frightened the hell out of NASA personnel as initial predictions indicated high likelihood of Earth impact. Further observations ruled out this catastrophe, but such is the orbit of Apophis that it will inevitably come around near Earth again and again – the next swing-by being Friday the 13th of April, 2029. This hair-raising encounter – below geosynchronous satellite altitude – will be a terrific opportunity for scientists, conveying to us a treasure trove of scientific data. So why not land a spacecraft on it?
But why land a spacecraft on it? Well my colleagues and I have the future existence and prosperity of human-kind in mind here because we feel if our project is funded by NIAC (NASA Initiative for Advanced Concepts), this would genuinely and materially contribute to the NASA planetary defence objectives.
Furthermore it would also support a far-sighted and visionary ambition to send miniature spacecraft to the nearest system to our own, Proxima Centauri, the Breakthrough Starshot Initiative (BSI). Rocket engineers would warn of the tyranny of the Tsiolkovsky equation here, but don’t fret, these spacecraft have no onboard rocket propellant, their thrust is provided by light, incident upon sails, and emitted by extremely powerful GW (gigawatt) Earth-based lasers. You could view the propellant not as a chemical, but as light itself, which can deliver a force – radiation pressure – when it reflects off any suitably interposed object. Laser sails must have a large area to weight ratio to maximise the acceleration delivered by the drive laser, and speeds of 0.2c are envisaged, 20% of the speed of light. With this initiative, humanity can explore our galactic environs without requiring huge and lumbering habitable world-ships, a concept which in my view should be confined to the realms of science-fiction. It is with robotic missions that humanity can explore the universe.
But how are laser-sails and planetary defence connected? This is where our NIAC proposal comes in. Let us imagine that a Potentially Hazardous Object, PHO, is discovered. Surely just as a boxer before entering the ring should know all his opponent’s strengths and vulnerabilities (if he is sensible); then so should humanity have the capability to scrutinise the exact nature and severity of the threat from any PHO, allowing commensurate responsive countermeasures to be undertaken. A rocket mission on stand-by would be logistically demanding, expensive and could not be tailored to the precise PHO in question, a costly broad-brush approach would need to be adopted, which probably couldn’t cover all eventualities.
Our team say that laser-sails can come to the rescue here, especially if the drive laser were mobile. You see, at least in principle, a laser-sail driven by an Earth-based laser can go distant places really fast, as long as the necessary laser infrastructure is available. Sure the technology doesn’t exist at the moment – the realisation of the BSI remains decades into the future and the technology is comparatively primitive at the moment compared to the predictions as to what can be eventually achieved.
But we have calculated that by 2029, the time of the celestial visitation, we can develop the tech to send a laser-sail up to the giddy heights of the Apophis closest approach, around 30,000km altitude. Way short of Proxima Centauri, true - but nevertheless not only a technological stepping-stone to humanity’s dream of interstellar travel but very importantly, it augurs the ability for rapid-response to threats to our own planet, to save us all from the prospect of oblivion.
Now I work with trajectories, and a while ago I developed some software, Optimum Interplanetary Trajectory Software (OITS). Not too far from the back of my mind was the problem of planetary defence and for this reason I decided to make the software available publicly (github) to democratise it – so that anyone could work out intercept trajectories to PHOs, should any candidates turn-up (allowing anyone to plot a trajectory to save the planet).
As far as the NIAC is concerned, I decided that maybe I could contribute something on the trajectory side but it was understood that the proposal hinged on development of the technology – the laser-sail – not necessarily the details of the mission, though the NIAC instructions to applicants suggests that everything must have a solid mission context. This was an excuse for me to develop some bespoke software, but first we need to look at the trajectory Apophis will follow as it encounters Earth – look at Figure 1 & Figure 2.
As far as the sail trajectory is concerned, the 15 MW (megawatt) drive laser required for this venture would need a helpful hand from a Black Brant sounding rocket. Historically, such rockets have been launched from Wallops Island on multiple occasions. You may observe that Wallops is not far off the ground track of Apophis, and at the time of passage by Wallops, has travelled beyond its perigee point (mid-Atlantic) and started its out-bound path away from the Earth, sweeping a curve almost right through the centre of the USA.
The trajectory of the laser sail after release by the Black Brant, including its initial acceleration by the drive laser which lasts precisely 1000s, can be seen in Figure 3.
But Wallops is not the only candidate for a launch and laser location, why not Spaceport America? Situated in New Mexico, and the location for the Virgin Galactic Hub, Spaceport America may seem a bit too far south of the asteroid’s ground track. It turns out not so – refer to Figure 4. Charts of reference data for the Wallops trajectory are shown in Figure 5.
But what about the cause of the westward motion of the sail after an initial eastward bearing? Note the yellow altitude bars are not drawn to scale, the altitudes reached by the sail are upwards of 35,000 km, and the direction of the sail acceleration delivered by the laser upon the sail’s release from the Black Brant (at 150km) is largely vertical with only a comparatively small Eastward component. This high vertical velocity component is ideal for a pseudo force seen in rotating reference frames such as the Earth’s – the Coriolis Force to take full effect and which indeed will act in a Westward direction. From the point of view of the sail it will rise to such a height that the Earth’s rotation will move the Earth Eastward with respect to it, hence the westward trajectories observed by someone located on Earth.
I end this blog with the i4is motto – Scientia ad Sidera – knowledge to the stars, but let’s reverse that sentiment. I firmly believe that it is through collective continual acquirement of knowledge, scientific knowledge, from what we observe and measure of our environment that we – humanity - shall find our salvation. It is through knowledge that we can understand any threats – celestial, viral, bacterial, terrestrial, and so on – and take responsive action to ensure our continued survival. If we didn’t have astronomy, we wouldn’t know about Apophis or other PHOs and so our extermination would be sudden and unavoidable. We have our science, let us use it positively to ensure our continued existence and to allow us to live comfortably in the space we inhabit.
