The multiple possible combinations of impactor composition, density, target density, and impact velocity control track morphology in a complex manner. For example, it is easy to demonstrate the effect of changing target density on the length and volume of Type A tracks in the relatively high density aerogels (60–180 kg m−3) described by Burchell et al. (2001, 2009a). This has now also been seen in lower density aerogels impacted at approximately 5 km s−1, where the same soda-lime glass beads penetrate almost twice as far into 13 kg m−3 aerogel as in a 30 kg m−3 target (Burchell et al. 2009a). Where robust impacting particles have similar physical properties, such as hardness, melting, and dissociation temperatures, for impacts at the same velocity, there does also seem to be a relationship between particle kinetic energy (dependent on size and density, hence mass), and the length of track and volume of aerogel displaced (Burchell et al. 2009a). However, does contrast between density of the impacting particle and a constant aerogel target exert the dominant control on track shape? Iida et al. (2010) and Niimi et al. (2011) have suggested that the larger scale features of aerogel track shape may directly reflect the impacting particle density, and that low-density particles are responsible for Type B and C tracks. Studies of impacts on Stardust-type aluminum foil by materials with a wide range of density, approximately 1 to approximately 8 g cm−3 (Kearsley et al. 2008, 2009), have shown that crater shape, particularly the ratio between depth and diameter, certainly is strongly influenced by impactor density, although both particle shape and internal structure can also be important. However, impact processes on a relatively dense (approximately 2.8 g cm−3) and ductile substrate such as aluminum foil are very different to those in low density aerogel. Although low density aggregate particles may create irregular shapes on foil (e.g., fig. 25 of Kearsley et al. 2008; fig. 6 of Kearsley et al. 2009), there is little opportunity for tightly packed constituent subgrains to separate and disperse widely to create further structural complexity, as occurs in aerogel. For impacts onto aerogel, there is good evidence that robust particles of higher density can make longer Type A aerogel tracks than lower density particles of the same size (Niimi et al. 2011), although the depth of penetration may also partly depend upon the degree of ablation and abrasion suffered (for example, compare the alumina impacts of Hörz et al.  with the glycine of Nixon et al. ), and depth may therefore not be related to the particle density alone. Crucially, our experimental creation of Type B and C tracks from relatively high density (approximately 2.4 g cm−3) natural and artificial aggregates and graphite, all impacted onto aerogel of low density (approximately 30 kg m−3) has now clearly demonstrated that bulk impactor density is not the only controlling factor over the track geometry. The overall density of our artificial aggregates is similar to that of soda-lime glass (known from many, many experiments to create Type A tracks). Hence, track shape alone cannot be a reliable indicator of the impactor density or porosity, as was implied by Iida et al. (2010) and Niimi et al. (2011). Instead, we have shown that impactor structure exerts a more significant control over track shape development. Interestingly, our experiments may also suggest that overall size of an impacting aggregate or organic particle can influence the resulting track shape, with some larger particles apparently creating elongate bulbous tracks with greater length to maximum width ratio (lower MW/TL) than smaller tracks in the same target (e.g., aggregates in Fig. 11b, coal in Figs. 12o and 12p). This may reflect pervasive flattening and/or break-up for the entire volume of a small weak, or soft, impactor during the initial portion of capture and track formation (i.e., near the aerogel surface). A larger particle of similar properties may experience only partial exfoliation and surface mechanical abrasion in the shallower part of the aerogel before it is slowed, thereby preventing complete disruption, and allowing the fragmenting core to penetrate to a relatively deep level, making a rather longer track.