Destructive impact probes

The mission of a destructive impact probe ends successfully with a vehicle (or even just a passive projectile) being destroyed on impact with the surface of another world. The first destructive impact probe was Luna 2, which, along with the launcher's upper stage, impacted the Moon in 1959. Luna 2 hit the surface at 3315 ms~ (Blagonravov, 1968), demonstrated by the loss of the radio signal. Rangers 6-9 impacted the Moon a few years later, obtaining (in the case of 7, 8, 9) close-up images of the lunar surface prior to impact at 2620-2680 ms-1 (e.g. Schurmeier et al., 1965; Hall, 1977). The craters made by these impacts were subsequently found in Lunar Orbiter and Apollo images. Discarded Apollo lunar module ascent stages and Saturn IVB rocket stages impacted the Moon and proved useful as artificial, well-characterised seismic sources (Latham et al., 1970, 1978).

Many years later, Lunar Prospector ended its successful mission by impacting the lunar surface at 1700 ms~ , in an attempt to detect water ice by means of telescopic observations of the ejecta plume from Earth. No plume was seen, however, but the exercise resulted in calculations of possible H2O ejecta cloud propagation that may be applicable to future events (Goldstein et al., 2001). The lunar orbiters Hiten and SMART-1 also ended their missions by impacting the lunar surface. NASA's LCROSS (Lunar CRater Observation and Sensing Satellite) is due to make another attempt to detect ice using the impact technique.

The destructive impact approach was employed in spectacular fashion by the Deep Impact mission, launched in 2004 (A'Hearn et al., 2000). The spacecraft comprised a flyby stage and an impactor, which separated prior to arrival at the target comet. The 370 kg impactor, most of the mass of which was copper, impacted the comet nucleus at 10.2 km s_1 to study the cratering process and nature of the comet nucleus sub-surface material. The flyby stage observed the comet and the impact event; simultaneous Earth-based observations were also made. The impactor spacecraft was instrumented with a camera and employed closed-loop targeting. The destructive impact probe approach had been proposed earlier, in the context of the never-implemented Clementine II mission, which planned to impact instrumented probes onto asteroid targets (Hope et al., 1997). Such an approach is one of the concepts proposed for mitigation of a threatening near-Earth object (NEO), where the momentum of the impactor is used to deflect the NEO's orbit slightly. A demonstration of NEO deflection has been under study by ESA as the Don Quijote mission.

In summary, destroying a spacecraft, rocket stage or other projectile by impacting it onto another world can be useful for one or more of the following reasons.

• Remote or in situ observations/measurements of the ejecta plume for physical and compositional measurements, either telescopically from Earth or by another spacecraft.

• Remote observations of the crater from another spacecraft, for crater scaling information and exposure of sub-surface material.

• Generation of an artificial seismic source for measurements elsewhere on the body.

It is also worth mentioning here the possibility of observing the glow from atmospheric entry probes. This has been proposed in the context of Huygens (Lorenz, 2002) and, as for some other missions, was only successful in establishing an upper limit on emission (Lorenz et al., 2006). The telescopic study of emission from bodies re-entering the Earth's atmosphere has also been the subject of recent work (e.g. in connection with the Genesis and Stardust return capsules, and the analysis of the ill-fated Space Shuttle Columbia) - these observations allow characterization of the thermal and non-thermal emissions from the shock layer, for a body with known mass and velocity. An additional serendipitous investigation was the encounter of the Mars Exploration rover Opportunity with part of its heat shield on the Martian surface, allowing at least partial documentation of the depth of charring of the thermal protection material. There are substantial and interesting intersections of the entry protection engineering disciplines with those associated with meteors and meteorites.

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