The SMART-1 lunar impact

Icarus 207 (2010) 28–38

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The SMART-1 lunar impact M.J. Burchell a,*, R. Robin-Williams a, B.H. Foing b,1 a b

Centre for Astrophysics and Planetary Science, University of Kent, Canterbury, Kent CT2 7NH, United Kingdom European Space Agency, Research & Science Support Dept., European Space Research & Technology Centre, NL-2200 AG Noordwijk, Netherlands

a r t i c l e

i n f o

Article history: Received 5 May 2009 Revised 21 August 2009 Accepted 7 October 2009 Available online 14 October 2009 Keywords: Impact processes Moon, Surface Cratering

a b s t r a c t The SMART-1 spacecraft impacted the Moon on 3rd September 2006 at a speed of 2 km s1 and at a very shallow angle of incidence (1°). The resulting impact crater is too small to be viewed from the Earth; accordingly, the general crater size and shape have been determined here by laboratory impact experiments at the same speed and angle of incidence combined with extrapolating to the correct size scale to match the SMART-1 impact. This predicts a highly asymmetric crater approximately 5.5–26 m long, 1.9–9 m wide, 0.23–1.5 m deep and 0.71–6.9 m3 volume. Some of the excavated mass will have gone into crater rim walls, but 0.64–6.3 m3 would have been ejecta on ballistic trajectories corresponding to a cloud of 2200–21,800 kg of lunar material moving away from the impact site. The shallow Messier crater on the Moon is similarly asymmetric and is usually taken as arising from a highly oblique impact. The light flash from the impact and the associated ejecta plume were observed from Earth, but the flash magnitude was not obtained, so it is not possible to obtain the luminous efficiency of the impact event. Ó 2009 Elsevier Inc. All rights reserved.

1. Introduction SMART-1, Small Mission for Advanced Research and Technology (Foing et al., 2001, 2003, 2006, 2007) was a European Space Agency mission to place a satellite in orbit around the Moon. Launched on 27th September 2003, it arrived in lunar capture orbit on 15th November 2004 and on baseline science orbit on 15th March 2005. This somewhat long transfer time was due to the successful use of an ion thruster as the propulsion system for the Earth–Moon transfer. The spacecraft was Europe’s first to orbit the Moon. As well as being a technology proving mission (cf. use of the ion thrusters in deep space), SMART-1 has also produced a series of scientific results, e.g. lunar composition and impact studies in general, see Foing et al. (2008), using a variety of instruments such as the D-CIXS spectrometer (Grande et al., 2007) and AMIE imager (Josset et al., 2006). At the end of its life, when its orbit was starting to decay to dangerously low altitudes, the spacecraft was deliberately placed on a trajectory to impact the lunar surface at a very shallow angle (1°) at 2 km s1. It did so at approx. 33.3°S and 46.2°W on 3rd September 2006 at 5 h 42 UT, and depending on the exact conditions of the lunar surface at the impact point will have resulted in a very highly oblique impact. This impact will have produced a new crater on the lunar surface. This is not the first man-made impact on the Moon; for example, during the 1960s and 1970s several craft deliberately impacted the Moon * Corresponding author. E-mail address: [email protected] (M.J. Burchell). 1 On behalf of the SMART-1 Impact Team (see also acknowledgements). 0019-1035/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2009.10.005

and produced observed craters (e.g. see reports in Whitaker (1972), Table 2 and Moore et al. (1980), Table III). The SMART-1 impact site (Fig. 1) was in darkness at the time of impact, permitting observation of the impact light flash (Ehrenfreund et al., 2007; Veillet and Foing, 2007). The resulting crater itself is too small to be seen from the Earth and awaits a future lunar mission to observe it. Therefore some other means is required to estimate the crater size and shape. Hydrocode modelling of large impacts is commonly done, but at highly oblique incidence many subtle effects occur making reliable modelling difficult. In laboratory experiments, it has previously long been noted that for non-normal incidence, craters remain circular in appearance until shallow incidence occurs (e.g. Gault and Wedekind (1978) suggest craters are circular until impacts are >60° from the normal incidence). By contrast, for impacts on both particulate and solid targets crater depth starts to reduce almost immediately impacts are non-vertical (e.g. Gault and Wedekind, 1978; Gault, 1972; Burchell and Whitehorn, 2003). Non-normal incidence impacts are also accompanied by reduced peak shock pressures (e.g. Pierazzo and Melosh, 2000; Dahl and Schultz, 2000, 2001a,b). However, for the very shallow angles of incidence used here, other effects such as projectile ricochet and impact decapitation (Schultz and Gault, 1990a,b) may occur, the latter in particular complicating crater formation and leading to very elongated craters and chains of craters (Gault and Wedekind, 1978). Thus at very shallow angles, angles impactors need to be treated as line-, rather than point-, sources (e.g. see Gault and Wedekind, 1978; Herrick and Forsberg, 1998; Anderson et al., 2004; Anderson and Schultz, 2006).

M.J. Burchell et al. / Icarus 207 (2010) 28–38

Fig. 1. Location of the SMART-1 impact on the Moon. (a) Global location (source ESA – C. Carreau). The impact area (white circle) is in a southern highland (volcanic) region, the ‘Lake of Excellence’. (b) Local map of impact region (produced by SMART-1/AMIE, 19 August 2006, from 1200 km altitude). Predicted orbital paths are shown (North to South) and the predicted impact point is given (white diamond). Source: ESA/Space-X (Space Exploration Institute).

Accordingly, here we predict the characteristics of the SMART-1 impact crater, including its shape and approximate size, based on laboratory experiments and appropriate scaling models. We also discuss the light flash generated by the impact event. 2. Methods The laboratory experiments were conducted at the University of Kent using a two stage light gas gun (Burchell et al., 1999). The gun fires projectiles carried in a sabot that is discarded in flight. The projectiles used here were 2.03 mm diameter alumin-

