Crash investigation

Crash investigation | Double Asteroid Redirection Test
Hera will provide humanity’s first view of a binary asteroid system, proceeding to map the entire surface of ‘Didymoon’ down to a size resolution of a few meters and the tenth of the surface surrounding the DART impact to better than 10cm, through a series of daring flybys © ESA - ScienceOffice.org

ESA’s Hera mission is designed to test deep space technology while exploring a distant asteroid and investigating a unique, man-made crater, testing a deflection method that may one day prove literally Earth-saving.

If all goes to plan, October 2022 will mark a significant achievement in the life of our species: the first time that Homo sapiens shifts the orbit of a body in the Solar System in a measureable way. The target is an approximately 170-m diameter asteroid – about the same size as the Great Pyramid of Giza – which is in orbit around another, larger asteroid: the 780m diameter Didymos (Greek for ‘twin’) near-Earth asteroid.The method is a NASA spacecraft called the Double Asteroid Redirection Test (DART), which will autonomously fly itself into the smaller body at 6km/s, nine times faster than a bullet.

The result of the collision with this refrigerator-sized DART spacecraft is expected to be an alteration in the tight 11.9-hour orbit of ‘Didymoon’ around its parent asteroid. This shift should be observable from Earth-based telescopes, because the Didymos binary pair will be on an unusually close approach to our planet at that point, coming just 11 million kilometres away at its nearest.

Didymoon’s degree of orbital shift will give researchers essential insights into the internal structure of asteroids and the potential of deflecting them as a means of planetary defence. But monitoring this historic event from a distance will not be sufficient by itself if we are to learn all its lessons.

By its very nature the Double Asteroid Redirection Test is a suicide mission, which has some unavoidable drawbacks. The last thing Earth will see in advance of the collision will be a close-up of Didymoon’s surface features – and then nothing. Potentially, DART might also carry a small ‘selfie-sat’ that it deploys beforehand in order to capture imagery of the moment of impact – but even so, past experience suggests nothing will be viewable directly at that point and only very limited data will be available on the surface properties of Didymoon.

Deep impact

On 4 July 2005, NASA’s Deep Impact spacecraft shot a 370kg copper impactor into comet Tempel 1. Shifting the orbit of this massive 7.6km × 4.9km body was never on the agenda; instead the aim was to expose the comet’s interior. However, in the impact’s aftermath millions of kilograms of dust and ice continued to outgas from the impact zone for days on end.

Deep Impact’s follow-on flyby showed nothing; it took a new visit by a separate spacecraft, NASA’s Stardust, in 2011 to finally measure the fresh 150m diameter crater scarring the comet’s surface. Plus, the distance involved means that terrestrial observatories’ measurement of Didymoon’s altered orbit will be stuck with a 10% residual uncertainty. The only way to improve on this, and really hone our understanding of this grand-scale space experiment, and see how the Double Asteroid Redirection Test impact has affected the surface of Didymoon, is to venture much, much nearer.

ESA’s Hera mission

That is the task of ESA’s Hera mission, the optimised design of which benefits from multiple ESA studies of asteroid missions across the last two decades – most recently the proposed Asteroid Impact Mission (AIM), which was planned in conjunction with the Double Asteroid Redirection Test. Hera is a small-scale mission in planetary terms, a large desk-sized spacecraft weighing in at less than 800kg fully fuelled (compared, for instance, to the van-sized, three tonne Rosetta comet-chaser). But it is also a highly agile, ambitious one.

Europe’s first deep-space CubeSat

In addition to its primary planetary defence objectives, Hera will demonstrate the ability to operate at close proximity around a low-gravity asteroid with some on-board autonomy similar in scope to a self-driving car, going on to deploy Europe’s first deep-space CubeSat, and potentially also a micro-lander, to test out a new multi-point intersatellite link technology.

Hera will also provide humanity’s first view of a binary asteroid system, proceeding to map the entire surface of Didymoon down to a size resolution of a few meters and the tenth of the surface surrounding the Double Asteroid Redirection Test impact to better than 10cm, through a series of daring flybys. How large a crater will Double Asteroid Redirection Test end up leaving? Will there be larger morphological effects, such as ground cracking, or stones and dust scattered widely compared to DART’s pre-impact images, implying post-collision quaking?

