NEOTωIST, an abbreviation for “Near Earth Object Transfer of angular momentum Spin Test”, is a concept for a low-cost, high figure of merit kinetic impactor demonstration mission which aims to give us an insight as to whether the kinetic impactor mitigation method is a viable way of protecting our planet from NEOs.

Motivation and Concept

Most of the kinetic impactor demonstration missions propose to quantify the momentum transfer by measuring a change in the heliocentric trajectory. The change is typically so small that it must be performed via radio-science from a second observer spacecraft which rendezvous with the NEO prior to impact. The NEOTωIST approach is different since it is designed to demonstrate the effectiveness of kinetic impact by measuring angular momentum transfer. This can be achieved by impacting the asteroid away from its rotation axis, making the asteroid rotate more/less frequently. This change in the asteroid’s rotational period can be measured from Earth via light curve observations; that allows quantification of the transferred momentum. Such approach promises comparatively low cost and features capabilities that are unique and valuable for an operational deflection mission.

near earth object anular momentum spin test

The basic mission, which can be completed with a kinetic impactor spacecraft and ground-based observations , can also be expanded upon depending on budget, creating multiple mission possibilities . Several small sub-spacecraft can be separated from the main impactor spacecraft shortly before impact. These sub-spacecraft allow observation of the impact event from multiple vantage points some of which are unique because their destruction is accepted.

near earth object anular momentum spin test

The possible mission configurations are:

  • Impactor only
  • Impactor + ejectable flyby subunit
  • Impactor + ejectable flyby subunit + chaser subunit

NEO target

The NEOTωIST target NEO is asteroid 25143 Itokawa. Since its elongated shape it is in fact easier to observe brightness changes. Moreover Itokawa is a well known NEO, it was in fact already visited by the Japanese spacecraft Hayabusa in late 2005.

This artist’s impression, based on detailed spacecraft observations, shows the strange peanut-shaped asteroid Itokawa. By making exquisitely precise timing measurements using ESO’s New Technology Telescope a team of astronomers has found that different parts of this asteroid have different densities. As well as revealing secrets about the asteroid’s formation, finding out what lies below the surface of asteroids may also shed light on what happens when bodies collide in the Solar System, and provide clues about how planets form.
Credit: JAXA, ESO/L. Calçada/M. Kornmesser/Nick Risinger (


Flyby unit

The flyby unit is released into a safe flyby course prior to impact. This unit performs observations of the impact from different positions along a trajectory roughly parallel to the impactor trajectory, including a view from 90° with respect to the impact velocity vector. This perspective yields more information about the geometry and dynamics of the ejecta cloud than for instance a view along the impact trajectory. After deployment, the unit also functions as a data buffering and re-transmission node for the other vehicles of the constellation.


The high velocities, geometries, and energy of the event make this a highly dynamic and challenging imaging task. In order to not rotate the whole spacecraft during the flyby event to follow the evolution of the ejecta cone a tracking mirror mechanism has been designed for this purpose.


Chaser unit

A single or multiple chasers unit are released from the Impactor, following it along its terminal trajectory. This vantage point potentially provides an additional view of the ejecta cloud along its central axis. However, the unique observation opportunity that the Chasers offer is that of observing the impact crater, which the FBM cannot do because of obscuration of the crater by ejecta at the time when it may be geometrically possible. To have a chance of observing the impact crater, the Chaser must follow the Impactor with a sufficient delay (10s of seconds) to allow dispersion of most of the ejecta for a clear view on the crater. Crater characterisation is useful because it can constrain the assumptions about total volume of ejected material. The ultimate fate of these units is to be destroyed either by impacting the NEO or debris from the impact.

Impactor unit


The kinetic impactor unit performs all functions associated with the interplanetary transfer, performs terminal homing on the target and act as a carrier for the sub-spacecraft.

The following animation represents the targeting sensor of the Deep Impact impactor spacecraft, that returned images up to 3 seconds before impact with the comet 9P/Tempel. Similar images are expected to be returned from the NEOTωIST impactor unit.


Further readings


Impact Calculator

The NEOTωIST – Impact Calculator was implemented during the Master Thesis ‘Instrumentation for an asteroid kinetic-impactor demonstration mission‘ from Steffen Weisenberger.

You can download and try it here.