29

ium spheres. Aluminium was chosen as it was a major component of the SMART-1 spacecraft main body, although the projectiles used here were solid and not porous like the spacecraft. The gun can fire at speeds in the range 1–8.5 km s1 and the speed in any particular shot is selected in advance by varying the conditions under which the gun is fired; here a nominal 2 km s1 was required, i.e. the SMART-1 impact speed on the Moon. The speed in each shot was measured by passage of the projectile through laser light curtains. There are two such light curtains (with known separation) along the length of the gun, each is traverse to the flight direction and focussed on a photodiode. The photodiode signals are read on a fast digital oscilloscope and the interruptions of the light signals (caused by passage of the projectile) are used to provide a timing signal. This combined with the separation between the light curtains gives a speed accurate to better than 1% in each shot. During a shot the target chamber was evacuated to typically 20 Pa to prevent deceleration of the projectile in flight. The target was composed of fine grained sand. There is a long tradition in high speed laboratory studies of impacts of using free flowing granular material as an analogue for solid surfaces and indeed much of the lunar surface is covered with a powered regolith. This permits flow typical of that which occurs in high speed impacts on planetary scales and yields craters with the characteristic bowl shape with raised rims that are also seen at larger scales. The sand used was composed of semi-rounded to rounded grains, 90– 150 lm diameter (>85% of grains were within this size range), free from silt, clay or organic materials. The sand was placed in a tray 70 cm long by 14 cm wide and 5 cm deep. When placed in the gun, the long axis of the tray was aligned with the direction of flight in the gun and was tilted relative to the (horizontal) direction of flight of the projectile. The angle of inclination of the target surface to the projectile direction of flight was measured to better than 0.1°. After a shot the resulting crater’s internal dimensions were measured relative to the original surface plane of the target. The diameter along the line of flight was taken as the length. The diameter transverse to the line of flight was the width. Crater depth was from the original surface plane to the deepest point in the crater. Rim wall height was measured above the plane. After these measurements the crater was filled in with more sand back to the level of the original surface plane. By calibration of sand mass against volume, the mass of sand required to infill the crater yielded crater volume. If any compaction beneath the crater is ignored, the excavated crater material has two components: that which flows into the rim walls and ejecta which is completely removed from the crater by flight. To separate these two components, in some cases the crater rim walls were pushed back into the crater before infilling; this way the volume removed from the crater by flight could be differentiated from the volume removed by flow into the crater rim walls. Down-range of the sand target was a thick aluminium witness plate, positioned perpendicular to the target surface. This was in line with the axis of the projectile flight. Any ricocheting projectile fragments would intercept this plate. Analysis after each shot showed that in each case a single crater resulted (indicating rebound of the impactor). The mean angle of flight from the centre of the sand crater to the centre of the witness plate crater was found in each shot. Three calibration shots using the same type of projectile as in the sand shots were carried out directly into aluminium witness plates at speeds of 1.10, 1.49 and 1.92 km s1. The dimensions of the craters made by ricocheting projectiles into similar aluminium targets were compared to these calibration shots to obtain an estimate of the ricochet speed. In one shot (see Section 3) this thick plate was replaced by a thin aluminium foil to determine the shape of the ricocheting projectile.

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The target was surrounded on three sides by collection boxes to collect the ejected sand. Each box was sub-divided into cells. The entrance to each cell was perpendicular to the target surface and faced the likely impact point. The interior of each cell sloped downward such that ejecta which entered was unlikely to bounce out again. To permit free passage of the projectile the up-range end of the tray did not feature such a box. The limited height of the target chamber meant only a restricted range of angles of elevation could be covered by these collection boxes. 3. Results Fourteen shots onto the sand target were made at a mean speed of (2.08 ± 0.08) km s1; details are given in Table 1. There were four impacts at an angle of incidence (from the horizontal) of 1°, with four more at 2°, four at 5° and two at 10°. The craters (see Figs. 2 and 3) were still circular in plan view at 10°, and only just deviated from circular at 5°. However, by 2° distinct non-circularity had become apparent. This became even more extreme at 1° incidence. At 2° itself, a range of crater shapes were seen in the experiments although there is very little variation (0.1°) in impact angle (equal to the uncertainty in angle) or speed (0.1 km s1) between the shots. Common features are that the up- and down-range crater rim walls were either virtually absent or at least significantly lower that the side walls and that a second, slightly smaller crater seems to emerge down-range, still connected to the main crater; the apparent variation in craters at this angle lies in how distinct this second crater is from the primary crater. These effects are clearer in the 1° shots (Fig. 3) where a series of 3 or 4 decreasing diameter, near-circular and slightly overlapping craters lie along the impact direction; the last crater in the series may be connected to the preceding ones by only a narrow neck (e.g. Fig. 3c). In most impacts, a small mound lies inside the main crater along the line of flight and close to the up-range wall. This is present in all craters except for one crater at 1°. In the craters at 1° this mound itself became elongated, along the direction of flight, into a short ridge. Also present in all the 1° impacts, but not at larger angles, were ridges visible on the surface of the sand radiating laterally to the line of flight from the up-range crater edge. Measurements of the craters normalised to the projectile size, 2.03 mm, or mass, 11.8 mg are given in Table 1 and shown in Fig. 4. In Fig. 4a crater length is shown. In Fig. 4a solid symbols show the total length of the impact feature. Although initially this

Table 1 Details of shots into the sand tray. Crater dimensions such as length, width and depth are given normalised to projectile diameter (2.03 mm) and mass is given normalised to projectile mass (11.8 mg). Where no entry is given the quantity was not measured in that shot. hrich is the angle to which the target surface of the ricocheting projectile as it continues down-range. Angle (°) from horizontal

Speed (km s1)

Total length

Main crater length

Width

Depth

Mass

hrich (°)

1 1 1 1.2 2 2 2.1 2.1 5 5 5.1 5.1 10 10.2

2.01 2.02 2.08 2.11 1.98 2.18 2.06 2.08 2.01 2.07 1.91 2.21 2.04 2.01

25.0 26.7 22.6 29.6 19.4 18.1 19.9 14.9 15.3 16.5 16.5 16.1 21.3 20.2

10.6 10.8 10.8 10.8 15.3 – 10.1 14.9 15.3 16.5 16.5 16.1 21.3 20.2

11.1 6.5 11.8 6.8 11.4 11.8 8.8 13.5 16.6 15.8 14.0 17.1 19.5 19.0

1.08 – – 1.50 1.47 1.97 1.53 – – 2.30 1.63 2.27 3.18 3.25

68 – – 76 58 – 70 70 – 288 144 203 525 356

1.0 1.0 0.5 0.6 1.0 2.9 0.8 1.9 2.4 1.9 2.4 1.5 2.4 2.5

decreased as the angle of impact became shallower, one consequence of the appearance of a multiple crater structure at very shallow angles was that total crater length started to increase again below 5°. However, where a primary crater was evident its length was also measured (open symbols in Fig. 4a) and this continued to decrease as angle of incidence decreased. So at these very shallow angles of incidence it is not appropriate to use total crater length to extrapolate the trends from larger impact angles (which will only apply to the primary crater). The greatest width of the craters was always found to occur in the primary crater and continued to decline as angle of incidence decreased (Fig. 4b). Crater depth is well known to start decreasing as soon as impact incidence becomes non-normal (e.g. see Burchell and Mackay (1998) for examples of this in impacts on ductile materials). Here, the craters were far from normal incidence and accordingly rather shallow (Fig. 4c). At 1° the average depth/width ratio was 0.12 ± 0.04. The rim walls were not high; over the range of incidence 1–10° they were at maximum approximately 17% of the crater depth, and, as stated above, at shallow incidence the rim walls had almost entirely vanished in the up- and down-range directions. The excavated crater mass (Fig. 4d) decreased continually as angle of incidence was reduced until <2° when the decrease stopped, i.e. when the down-range craters appear. A power law fit to the data is shown in Fig. 4d. However, the mass vs. angle of incidence can also be fit by an a sinn h dependence, where a = (2750 ± 1080) and n = (1.06 ± 0.19). Whilst not limiting n precisely, the result is compatible with Gault and Wedekind (1978) who suggested that n = 1 was appropriate for impacts on granular materials in the laboratory. However, this sin h fit systematically under-estimates the crater mass by a factor of almost two at 1° incidence, suggesting that the emergence of the down-range craters is increasing crater mass at shallow angles of incidence beyond that expected by a simple sin h dependence. The ratio of crater length to width, known as ellipticity or circularity, is often used to illustrate how circular crater shape is. A value of 1 is a circular crater and a value greater than 1 indicates a crater elongated along the direction of the impact. Here however, the appearance of down-range craters overlapping the primary complicates matters; nevertheless, the ellipticity is plotted in Fig. 5. In Fig. 5 the results for the main and total impact feature are shown separately. The main crater did not have to be fully separated from the down-range craters for these values to be obtained; values were obtained when the majority of the shape of the main crater could be discerned and a separate ellipse fitted. It can be seen from Fig. 5 that the main crater and down-range craters were not separable until the angle of incidence was <5° and also that the main crater was still roughly circular until this threshold was passed. At the shallower angles the total feature had an ellipticity of up to 4–4½, but the main crater itself was still often circular or had an ellipticity of just 1½. The mass of material excavated from the crater has two immediate destinations, as material that was ejected from the site by flight or as material that flowed into the raised rim walls around the crater (again ignoring any material displaced downwards and effectively increasing the sand density beneath the crater by compaction). At normal incidence an equal partitioning of the excavated material between flow and ejection is often assumed. To obtain the fraction that went into the two routes, in some shots at 1° and 2° the crater infilling method, which was used to obtain crater mass, only took place after the raised rims had been pushed back into the crater, i.e. in these cases the measured infill mass was only from the flight ejected component. This was found to be 92% of the average total excavated mass at 1°, and 91% at 2°, indicating that very little material flowed into the raised rim walls at these shallow angles of incidence. This is consistent with the observed

