Peering inside Near Earth Objects – ISHTAR
The history of life on earth has been shaped by the impact of comets and asteroids with our planet to a degree that we have only recently started to appreciate. Geological records have illustrated the devastating effects that can result from an asteroid collision, in some cases altering the evolution of the planet.
The risk posed to human civilization from the impact of a Near-Earth Object (NEO) cannot be ignored. According to safety standards in use across Europe, the level of threat that any given scenario produces is judged by both the probability of the event happening and the amount of fatalities that it would produce. An impact of a ~1km sized body has a probability of occurring every 100,000 years or so but would result in an estimated
1.5 billion fatalities. If an industrial plant operator posed a comparable threat, they
The ISHTAR mission plans to rendezvous with two asteroids and measure key physical properties like mass, density and internal structure. These parameters will provide insight into possible mitigation techniques, as well as knowledge about their origin and evolution. would be required to take steps to reduce the risk to levels that were deemed tolerable. So too the threat from NEOs must be acted upon.
At the present time, Earth is defenceless against the impact of a NEO. However, numerous theoretical ‘saviour strategies’ have been outlined ranging from the proximity detonation of a powerful nuclear device, to the gentle push supplied by attaching a low thrust, high specific impulse engine to the threatening body. The appropriate mitigation technique to utilize in a given situation is highly dependent on the structure of the incoming NEO. For example, a high-powered explosion or a hypervelocity impact with a NEO would most likely result in the production of multiple, smaller NEOs on a similar trajectory to their parent asteroid. This, almost certainly would not remove the threat, but rather generate a different type of problem. Similarly, the deployment of a propulsion system on the asteroid (e.g. solar sail, ion engine, etc.) would also be affected by the internal structure of the
NEO: for a body consisting of a loose amalgamation of separate entities, the issue of where and how to apply the thrust without disrupting the NEO structure is critical.
Quite simply, we need to know how NEOs are constructed and what they are made of before we can plan a defence against them. Knowledge of asteroid structure will also increase scientific understanding of the origin and evolution of these planetary bodies, and of the whole solar system.
Mission overview
The ISHTAR (Internal Structure High-resolution Tomography by Asteroid Rendezvous) mission provides a unique opportunity to address key, fundamental issues necessary to understand NEOs both from a scientific and hazard mitigation viewpoint. These key issues translate to certain critical
NEO properties that require measurement, the most important of this information being:
 bulk mass and density
 internal structure, cohesion, mass distribution
 detailed shape
 specification of spin rate, including pole orientation and precession (if any).
 detailed surface geology, including characterization of regolith.
To make a measurement of these properties requires utilization of a range of instruments. Powerful radio science instrumentation is required to measure the mass of the NEO via the detections of

gravity deflections of the spacecraft. Tiny variations in the Doppler shift of the radio signal transmitted to earth are an indication of slight changes in the spacecraft velocity caused by different mass concentrations in the asteroid body. This can enable the external gravitational field to be reconstructed, which, in conjunction with internal structure measurements, can result in formation of a detailed internal model of the asteroid.
Obtaining internal structure measurements of the asteroid requires a combination of techniques, centred around a radar tomographic instrument. The imaging of the internal structure uses electromagnetic radiation in the radio range. The low radio frequencies can penetrate deep below the asteroid surface (up to ~1km or more) before being reflected back and picked up by the radar antenna on board the spacecraft. By taking measurements from different points around the asteroid it is then possible to reconstruct what the interior of the object looks like, using techniques similar to those used for medical magnetic tomography scans. ISHTAR will be the first mission to make measurements of the internal structure of an asteroid.
Specification of surface properties, along with the shape and volume of the body require an imager to be included in the payload. A multispectral capability will add global composition information, while specific mineralogical spectral features would require an infra-red spectrometer.
Astrium and its partners have extensive experience in these instrument areas. For example, in the radar field, Astrium’s Advanced Synthetic Aperture Radar (ASAR) has provided detailed images of the
Earth from on board ENVISAT. This heritage will be exploited for the ISHTAR radar payload and other instruments.
Radar Tomography
The inclusion of a radar tomography instrument in ISHTAR’s payload opens up the possibility of making crucial internal structure measurements – information that is key to planning a mitigation strategy. Deep imaging is possible because low-frequency radar has the capability of penetrating below ground.
The radio waves penetrate the rocks of the asteroid until they are reflected back by a change in material composition, density or electrical properties.
By
The consequences of a NEO impact are strongly dependent on its internal strength and composition. The Meteor crater (shown) would have been much smaller if the asteroid had been made of stone instead of iron. collecting these reflected echoes it is possible to draw a map of the interior, including the presence of voids, fractures, metal, ice, etc. The extent to which such a
3D profile can be achieved depends on the absorption characteristics of the body and the wavelength of the radiation being used. The internal structure information is reconstructed with algorithms similar to those used in the medical or geophysical tomographic fields after many orbits of the object have been executed.
Target asteroid
The NEO population is dominated by asteroids belonging to two spectral classes: stony asteroids
(class S) and primordial asteroids (class C). ISHTAR ideally would survey two asteroids, one example of each class, armed with the above instrumentation. A single survey of one asteroid of one of the other minor classes would still provide extremely useful information, however. The typical target profile is an asteroid with size ~1km. The journey to rendezvous will take an average of one to two years. A number of potential targets for the mission have now been identified. These include objects like asteroid (4660) Nereus, which is predicted to pass within 3 lunar distances of the Earth in 2060, a near miss in astronomic terms!