11
superheated steam, and an impact at 15 km s−1 (roughly 18 eV / water molecule), which will produce an ionized
plasma. Impacts with energies / atom ≳ 20 eV can thus be usefully described as extreme hypervelocity impacts, and
ISO impact experiments will alnost entirely be extreme hypervelocity impacts. The resulting ionized prompt plumes
will producing radiation at Extreme UltraViolet (EUV, 10 to 120 nm) and soft X-ray (0.1 to 10 nm) and even hard
X-ray wavelengths (≤0.1 nm), depending on the collision energy, which can be used to investigate the physics of the
impact and the composition of the impacted bodies. Impacts at these velocities will strip off of multiple electron shells,
creating highly ionized atoms and yielding prompt radiation radiation containing multiple electron recombination lines
(Eubanks et al. 2020), but will not be energetic enough to cause nuclear reactions.
6.1. The Physics of Hypervelocity Impacts
The hypervelocity impact technique was pioneered in 2005 by the Deep Impact (DI) mission, which struck the comet
Tempel 1 with an impactor at an impact velocity of ∼10.3 km s−1 (A’Hearn et al. 2005). The DI impactor largely
consisted of a 178.4 kg copper mass. Here, we model impacts with a probe, assumed to be made of pure 65Cu to avoid
contamination of the prompt plume spectra, and determine the energies reached for various atomic species as function
of the impact velocity.
A small impactor will not change the velocity of an impacted ISO by more than a few mm s−1, and so a reference
frame fixed in the the ISO can be viewed as an inertial frame, and the atomic constituents of the ISO can be viewed
as initially at rest in that frame. Assume, as a first order approximation, a non-relativistic head-on elastic atomic
collision between an atom in the impactor, of mass mi and initial velocity viin , and an atom in the ISO, with mass
mISO and zero velocity in the ISO rest frame. Then the post-collision velocities in the ISO rest-frame are given by
viout =
mi − mISO
mi + mISO
viin
(10)
and
vISOout =
2 mi
mi + mISO
viin .
(11)
Atoms with small atomic mass compared to the constituents of the impactor will receive a large velocity change (up
to twice the impact velocity) but a relatively small fraction of the incoming atom’s Kinetic Energy (KE), while more
massive atoms will have a smaller velocity change, but can absorb more of the incoming atom’s KE. The energies
considered in this paper are not large enough to initiate most nuclear reactions, but it is reasonable to assume that
cohesive and molecular bonds will be broken, and electrons removed, up to the maximum amount of energy available.
Figure 7 shows impact energies from Equation 11 for Hydrogen, Helium, Carbon and Oxygen, common constituents
in solar system comets and asteroids, assuming an impact by a 65Cu probe at the indicated impact velocity.
Although there is one impact velocity for any given impact, the different atomic masses of the various ISO constituents
mean that these atoms will gain different amounts of energy per collision, and thus will be at different temperatures.
Once the prompt impact plasma forms, the temperatures will be rapidly equalized through equipartition of energy,
which will increase the kinetic energy of light elements and decrease the energy of the heavier elements in a given
composition. This warming of the light elements should be sufficient to produce the Lyman alpha transition for
Hydrogen at 121.6 nm (10.2 eV) for almost any ISO fast flyby (Eubanks et al. 2020). The prompt energies shown in
Figure 7 are large enough for collisions at velocities ≥ 100 km s−1 to general K-alpha X-ray spectral lines for many
of the elements likely to be common in ISOs. Instruments such as the ALICE Ultraviolet Imaging Spectrograph, with
sensitivity down to 52 nm (23.8 eV) (Stern et al. 2008) could be adopted to observe the ultraviolet spectra from ISO
impacts but it will probably be necessary to develop special purpose X-ray telescopes to properly observe the full
impact spectrum.