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31

Fig. 2. Crater shape vs. impact angle of incidence. (a) 10° incidence, (b) 5° incidence, (c) 2° incidence. Impacts are from the left. One centimeter scale bars are shown.

Fig. 3. Crater shape at shallow incidence in the laboratory. (a) 1° incidence, (b) 1° incidence, (c) 1.2° incidence. Impacts are from the left and 1 cm scale bars are shown. Multiple craters are seen along the flight direction. Shock waves in the sand targets are just visible radiating transversely from the up-range edge of the craters (shown arrowed).

very low rim walls which do not form a complete circuit of the crater. Capture cells were placed lateral to and down-range of the target to monitor the ejecta, to permit entry of the projectile, a ±5.5°

region in the up-range direction, as seen from the impact point, was left open. Lateral to the impact direction elevations of 0–56° were sampled, but only 0–19° in the down-range direction due to space limitations in the target chamber. For impacts at 1° inci-

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20

30

0.26±0.05

y = (10.8±0.8)x 2 r = 0.7974

20 10

(-0.35±0.08)

y = (26.4±1.7)x 2 r = 0.7407

0 0

Normalised crater depth

Primary Total

2

Normalised crater width

(a)

4 6 8 10 Angle of incidence (o)

(b)

15 10 0.33±0.05

y = (9.0±0.7)x 2 r = 0.8032

5 0 0

4

2

8 4 6 10 Angle of incidence (o)

600

(c) 3 2 1

0.45±0.07

y = (1.1±0.1)x 2 r = 0.8875

0 0

2

10 8 4 6 Angle of incidence (o)

Normalised crater mass

Normalised crater length

40

(d) y = (69±20)ex/(5.4±1.0) 2

r = 0.8393

450 300 150 0 0

2

10 4 6 8 Angle of incidence (o)

Fig. 4. Measured crater data vs. angle of incidence. (a) Total crater length (solid squares and solid line) and primary crater only (open triangles and dashed line). (b) Crater width. (c) Crater depth. (d) Excavated crater mass. ((a–c) are normalised to projectile diameter, (d) is normalised to projectile mass.)

5

Total length / width Main crater length / width

Circularity

4 3 2 1 0 0

2

4 6 8 Angle of incidence (o)

10

Fig. 5. Crater ellipticity (or circularity) defined as crater length divided by width. Solid symbols () are calculated using the total length of the impact feature and the width of the main crater. Open symbols (s) are shown where it was possible to define a main crater whose length could be found distinct from the total feature length. A dotted line shows ellipticity = 1, as expected for circular craters. Below impact angles of 5° the ellipticity for the total impact feature diverges increasingly from that for a circle. The main crater however behaves differently, even at low angles of incidence some of the main craters are still circular.

dence only 22% of the flight ejected sand was captured. If the possibility of significant up-range ejection of material is considered minimal, the suggestion is thus that the majority of the ejected material did so at angles of ejection above 19° in the forward direction or 56° in the lateral direction these being the maximum angles of elevation covered by the capture system. This ignores the possibility that displaced material has gone into significant compaction

of the sand beneath the target which was not measured here. Anderson et al. (2004) report that in laboratory experiments of oblique impacts on granular targets, the up-range ejecta was reduced in volume as impacts become more oblique and that the downrange ejecta was ejected at lower angles than the material ejected laterally. Substantial fractions of the projectile were found to ricochet from the target and travel down-range as a single entity in all the shots. In one shot at 1° a thin aluminium foil was placed down-range of the target. The entrance hole made in this foil was circular to a high degree, with a diameter of 2.06 mm (compared with the 2.032 mm pre-shot diameter of the projectile). This strongly suggests that the projectile was not significantly deformed during impact on the sand. In the other 13 shots a thick aluminium plate was positioned down-range and the ricocheting projectile made a crater in this plate. The centre of this crater as seen from the centre of the main crater, gave the ricochet angle of the outgoing projectile (shown in Fig. 6). This ricochet angle does not equal the angle of incidence in the impact and is, on average, significantly shallower. At 1–2° incidence the ricochet angle had a mean value of (0.85° ± 0.14°). By firing projectiles directly into similar aluminium plates a crater size vs. impact speed calibration was obtained (Table 2). The speed of the ricocheting projectile (Fig. 7) was obtained from the depth of the crater formed in the subsequent impact on a thick aluminium plate (compared to the calibration data). If the crater diameter were used instead of depth, the resultant speed varied slightly (but not systematically) by typically ±9%. A linear fit to the data in Fig. 7 indicates a trend towards faster ricochet speeds at lower angles of incidence (as previously shown by Schultz and Gault (1990b)) but this requires more data at higher angles to fully determine the nature of the dependence here. A mean ricochet speed of 1.8 ± 0.1 km s1 was obtained from all the data and 1.9 ± 0.1 km s1 from data at

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2.5

y=x

(a)

-1

Ricochet speed (km s )

Ricochet angle (o)

3

2

1

y = (0.85±0.14)x 2 r = 0.7566

0.49±0.09

0 2

4 6 8 Angle of incidence (o)

1.5 y = (1.87±0.05) - (0.018±0.011)x 2 r = 0.7385

1.0

10

0

2

4

6

8

10

o

Angle of incidence ( )

Fig. 6. Angle of ricochet. The ricochet angle can be seen to be in general smaller than the impact angle (a power law fit is shown as a solid line). A dashed line shows the behaviour if the two angles were equal in an impact. Similar behaviour was reported by Gault and Wedekind (1978).

Table 2 Details of calibration shots onto thick aluminium targets at normal incidence. Speed (km s1)

Crater diameter (mm)

Crater depth (mm)

1.10 1.49 1.92

2.72 ± 0.05 3.16 ± 0.05 3.80 ± 0.05

0.06 ± 0.01 0.98 ± 0.01 2.01 ± 0.01

just 1° and 2° incidence (i.e. 87% and 91% of the impact speed, respectively).