Planetary defence

Precise mapping of Didymoon’s volume will be combined with radio science experiments to assess how Didymoon’s gravity influences the spacecraft’s trajectory, to derive the asteroid’s density and constrain our models of its internal structure. Hera will also be a pioneer in the novel field of planetary defence: by pinpointing the shift in Didymoon’s orbit to a much greater precision than is achievable from Earth, the mission will give the fullest possible insight into the end result of the Double Asteroid Redirection Test collision – serving up hard data that might one day be used to safeguard Earth, demonstrating how to divert an incoming body before it becomes a threat.

What is Hera’s Asteroid Framing Camera (AFC)?

Hera’s baseline payload begins with an instrument called the Asteroid Framing Camera (AFC), to be used for guidance and navigation as well as scientific observation, which is an already-existing flight spare of a German contribution to NASA’s Dawn mission to the asteroid belt. This camera has been distinguished by returning remarkable images of the largest single asteroid, Ceres, and its mysterious bright spots.

Now, its sister camera is set to survey the smallest asteroid humankind has visited as well. The AFC is joined by a compact lidar (or ‘laser radar’) instrument to be used for measuring surface altimetry, plus one or more deployable six-unit CubeSat nanosatellites to carry a hyperspectral imager and a second instrument still to be finalised.

At the time of writing, Hera still has another 40kg of payload capacity available, which could take the shape of a high-frequency radar for measurement of subsurface properties, a mini-impactor proposed by Japan (a twin of the version currently in flight on Japan’s Hayabusa-2 asteroid mission, see below) or a mini-lander, currently under study by Airbus and DLR, the German Aerospace Center (based on a version also in flight aboard Hayabusa-2).

Space servicing vehicles

ESA has a long tradition of technology-testing missions being used for ambitious science goals, exemplified since the turn of the century by the Proba series of minisatellites, variously tasked with gathering data for environmental and solar science. Hera follows the same philosophy, even though it will go one better than the Proba family by departing Earth orbit entirely.

The single most significant technology Hera will demonstrate during its mission to the Didymos binary is intangible in nature, a software algorithm rather than physical hardware, but one seen as essential to a coming class of autonomous ‘space servicing vehicles’.

Hera’s streamlined nature means it will perform its guidance, navigation and control (GNC) activities through an innovative data fusion strategy, combining inputs from multiple sensors to build up a detailed picture of its surroundings in space. That would mean the bringing together second-by-second of visual tracking of distinctive features on the asteroid surface with altimeter distances plus onboard inertial and star tracker measurements. Future servicing vehicles would need to perform comparable data fusion when it comes to rendezvous and docking with satellites intended to be refurbished, refuelled or potentially deorbited. Any mistake in this scenario would lead to catastrophic collision, and plentiful space debris.

Failure is not an option

In the case of Hera, failure will not be an option when it comes to key manoeuvres such as CubeSat (and possibly lander) deployment or close Didymoon flybys, down to a matter of metres above the surface. But what if one or more of the sensor inputs is in error or an actuator delivers the wrong correction to the spacecraft trajectory or attitude? That is where Hera’s ‘Fault Detection, Isolation and Recovery’ (FDIR) technique comes in.

FDIR is an approach widely applied in space engineering, ranging from protecting individual electronic components to safeguarding the entire spacecraft: for example, modern space computer chips seeking to make up for memory ‘bit flips’ due to space radiation can perform calculations on a multiple, parallel basis, sometimes voting to decide the most likely truthful result. In a similar fashion, Hera’s data-fusion-based GNC FDIR is designed to identify errors in real time through ongoing sensor cross-checks, isolating them and then making up for them by triggering sensor or actuator reconfigurations or even, in case of extreme emergency, triggering an autonomous collision avoidance manoeuvre.

The combination of GNC and FDIR using vision-based sensing was achieved by ESA for the first time in the relatively straightforward but safety-critical case of semi-autonomous docking by the Automated Transfer Vehicle cargo spacecraft to the International Space Station (ISS). Expanding the technique to more challenging rendezvouses in space and increasing its degree of autonomy has been worked on for years in the context of this mission, most recently by GMV in Spain. Success will mark a giant leap forward for mission-critical autonomy.

What new discoveries will asteroid missions make?

Plenty of new discoveries can be expected from Hera. Each fresh close encounter with an asteroid has led to a fresh transformation in our understanding. A decade ago Europe took its first asteroid close-up, as ESA’s Rosetta probe performed a flyby of 2867 Šteins, a Gibraltar-sized diamond-shaped asteroid in the main Asteroid Belt. Dozens of craters were seen, including a gaping hole at the south pole of Steins – a large impact crater about 2km wide and nearly 300 m deep. A chain of several craters ran towards the north pole from this crater. The low density of Šteins suggests it is a ‘rubble pile’ asteroid, broken apart by previous impacts and held together weakly by its gravity – and probably fated to one day break apart. A second Main Belt asteroid flyby took place in 2010, as Rosetta passed the mammoth 100km 21 Lutetia. This higher-density asteroid was similarly studded with craters, confirming that collision is the main shaper of these primitive bodies.

