Based on the recommendations of asteroid experts, ESA hasselected two target asteroids for its Near-Earth Object deflectingmission, Don Quijote.
Don Quijote is an asteroid-deflectingmission currently under study by ESA’s Advanced Concepts Team (ACT).Earlier this year the NEO Mission Advisory Panel (NEOMAP), consistingof well-known experts in the field, delivered to ESA a target selectionreport for Europe’s future asteroid mitigation missions, identifyingthe relevant criteria for selecting a target and picking up two objectsthat meet most of those criteria. The asteroids’ temporary designationsare 2002 AT4 and 1989 ML.
With this input and the support ofESA’s Concurrent Design Facility (CDF) experts, the Advanced ConceptsTeam has now completed an extensive assessment of suitable missionarchitectures, launch strategies, propulsion system options andexperiments.
The current scenario envisages two spacecraft inseparate interplanetary trajectories. One spacecraft (Hidalgo) willimpact an asteroid, the other (Sancho) will arrive earlier at thetarget asteroid, rendezvous and orbit the asteroid for several months,observing it before and after the impact to detect any changes in itsorbit.
Industrial studies are now about to start; it will be downto European experts to propose alternative solutions for the design ofthe low-cost NEO precursor mission. This will be the first step towardsthe development of a means to tackle asteroid impacts – one of the fewnatural disasters that our technology can prevent.
A near miss?
Whilethe eyes of the world were on the Asian tsunami last Christmas, onegroup of scientists were watching uneasily for another potentialnatural disaster – the threat of an asteroid impact.
On 19December 2004 MN4, an asteroid of about 400 m, lost since its discoverysix months earlier, was observed again and its orbit was computed. Itimmediately became clear that the chances that it could hit the Earthduring a close encounter in 2029 were unusually high. As the dayspassed the probability did not decrease and the asteroid becamenotorious for surpassing all previous records in the Torino and Palermoimpact risk scales - scales that measure the risk of an asteroid impactjust as the Richter scale quantifies the size of an earthquake.
Onlyafter earlier observations of the object were found and a more accuratetrajectory was computed did it become clear that it would not impactthe Earth – at least not in 2029. Impacts on later dates, thoughunlikely, have not been totally ruled out. It is extremely difficult totell what will happen unless we come up with a better way to track thisor other NEOs and if necessary take steps to tackle them.
Mostworld experts agree that this capability is now within our reach. Amission like ESA’s Don Quijote could provide a means to assess athreatening NEO and take concrete steps to deflect it away from theEarth.
But every good performance needs rehearsing and in orderto be ready for such a threat, we should try our hardware on a harmlessasteroid first. Don Quijote would be the first mission to make such anattempt. The big question was: which asteroid and what should it belike?
Looking for the perfect target
The NEOpopulation contains a confusing variety of objects, and deciding whichphysical parameters are most relevant for mitigation considerations isno trivial task. But the NEOMAP experts took on the challenge and inFebruary 2005 provided ESA with their recommendations on the asteroidselection criteria for ESA’s deflection rehearsal.
People mightwonder whether performing a deflection test, such as that planned forDon Quijote, represents any risk to our planet. What if things gowrong? Could we create a problem, rather than learn how to avoid one?
Expertsworld-wide say the answer is no. Even a very dramatic impact of a heavyspacecraft on a small asteroid would only result in a minusculemodification of the object’s orbit. In fact the change would be sosmall that the Don Quijote mission requires two spacecraft – one tomonitor the impact of the other. The second spacecraft measures thesubtle variation of the object’s orbital parameters that would not benoticeable from Earth.
Target objects can also be selected sothat all possible concerns are avoided altogether, by looking into theway the distance between the asteroid’s and the Earth’s orbits changeswith time. If the target asteroid is not an ‘Earth crosser’, as is thecase with NEOs in the ‘Amor’ class (which have orbits with periheliondistance well in excess of 1 AU), testing a deflection manoeuvrerepresents no risk to the Earth.
Other considerations related tothe orbit of the target asteroid are also important, especially thechange of orbital velocity that is required by the spacecraft to ‘catchup’ with the target asteroid – the so-called ‘delta V’. This should besufficiently small to minimise the required amount of spacecraftpropellant and enable the use of cheaper launchers, but high enough toallow the same spacecraft to be used with a number of possible targets.
Navigationand deflection measurements requirements set some heavy constraints onthe target selection. The shape, density, and size are all importantfactors, but are often poorly known. A spacecraft orbiting an asteroidneeds to know about the object’s gravitational field in order tonavigate. The ‘impactor spacecraft’ must know the position of thecentre of mass to define the point it is aiming for.
Asteroidscome in all sort of flavours, but as far as composition is concernedtwo main types dominate. Our still rudimentary knowledge of theabundance of asteroids of different types in the near-Earth asteroidpopulation indicates that the next hazardous asteroid is more likely tobe a ‘C-type’, than an ‘S-type’. C-types have dark surfaces with acarbonaceous spectral signature, while S-types have brighter surfaces,their spectra matching closely that of silicates. The surfaceproperties of the target asteroid -and in particular the percentage oflight that it reflects - are a critical factor in the final phase ofthe impactor spacecraft navigation. The brighter it looks the easier itis to aim at. However for a rehearsal the target should not be too easy.
ESAhas selected asteroids 2002 AT4 and (10302) 1989 ML as mission targetsbecause they represent best compromise among all the (sometimesconflicting) selection criteria. A decision on which of the two willbecome the final destination of both Sancho and Hidalgo spacecraft willbe made in 2007.
Don Quijote – the knight errant rides again
Thephase of internal studies on the Don Quijote mission is now over, andit is time for the space industry to suggest suitable design solutions.ESA has made an open invitation to European space companies to submitproposals on possible designs. The selection of the most promising oneswill take place towards the end of the year. In early 2006, two teamsshould start working on their interpretations of this technologydemonstration mission. A year later, once the results are available,ESA will select the final design to be implemented, and then DonQuijote will be ready to take on an asteroid!
DonQuijote is a NEO deflection test mission based entirely on conventionalspacecraft technologies. It would comprise two spacecraft - one of them(Hidalgo) impacting an asteroid at a very high relative speed while asecond one (Sancho) would arrive earlier at the same asteroid andremain in its vicinity before and after the impact to measure thevariation on the asteroid’s orbital parameters, as well as to study theobject.
Asteroid 2004 MN has now been given an officialdesignation, (99942) Apophis. Recent observations using Doppler radarusing Arecibo radio telescope in Puerto Rico have reduced the impactprobability during future encounters to very small levels, though theyhave not totally ruled out an Earth impact. In 2029, the asteroid willhave the closest approach ever witnessed for an object of this size,swinging by the Earth at a distance of around 32,000 kilometres. Itstrajectory will be well within the geosynchronous orbit used by mosttelecommunications and weather satellites, and the object will bevisible to the naked eye. Further radar measurements are expected in2013.
Don Quijote target asteroids 2002 AT4 and (10302) 1989 ML do not represent any danger to our planet.
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