(b)

1.0

Ricochet speed / impact speed

0

2.0

0.8 0.6 0.4 0.2

y = (0.92±0.02) - (0.006±0.005)x 2 r = 0.6251

0.0 0

2

4

6

8

10

o

Angle of incidence ( )

4. Discussion The laboratory experiments are at much smaller size than the SMART-1 impact event and have assumed a slightly inclined, smooth target surface (the true nature of the impact site is still unknown). We consider the central body of the SMART-1 spacecraft (ignoring its wing like solar panels) to be a cube 1 m in extent with mass at impact of 285 kg. A variety of scaling methods exist to permit data from one size to be compared to that at another. The simplest (most naïve) method is to assume the crater dimensions normalised to impactor size are constant in both the laboratory and for SMART-1. Taking 1° as the impact angle, the resulting prediction is for a lunar impact crater with a shape similar to those in Fig. 3, total length 26 ± 2 m long, primary crater length 10.8 ± 0.8 m, maximum width 9.0 ± 0.7 m and depth 1.1 ± 0.1 m. The total excavated crater mass would be 23,700 kg, of which 21,800 kg would have been ejecta. However, such scaling ignores the effect of amongst other things gravity and should be used with caution. A more sophisticated scaling method, p scaling, is based on similarity analysis (e.g. Holsapple and Schmidt (1982) or see Melosh (1989) for a discussion). For crater diameter D and volume V, gravity g, impact speed v, projectile mass m and diameter L and target density q we have

p2 ¼ 1:61gL=v 2 pD ¼ Dðq=mÞ1=3 pV ¼ qV=m

ð1Þ ð2Þ ð3Þ

with assumed relationships of the form

pD ¼ C D pb 2 pV ¼ C V p2 c

ð4Þ ð5Þ

Fig. 7. Speed of ricocheting projectile based on depth of the crater from a subsequent impact on a thick aluminium sheet. (a) Absolute speeds are shown. (b) Ricochet speed is normalized to incidence speed. There is some evidence of an increase in speed at low angles but more data would be needed at higher angles to confirm this.

In Melosh (1989) values of the constants and coefficients are given for sand, where CD = 1.4–1.68, b = 0.16–0.17, CV = 0.4–0.24 and c = 0.49–0.51. This is for normal incidence. To allow for oblique incidence, various authors (e.g. Gault and Wedekind (1978) and Schultz and Gault (1990a,b) and see the review of Pierazzo and Melosh (2000) for a detailed discussion) suggest introducing a sin h dependence into the definition of pV or alternatively replacing v with v sin h. Given the values for b and c these approaches are equivalent. However, they are based on assumptions which perforce ignore the on-set of new behaviour as seen here at very small angles (when the crater becomes elongated due to an increase in total length). However, here we are only interested in scaling between two size regimes at the same angle of incidence, so we assert that the exact form of the dependence on h is irrelevant and ignore it; i.e. at a fixed value of h, values of CD0 and CV0 are obtained from the laboratory data and are considered to include whatever is the appropriate functional dependence on h which is then taken as being scale independent. Then, assuming unchanged values for b and c and given a value of p2 taken at the SMART-1 scale (1° incidence, spacecraft length 1 m, mass 285 kg, mean lunar density 3340 kg m3, lunar gravity 1.62 m s2 and impact speed 2 km s1) the values of CD0 and CV0 are used to obtain total crater length 5.5 m, primary crater length 2.3 m, width 1.9 m and crater volume 0.73 m3. Note that we use the bulk lunar density, not that of the more porous regolith upper layer. Applying the correction for rim

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walls vs. ejecta gives an ejecta volume of 0.66 m3 (or 2200 kg). Taking crater depth/diameter as a constant at both scales, a depth of 0.23 m is obtained. As well as ejected target material, it was found that the projectile ricocheted off the sand in the laboratory experiments. As indicated above, the ricochet angle did not equal the angle of incidence, tending to fall below the incident angle (also noted by Gault and Wedekind (1978)) and was (0.85 ± 0.14)° with a speed of 1.9 ± 0.1 km s1. The implication is that on the Moon, the SMART-1 spacecraft may have ricocheted off the lunar surface at high speed and shallow angle of ascent. The laboratory scale results also show ridge-like features in the targets after impact. It has previously been noted that near simultaneous, neighbouring impacts on granular targets can produce such ridge-like features (Oberbeck and Morrison, 1974) and this has been assigned to the collision of material ejected from the adjacent craters. The interior angle made by the ridge is highly sensitive to relative time of impact and separation of the craters, as well as the angle of incidence. A similar effect has also previously been noted for overlapping impacts (Schultz and Gault, 1985). Taken together, the results suggest that the SMART-1 lunar impact will have left a shallow, asymmetric crater on the Moon. The size will be of order 10 m long. The estimates here range from 5.5 m (p scaling which includes an explicit gravity dependence) to 26 m (simple scaling of crater dimensions with impactor size). The primary crater will be smaller, 2.3–11 m long. Crater width will be 1.9–9 m, with crater depth 0.23–0.11 m. The crater rims walls will be low, typically less than the crater depth, with gaps in the up and down-range directions. There would have been some 2200– 22,000 kg of ejected lunar material, most of it at high angles of ejection and probably in the down-range direction. In addition, there would have been a very low angle ricochet of the spacecraft itself at high speed leading to a second impact down-range. It should be borne in mind that the work here used a solid sphere as the projectile, whereas the real spacecraft had a more complicated shape and internal structure including void space, both of which would alter the resulting impact process. Nevertheless, the predicted crater diameters match well to those measured (13–15.5 m) for impacts of the Ranger 7, 8, 9 spacecraft (mass 366–370 kg) on the lunar surface at speeds of 2.6–2.7 km s1 (Whitaker, 1972). Given the shallow nature of the impact crater and its rim walls, it will be hard to see except with highly oblique illumination, unless the exhumed sub-surface material appears different to the original surface either as an ejecta blanket or rays on the surface (e.g. see Gault and Wedekind, 1978; Oberbeck, 1971; Hawke et al., 2004). However, once observed it should be distinct from typical lunar craters. Since impacts on bodies in space occur with an angular distribution where the probability at any angle h to h + dh is 2sin h cos h dh distribution (Shoemaker (1962) and see Pierazzo and Melosh (2000) for a detailed review), it follows that very few impacts are at highly oblique incidence (e.g. see Bottke et al., 2000). Further, most oblique impacts still cause fairly circular craters in plan view (until impact angles exceed 60° or 70° from the vertical). Accordingly, a variety of methods have been proposed to identify highly oblique craters. Crater ejecta blankets are often held to be the most sensitive indicators of impact angle and direction (e.g. Gault and Wedekind, 1978) as this shows asymmetries forming a ‘‘butterfly” wing pattern. In addition, at very shallow angles there is a long streak in the down-range direction on the target surface (there is some evidence for that in the impacts here and this is very pronounced in impacts on the same gun using metal targets, see Burchell and Mackay, 1998). Crater shape (i.e. non-circular appearance) has also been used to identify oblique impacts on planetary bodies (e.g. Schultz, 1992; Schultz and Lutz-Garihan, 1982; Schultz and D’Hondt, 1996; Bottke et al., 2000) and in some cases (e.g. Schultz and Lutz-Garihan, 1982) use of ejecta asymme-