Europe plays a key role in a new asteroid encounter scheduled for this July, when Japan’s Hayabusa 2 reaches near-Earth asteroid 162173 Ryugu. It will put down a micro-lander called the Mobile Asteroid Surface Scout (Mascot), developed by the German Aerospace Center (who previously contributed the Philae lander to Rosetta) and French space agency CNES, carrying an infrared spectrometer, a magnetometer, a radiometer and camera. A follow-on version of the Mascot lander, known as Mascot+, is currently under study to be carried by Hera.

Additionally Hayabusa 2 will perform its own miniature version of an impactor experiment, called the Small Carry-on Impactor (SCI), consisting of a small 2.5kg copper projectile given added force by a high-explosive charge. SCI will strike with a velocity of 2km/s, offering a valuable bridge between the kind of simulated impact tests performed in terrestrial labs and the full-scale Double Asteroid Redirection Test collision, allowing the testing of impact scaling laws. A follow-up SCI payload is also being considered for Hera, not to attempt to change Didymoon’s trajectory any further but to produce a second crater at a different energy level than DART. This experiment will provide invaluable data to fully validate numerical impact algorithms that will be key to designing any future planetary defence missions.

Exploration of these asteroids, and the many others surveyed so far, have highlighted their striking variety in terms of size, shape, surface characteristics and constituent materials. Similarly, asteroids rotate in various ways, from simple rotation to slow precession or rapid tumbling. It is possible that asteroid rotation is constrained by fundamental ‘spin limits’, beyond which centrifugal acceleration would lead material to escape from the surface of rubble-pile bodies. Indeed, such escapes might explain the origin of many binary asteroid systems, which make up 15% of the known total.

New light on collisional dynamics

The internal structure of asteroids remains a blank spot in scientific understanding. Are there large voids within their deep interior, or are they composed of loose regolith or conglomerates of monolithic rock? In particular, there is no way of knowing how an actual asteroid would respond to the specific external stimulus of an impact – short of trying it for real.

By shedding new light on collisional dynamics, the combination of the Double Asteroid Redirection Test plus Hera will add to our understanding not just of asteroid formation and evolution but the creation and ongoing history of our entire Solar System – a story etched in impacts.

Down at smaller scales, Hera’s surface observations will reveal the range of physical phenomena other than gravity that govern asteroid surfaces, influence their material properties and keep them bound together. What are the relative roles of electrostatic and Van der Waals forces, for instance? One proposal is that the most finely-grained asteroids might resemble ‘fairy castles’, crumbling to the touch. Such findings would hold relevance for asteroid mining as well as planetary defence, while also offering insight into the very earliest microscopic-scale processes of accretion, right back at the dawn of this and other planetary systems.

Historic moment

Hera is currently preparing for its Phase B1 study, along with a set of technology developments. The decision on whether the mission will progress to flight will be taken by Europe’s leaders at the end of next year. But certainly planetary defence is a global responsibility, and ESA is currently readying a new programme to be presented at the next Ministerial Council called Space Safety, that places planetary defence together with related topics such as space debris and space weather.

DART and Hera were originally conceived as one – the origin of the two missions can be traced back to an ESA 2002 study of a double spacecraft asteroid deflection mission called Don Quijote. If approved, Hera is on track for a 2023 launch, arriving at Didymos for its ‘crime scene investigation’ a couple of years later. The experience of the Stardust crater – as well as the recently discovered crater of ESA’s Smart-1 spacecraft on the Moon – suggests DART’s impact point will be largely unchanged from the moment of collision. Or, in the event of a delay in the Double Asteroid Redirection Test mission, then the pair might reach Didymos at the same time. Either way, a historic moment is coming in the shape of the DART impact. Humankind will draw maximum benefit from it through a close-up view.

Ian Carnelli

Programme Manager
General Studies Programme (GSP)

European Space Agency (ESA)

http://www.esa.int/ESA
http://www.esa.int/Our_Activities/Space_Engineering_Technology/Asteroid_Impact_Mission/Mission_profile

This article will appear in SciTech Europa Quarterly, issue 26 which will be published in March, 2018.

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