tries was also made. In some cases the non-circular appearance is coupled with steeper crater rim walls on the up-range interior (Gault and Wedekind, 1978) or offset central mounds in the uprange direction (Schultz, 1992; Hawke et al., 2004), although this latter possibility is disputed by Ekholm and Melosh (2001) based on a study of venusian craters on a variety of terrains (flat, sloping, etc.) some of which may influence crater topography. For large craters, modification which occurs after the initial crater formed can also obscure those features of crater shape which are the most sensitive to impact direction (e.g. see Schultz and Anderson (1996) for a discussion). In Wallis et al. (2005) detailed 3-D mapping of craters created in the laboratory for impacts on ductile metal targets was used to show that for craters which have not been modified or partially in-filled, the fine detail of the (un-modified) interior crater shape is very sensitive to obliquity of impact even for near normal incidence, but this has not been fully demonstrated using craters on Solar System bodies. There should be few problems in identifying the SMART-1 impact crater. For example, its location is well known and there may be newly exposed material at the site (e.g. see Whitaker (1972) for identification of man-made impact sites in the Apollo era). Further, it will be significantly asymmetric. There may also be influences on the crater shape from the local conditions (e.g. slope of surface at the impact point) and the non-uniform shape of the spacecraft. As an analogue of what to expect, we consider crater Messier on the Moon. This has an asymmetric shape (Fig. 8) assumed to arise from a very shallow impact and a second crater (Messier A, Fig. 8b), just down-range of the main crater which may be associated with it from ricochet (Hawke et al., 2004; Forsberg et al., 1998). In Fig. 8a note the butterfly shaped lateral ejecta blanket and the long surface streaking emerging downrange of the craters (also see Schultz and Gault, 1990b). In the case of Messier, it also appears that, unlike in the laboratory experiments, it is the detached down-range crater that has the greater width. The primary crater has only a very low rim wall.

5. Impact light flash It has long been known that in hypervelocity impacts a small fraction of the incident kinetic energy is emitted as a visible light flash. Hypervelocity impact light flash has been studied in the laboratory since many years, e.g. see Atkins (1955) who observed impacts of 1.27 mm diameter steel spheres onto steel plates at 1.5 km s1. Ignoring any issues of scaling with size (or mass) beyond the experimental range used, it is possible to predict light flash energy for impacts on Solar System bodies. When trying to predict impact light flash intensity, use is often made of results from laboratory experiments. However, these show a range of different observations (see Table 3). This can partly be ascribed to their covering a wide range of impact conditions, i.e. speed, angle of impact, projectile and target materials, ambient pressure, etc. One of the key results from such experiments is the measurement of the luminous efficiency g, namely the fraction of the incident kinetic energy emitted in the light flash. Whilst some authors give g as a constant, others observe that it has a strong dependence on impact speed (v) and hence give total light intensity as a function of v; where this occurs, the equivalent dependence of g on v is given in Table 3. Given this dependence, impact light flash from events such as SMART-1 hitting the Moon at 2 km s1 will have a much lower g value than higher speed meteoroid impacts. For example, based on the results of Burchell et al. (1996b), for impacts onto ice g = 6  106 at 2 km s1, rising to 1  104 at 10 km s1 and 2.4  103 at 72 km s1. Several experiments have also reported on the temperatures observed in impact light flashes. These seem to be in the range

M.J. Burchell et al. / Icarus 207 (2010) 28–38

Fig. 8. Crater Messier and Messier A. (a) Large area view, showing ejecta blankets (Apollo 15, AS15-M-2404). (b) Higher resolution image (Apollo 11, AS11-42-6304). In both cases the impact direction is bottom right to top left. The main crater is approximately 11 km in length along its major axis. Source NASA Apollo image gallery.

2000–5000 K, with the lower values occurring at the lower speeds such that for SMART-1 impacting the Moon. There is a difference in the literature as to how temperature evolves with impact angle: for granular (porous) targets it increases in non-normal incidence impacts (Ernst and Schultz, 2004), whereas on rigid, solid targets it decreases (Sugita et al., 1998). It is particularly important to note that at lower speeds, the lower temperatures generally obtained indicate that much of the emission will be in the IR, as well as the visible. Also of concern is whether the emission is continuum or dominated by line emission and hence if g really has similar behaviour over a wide range of impact conditions. Models of the light emis-

35

sion (e.g. Sugita et al., 2003) suggest that whilst emission from individual atomic lines and molecular bands has widely varying power law dependencies, changing the impact speed excites different lines giving a mean light intensity which (for a given combination of projectile and target materials and hence a given set of atomic lines or molecular bands) has a single, mean power law dependence on impact speed over wide range of speeds. We also note that the results of Eichhorn (1975), show a relationship between ambient pressure and light intensity. If this also holds at larger projectile masses, an enhanced impact flash would have been obtained in those experiments which only took data at relatively high ambient pressures, artificially increasing the measured value of g. However for lunar impacts onto an assumed granular-like regolith, Ernst and Schultz (2004) discount this citing the results of Gehring and Warnica (1963) for impacts onto granular materials (as distinct to those for impacts on metals). Overall, it is difficult to use laboratory experiments to predict what g and the temperature should have been for the SMART-1 impact, as it is not known how impact light flash evolves in extremely oblique impacts. More generally for lunar impacts however, Gehring and Warnica (1963) for example, predicted that meteoroid impacts on the near face of the Moon (when dark) should be visible from the Earth with a 12-in. telescope, i.e. in regions undergoing solar illumination the flash would be too faint to be seen above the Moon-shine of reflected sunlight, but would be visible if the background light was only Earth-shine illuminating the lunar surface. They also concluded that an impact by craft such as a Ranger lunar probe, size approximately a metre, impacting at 2.4 km s1, should produce a visible flash. However, after the early optimism of the 1960s and 1970s, belief in observing impact light flashes on the Moon waned. Several observing programmes failed to detect meteoroid impacts for example. Further, during the Apollo era, when lunar light flashes were reported they did not coincide with signals from seismometers left on the lunar surface by visiting astronauts. This situation continued until the end of the 1990s when, during the 1999 Leonid shower, Dunham et al. (1999) reported impact flashes which were observed simultaneously by several different telescopes. More reports rapidly emerged, e.g. Ortiz et al. (2000) and Bellot-Rubio et al. (2000). Bellot-Rubbio et al. (2000) gave g for Leonid impacts on the Moon at speeds of some 72 km s1 of 2  103 (with an uncertainty of 1 order of magnitude) and a flash duration of typically 20 ms. Subsequent observing programmes during the 2001 and 2004 Leonids, and the 2004 and 2005 Perseids (see Yanagisawa et al. (2006) for first reports of lunar impacts during a Perseid shower where mean lunar impact speed is 59 km s1), have produced further observations of lunar impact flashes. As well as observations of impacts made during meteoroid showers, sporadic lunar impacts are also now reported using redundant detection techniques (Ortiz et al., 2006). Based on their observed impact rates and the terrestrial in-fall rate of large meteoroids (0.1–10 m), for mean impact speeds of 16–20 km s1 and 40 ms flash durations, Ortiz et al. (2006) predicted g for lunar impacts of >102, higher than expected, or required an impact flux rate greater than that currently predicted, i.e. they were observing impacts by higher mass objects than expected on average. They also predicted before the event, that the SMART-1 impact flash would be visible from the Earth. Separate to this, the phenomenon of impact flash caused by spacecraft impacting Solar System bodies was successfully reported by the Deep Impact mission which impacted comet Tempel 1 with a 370 kg mass at 10.2 km s1 in 2005 (A’Hearn et al., 2005). The event was observed by a world wide coordinated campaign (Meech et al., 2005) and was backed up by extensive laboratory impact studies (e.g. see Schultz et al., 2005, 2007; Ernst and Schultz, 2007). The value of g which was obtained for that event (by taking the known spacecraft kinetic energy and comparing to

36

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Table 3 Laboratory observations of impact light flash for different projectile (proj.) and target materials. Where a velocity (v) dependence is given,

v is in km s1.

Proj. type

Proj. size

Target

Vacuum (Pa)

Speed (km s1)

Luminous efficiency g

Temp. (K)

Comment

Ref.

Metals, glass, nylon Al

mm

Sand, granite and metal

0.13–105

2.1–7.3

v1.5–6.3

–

For impacts on aluminium flash intensity rose for vacuum >133 Pa

Gehring and Warnica (1963)

mm

Al

0.05–27

2.5

104

–

Flash intensity dependent on ambient pressure

Pyrex

20– 100 lm
Al, lead

Not given

4–15

v2.5

Temperature increases with

Metal

103–100

1–30

v1.5–6.3

2500– 5000 2500– 5000 –

MacCormack (1963) Rosen and Scully (1965) Eichhorn (1975, 1976, 1978) Burchell et al. (1996a) Burchell et al. (1996a) Burchell et al. (1996b) Tsembelis et al. (2008) Sugita et al. (1998) Ernst and Schultz (2002) Ernst and Schultz (2004)

Iron

1.9

v

Flash intensity dependent on ambient pressure, with a peak between 0.13 and 13 Pa Ambient pressure was constant

Iron


Molybdenum

10

1–42

v

Iron


Al

104

1–42

v1.5

–

Ambient pressure was constant

Iron


Ice

104

1–70

(2.1  106)v1.65

–

Ambient pressure was constant

2600 ± 400

Temp. independent of

4000– 6500 1850– 2700 3800– 4000 4500 –

Temp. v ? 0:3 where v ? is the normal component of the velocity Temp. v0.75

4

1.3

Iron


Glass

10

5–20

v

Quartz

mm

Dolomite

Not given

4.7–5.6

–

Pyrex

mm

Pumice

<67

–

Pyrex

mm

Pumice

<67

4.05– 5.76 4.5–6.1

–

Pyrex

mm

Pumice

<67

5.2–5.5

(2–5)  105

4

the observed light flash energy) was found to be of order 108, significantly lower than the value of 8  104 predicted pre-impact (Schultz et al., 2005), the value of 1  104 for impacts on solid ice (from Burchell et al., 1996b) and the values for lunar impacts suggested by Ortiz et al. (2006). However, it should be noted that a comet such as Tempel 1 is not only made of ice dominated material, but is also highly porous. The porosity may direct some of the incident kinetic energy into collapsing internal pore space and lead to development of much of the impact event inside a cavity beneath the target surface, thus limiting both the amount of light generated and that observed externally (see Ernst and Schultz (2007) for a discussion). Based on the evidence from a variety of sources (above) indicating a light flash could be generated, a SMART-1 impact observation campaign was mounted by telescope from the Earth (Ehrenfreund et al., 2007). However, viewing conditions (bad weather at some sites, plus difficulties in observing a light flash close to the solar illuminated portion of the lunar surface) restricted the reports to just one definite and one possible sighting of the flash (Veillet and Foing, 2007). The definite observation was made using a relatively long exposure (10 s) compared to the tens to hundreds of ms expected for the flash itself and the relevant part of the image was unfortunately over-exposed. Thus, although the flash was observed, getting a magnitude for the flash was not immediately possible and hence an estimate of the luminous efficiency can only be made by modelling the wings of the over-exposed signal peak which is still in progress. Of interest to later missions however, was that the plume of ejecta from the impact was also observed telescopically (Veillet and Foing, 2007; Ehrenfreund et al., 2007). One complication for SMART-1 however would have been the presence of about 1 kg of un-used propellant (hydrazine) on the craft that had not all been vented prior to impact. Emission from decomposition of this material at high temperatures could have added to the flash magnitude. Other man-made lunar impacts have recently occurred or are in hand. The Indian Chandrayaan-1 mission to the Moon, released a small Moon Impact Probe (MIP) onto the surface on 14 Novem-

v

Normal incidence Impacts at 30° from normal (i.e. temp. is greater for non-normal incidence) g is given for visible wavelengths, it is greater in the IR where g is (0.5–2)  104

Ernst and Schultz (2005)

ber 2008 that did not produce a visible impact flash. There are no optical observations reported for Chang’E 1 impact on 1 March 2009. The large Kaguya spacecraft impacted the Moon on 10 June 2009 at 18:25 UT at lunar coordinates 80.4°E and 65.5°S and early reports indicate a flash was observed (in the near-IR). The upcoming NASA LCROSS mission (launch date 2009, see Colaprete et al., 2009) will deliberately impact the lunar surface, in a permanently shadowed crater in a steep angle impact (70° from the horizontal is the mission baseline). The impact will be by a 2300 kg Centaur upper stage at 2.5 km s1. This will produce an ejecta plume to be observed by several instruments on a second craft (mass 700 kg) which will follow the impactor and itself hit the Moon several minutes later. It should be possible to see both the impact flash and the ejecta plume from Earth. Pre-impact, the main craft will have been deliberately vented of propellants to remove the issue of extra light emission from the propellant. Preliminary modelling of the impact has already been carried out by several groups focussing mainly on the crater and ejecta plume formation (e.g. Korycansky et al., 2008; Shuvalov and Trubetskaya, 2008) and laboratory experiments are also underway (e.g. Schultz, 2006; Hermalyn et al., 2009). The mission is intended to excavate and expose potential sub-surface lunar ices and permit their analysis (looking for evidence of any ice, hydrocarbons, etc.) from observations of the ejecta plume via cameras and spectrometers on the following, shepherding, spacecraft. Laboratory experiments on ice targets show that under hypervelocity impacts even organic compounds present in ice can still be detected in the ejecta plumes, although with concentrations that vary with angle of ejection (Bowden et al., 2009). Based on the estimates of luminous efficiency given above, taking the results for ice (Burchell et al., 1996b) as setting one bound and dry particulates such as pumice another (Ernst and Schultz, 2005) another, a value for g as low as 105 may be found at the low impact speed to be used by LCROSS. When combined with the materials characterisation results from the mission, LCROSS will provide an opportunity to directly obtain the lunar luminous efficiency in a well characterised hypervelocity impact.

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6. Conclusions The SMART-1 impact crater joins that from the Deep Impact mission’s impact on comet Tempel 1 as man-made craters under known impact conditions on Solar System bodies; although both resultant craters are still un-observed. In both cases an impact light flash accompanied the events. It is to be hoped that imaging of the SMART-1 crater will occur eventually and that the Deep Impact crater on Tempel 1 will be imaged in 2011 by the NASA Stardust NExT mission (using the still operational Stardust spacecraft which visited comet Wild 2 in 2004). Although in the case of SMART-1, the local surface topography, the extremely shallow impact angle and the spacecraft shape all complicate the resulting crater formation, it offers one of the few chances to see the results of a manmade impact crater on a Solar System body where the impact conditions were well constrained. In addition, the impact light flash from SMART-1 was observed from Earth, and it is hoped that ongoing analysis of the data may set at least a limit on the luminous efficiency in lunar impacts. The discovery of the crater is awaited. Acknowledgments We thank STFC/PPARC for financial support to operate the Kent light gas gun. We acknowledge the contributions from SMART-1 Project Team, SMART-1 Science and Technology Working Team (STWT) and SMART-1 Impact Campaign Team for discussions (see full list in cited papers Foing et al. (2007) and Ehrenfreund et al. (2007)). We thank J.L.B. Anderson and an anonymous referee for insightful comments concerning the manuscript. References A’Hearn, M.F., and 32 colleagues, 2005. Deep Impact: Excavating Comet Tempel 1. Science 310, 258–264. Anderson, J.L.B., Schultz, P.H., 2006. Flow-field center migration during vertical and oblique impacts. Int. J. Impact Eng. 33, 35–44. Anderson, J.L.B., Schultz, P.H., Heineck, J.T., 2004. Experimental ejection angles for oblique impacts: Implications for the subsurface flow-field. Meteor. Planet. Sci. 39, 303–320. Atkins, W.W., 1955. Flash associated with high velocity impact on aluminium. J. Appl. Phys. 26, L126–L127. Bellot-Rubio, L.R., Ortiz, J.L., Sada, P.V., 2000. Luminous efficiency in hypervelocity impacts from the 1999 lunar Leonids. Astrophys. J. 542, L65–L68. Bottke, W.F., Love, S.G., Tyell, D., Glotch, T., 2000. Interpreting the elliptical crater populations on Mars, Venus and the Moon. Icarus 145, 108–121. Bowden, S.A., Parnell, J., Burchell, M.J., 2009. Survival of organic compounds in ejecta from hypervelocity impacts on ice. Int. J. Astrobiol. 8, 19–25. Burchell, M.J., Mackay, N., 1998. Crater ellipticity in hypervelocity impact on metals. J. Geophys. Res. 103E, 22761–22774. Burchell, M.J., Whitehorn, L., 2003. Oblique incidence hypervelocity impacts on rocks. Mon. Not. R. Astron. Soc. 341, 192–198. Burchell, M.J., Kay, L., Ratcliff, P.R., 1996a. Use of combined light flash and plasma measurements to study hypervelocity impact processes. Adv. Space Res. 17, 141–145. Burchell, M.J., Cole, M.J., Ratcliff, P.R., 1996b. Light flash and ionization from hypervelocity impacts on ice. Icarus 122, 359–365. Burchell, M.J., Cole, M.J., McDonnell, J.A.M., Zarnecki, J.C., 1999. Hypervelocity impact studies using the 2 MV Van de Graaff dust accelerator and two stage light gas gun of the University of Kent at Canterbury. Meas. Sci. Technol. 10, 41– 50. Colaprete, A., and 11 colleagues, 2009. An overview of the Lunar Crater Observation and Sensing Satellite (LCROSS) Mission – An ESMD mission to investigate lunar polar hydrogen. LPSC XXXX. Abstract 1861. Dahl, J.M., Schultz, P.H., 2000. Strain rate measurements in vertical and oblique projectile impact experiments. LPSC XXXI. Abstract 1901. Dahl, J.M., Schultz, P.H., 2001a. Measurement of oblique impact-generated shear waves. LPSC XXXII. Abstract 1429. Dahl, J.M., Schultz, P.H., 2001b. Measurement of stress wave asymmetries in hypervelocity projectile impact experiments. Int. J. Impact Eng. 26, 145–155. Dunham, D.W., Cudnik, B., Hendrix, S., Asher, D.J., 1999. Lunar Leonid Meteors. IAUC 7320. Ehrenfreund, P., and over 50 colleagues, 2007. SMART-1 Impact ground-based campaign. LPSC XXXVIII. Abstract 2446. Eichhorn, G., 1975. Measurement of the light flash produced by high velocity particle impact. Planet. Space Sci. 23, 1519–1525.

37

Eichhorn, G., 1976. Analysis of the hypervelocity impact process from impact flash measurements. Planet. Space Sci. 24, 771–781. Eichhorn, G., 1978. Heating and vaporization during hypervelocity particle impact. Planet. Space Sci. 26, 463–467. Ekholm, A.G., Melosh, H.J., 2001. Crater features diagnostic of oblique impacts: The size and position of the central peak. Geophys. Res. Lett. 28, 623–626. Ernst, C.M., Schultz, P.H., 2002. Effect of velocity and angle on light intensity generated by hypervelocity impacts. LPSC XXXV. Abstract 1782. Ernst, C.M., Schultz, P.H., 2004. Early-time temperature evolution of the impact flash and beyond. LPSC XXXV. Abstract 1721. Ernst, C.M., Schultz, P.H., 2005. Investigations of the luminous energy and luminous efficiency of experimental impacts into particulate targets. LPSC XXXVI. Abstract 1475. Ernst, C.M., Schultz, P.H., 2007. Evolution of the Deep Impact flash: Implications for the nucleus surface based on laboratory experiments. Icarus 190, 334–344. Foing, B.H., Heather, D.J., Almeida, M., 2001. The science goals of ESA’s SMART-1 mission to the Moon. Earth Moon Planets, 523–531. Foing, B.H., and 16 colleagues, 2003. P. SMART-1 mission to the Moon: Technology and science goals. Adv. Space Res. 31(11), 2323–2333. Foing, B.H., and 21 colleagues, 2006. SMART-1 mission to the Moon: Status, first results and goals. Adv. Space Res. 37(1), 6–13. Foing, B.H., and 22 colleagues, 2007. SMART-1 mission overview from launch, lunar orbit to impact. LPSC XXXVIII. Abstract 1915. Foing, B.H., and 18 colleagues, 2008. SMART-1 highlights and relevant studies on early bombardment and geological processes on rocky planets. Phys. Scripta T130 (article no. 014026). Forsberg, N.K., Herrick, R.R., Bussey, B., 1998. The effects of impact angle on the shape of lunar craters. LPSC XXIX. Abstract 1691. Gault, D.E., 1972. Displaced mass, depth, diameter, and effects of oblique trajectories for impact craters formed in dense crystalline rocks. Moon 6, 32–44. Gault, D.E., Wedekind, J.A., 1978. Experimental studies of oblique impact. In: Proceedings of the 9th Lunar and Planetary Science Conference, Pergamon, New York, pp. 3843–3875. Gehring, J.W., Warnica, R.L., 1963. An investigation of the phenomena of impact flash and its potential use as hit detection and target discrimination technique. Proc. Sixth Symp. Hypervelocity Impact 2, 627–681. Grande, M., and 30 colleagues, 2007. The D-CIXS X-ray spectrometer on the SMART1 mission to the Moon – First results. Planet. Space Sci. 55, 494–502. Hawke, B.R., Blewett, D.T., Lucy, P.G., Smith, G.A., Bell III, J.F., Campbell, B.A., Robinson, M.S., 2004. The origin of lunar ray craters. Icarus 170, 1–16. Hermalyn, B., Schultz, P.H., Heineck, J.T., 2009. LCROSS early-time ejecta distribution: Predictions from experiments. LPSC XXXX. Abstract 2416. Herrick, R.R., Forsberg, N.K., 1998. The topography of oblique impact craters on Venus and the Moon. LPSC XXIX. Abstract 1525. Holsapple, K.A., Schmidt, R.M., 1982. On the scaling of crater dimensions. 2: Impact processes. J. Geophys. Res. 87 (B3), 1849–1870. Josset, J.L., and 18 colleagues, 2006. Science objectives and first results from the SMART-1/AMIE multicolour micro-camera. Adv. Space Res. 37(1), 14–20. Korycansky, D.G., Plesko, C.S., Asphaug, E., 2008. LCROSS impact predictions. LPSC XXXIX. Abstract 1963. MacCormack, R.W., 1963. Investigation of impact flash at low ambient pressures. Proc. Sixth Symp. Hypervelocity Impact 2, 613–625. Meech, K.J., and 208 colleagues, 2005. Deep Impact: Observations from a worldwide Earth-based campaign. Science 310(5746), 265–269. Melosh, H.J., 1989. Impact Cratering. Oxford University Press, Oxford, UK. Moore, H.J., Boyce, J.M., Hahn, D.A., 1980. Small impact craters in the lunar regolith – Their morphologies, relative ages, and rates of formation. Moon Planets 23, 231–252. Oberbeck, V.R., 1971. Laboratory simulation of impact cratering with high explosives. Moon 2, 263–278. Oberbeck, V.R., Morrison, R.H., 1974. Laboratory simulation of the herringbone pattern associated with lunar secondary crater chains. Moon 9, 415–455. Ortiz, J.L., Sada, P.V., Vellot Rubio, L.R., Aceituno, F.J., Aceituno, J., Gutiérrez, P.J., Thiele, U., 2000. Optical detection of meteoroidal impacts on the Moon. Nature 405, 921–923. Ortiz, J.L., and 10 colleagues, 2006. Detection of sporadic impact flashes on the Moon: Implications for the luminous efficiency of hypervelocity impacts and derived terrestrial impact rates. Icarus 184, 319–326. Pierazzo, E., Melosh, H.J., 2000. Understanding oblique impacts from experiments, observations, and modeling. Ann. Rev. Earth. Planet. Sci. 28, 141–167. Rosen, F.D., Scully, C.N., 1965. Impact flash investigations to 15.4 km/sec. Proc. 7th Symp. Hypervelocity Impact 6, 109–140. Schultz, P., 1992. Atmospheric effects on ejecta emplacement and crater formation on Venus from Magellan. J. Geophys. Res. 97, 16183–16248. Schultz, P.H., 2006. Shooting the Moon: Constraints on LCROSS targeting. Workshop on Lunar Crater Observing and Sensing Satellite (LCROSS) Site Selection October 16, 2006. NASA Ames Research Center, Moffett Field, CA. LPI Contribution No. 1327, pp. 14–15. Schultz, P.H., Anderson, R.R., 1996. Asymmetry of the Manson impact structure: Evidence for impact angle and direction. In: Koeberl, C., Anderson, R.R. (Eds.), The Manson Impact Structure, Iowa: Anatomy of an Impact Crater. Geological Society of America, pp. 397–417 (special paper 302). Schultz, P.H., D’Hondt, S., 1996. Cretaceous-tertiary (Chicxulub) impact angle and its consequences. Geology 24, 963–967. Schultz, P.H., Gault, D.E., 1985. Clustered impacts: Experiments and implications. J. Geophys. Res. 90 (B5), 3701–3732.

38

M.J. Burchell et al. / Icarus 207 (2010) 28–38

Schultz, P.H., Gault, D.E., 1990a. Decapitated impactors in the laboratory and on the planets. LPSC XXI, 1099–1100. Schultz, P.H., Gault, D.E., 1990b. Prolonged global catastrophes from oblique impacts. In: Sharpton, V.L., Ward, P.D. (Eds.), Global Catastrophes in Earth History: An Interdisciplinary Conference on Impacts, Volcanism and Mass Mortality. Geological Society of America, pp. 239–261 (special paper 247). Schultz, P.H., Lutz-Garihan, A.B., 1982. Grazing impacts on Mars – A record of lost satellites. J. Geophys. Res. 87, A84–A96. Schultz, P.H., Ernst, C.M., Anderson, J.L.B., 2005. Expectations for crater size and photometric evolution from the Deep Impact collision. Space Sci. Rev. 117, 207– 239. Schultz, P.H., Eberhardy, C.A., Ernst, C.M., A’Hearn, M.F., Sunshine, J.M., Lisse, C.M., 2007. The Deep Impact oblique impact cratering experiment. Icarus 190, 295– 333. Shoemaker, E.M., 1962. Interpretation of lunar craters. In: Kopal, Z. (Ed.), Physics and Astronomy of the Moon. Academic, New York. pp. 283–359 (538 pp.). Shuvalov, V.V., Trubetskaya, I.A., 2008. Numerical simulation of the LCROSS impact experiment. Sol Syst. Res. 42 (1), 1–7.

Sugita, S., Schultz, P.H., Adams, M.A., 1998. Spectroscopic measurements of vapor clouds due to oblique impacts. J. Geophys. Res. 103 (E8), 19427–19441. Sugita, S., Schultz, P.H., Hasegawa, S., 2003. Intensities of atomic lines and molecular bands observed in impact-induced luminescence. J. Geophys. Res. 108 (E12), 5140. doi:10.1029/2003JE002156. Tsembelis, K., Burchell, M.J., Cole, M.J., Margaritis, N., 2008. Residual temperature measurements of light flash under hypervelocity impact. Int. J. Impact Eng. 35, 1368–1373. Veillet, C., Foing, B., 2007. SMART-1 impact observation at the Canada–France– Hawaii telescope. LPSC XXXVIII. Abstract 1520. Wallis, D., Burchell, M.J., Cook, A.C., Solomon, C.J., McBride, N., 2005. Azimuthal impact directions from oblique impact crater morphology. Mon. Not. R. Astron. Soc. 359, 1137–1149. Whitaker, E.A., 1972. Artificial lunar impact craters: Four new identifications. Apollo 16 Preliminary Science Report Part I, NASA SP-315, pp. 29(39)–29(44). Yanagisawa, M., Ohnishi, K., Takamura, Y., Masuada, H., Ida, H., Ishida, M., Ishida, M., 2006. The first confirmed Perseid lunar impact flash. Icarus 182, 489–495.