Contents at a Glance
The principle of the kinetic impactor mitigation method is that the NEO or Asteroid is deflected following an impact from an impactor spacecraft. The principle of momentum transfer is used, as the impactor crashes into the NEO at a very high velocity of 10km/s or more. The mass and velocity of the impactor (the momentum) are transferred to the NEO, causing a change in velocity, causing it to deviate from its course slightly.
A reconnaissance spacecraft is also required, which allows accurate measurement of the NEO’s orbit before the impact. Additionally, the reconnaissance spacecraft can map the NEO, providing the impactor with vital information such as its exact size, shape, rotation speed and even chemical composition.
Following the impact, the reconnaissance spacecraft once again accurately measures the orbit of the NEO to confirm the success of the impact in changing the NEO’s course.
The behavior of the impact itself, and how the momentum is transferred to the NEO, is a key research topic for the scientific side of the NEOShield project.
For example, if the impact blasts material from the surface of the NEO out into space this increases the efficiency of the impact because this material transfers additional momentum to the NEO.
The kinetic impactor concept has been studied to phase A level within Europe in the frame of the Don Quijote mission study. The experience gained from this study has been used during the NEOShield project in developing the detailed test mission design for the kinetic impactor mitigation method.
A target NEO suitable for the NEOShield kinetic impactor test mission has been selected based on the following criteria:
- “Earth safety first!“, meaning that under no circumstances the planetary threat after completion of the mission must be higher than before.
- The deflection of the target NEO owing to the deflection action must be significant enough to be clearly detectable. This excludes NEOs above a certain size.
- Duration of the kinetic impactor test mission not to exceed 7 years
As NEO suited best under the above criteria the asteroid 2001QC34 has been identified. This NEO is a Potentially Hazardous Asteroid but is presently predicted to miss the Earth orbit by more than eleven lunar distances.
|Properties of Target NEO 2001 QC34|
|Mass||31 million tonnes|
|Shape||potato-like, with a length-to-width ratio of about 1.5|
|Mineral composition||silicates, metals|
|Minimum distance between Earth orbit and target NEO orbit||11.4 times the Earth -Moon distance (about 3% of the Sun-Earth distance)|
In the figure above the constellation of minimum separation between Earth and NEO orbit is indicated by a small blue and a small red dot, respectively.
Note that 2001 QC34 passes here the Earth within the Earth’s orbit and not too far from the point where 2001 QC34 comes closest to the Sun (the NEO’s perihelion).
In order to increase the separation between Earth and NEO orbit it is consequently necessary to reduce the semi-major axis of the NEO’s orbital ellipse.
This can be achieved by reducing the orbital velocity of 2001 QC34 by a kinetic impact transferring momentum to the NEO in anti-flight direction.
The preferred mission scenario for the NEOShield kinetic impactor test mission is based on launching an reconnaissance spacecraft and an impactor spacecraft together as a stack on a Soyuz rocket from Kourou in French Guiana. In launch configuration the reconnaissance spacecraft will be mounted on top of the impactor spacecraft.
The latter remains throughout the mission mated with Fregat, the upper stage of the Soyuz launcher. Even after having used up its propellant the significant dry mass of the Fregat substantially helps increasing the momentum that is imparted to the target NEO when the impactor crashes into it.
After having reached interplanetary space the reconnaissance spacecraft is separated from the impactor. From now on both spacecraft have to follow separate trajectories.
The reconnaissance spacecraft has to reach the target NEO several months earlier than the impactor in order to leave sufficient time for NEO exploration and orbit determination. The trajectory of the impactor is designed such as to maximize the velocity relative to the target NEO.
Before the impactor reaches the target NEO the reconnaissance spacecraft is placed in a position which is favorable for observing the impact but at a distance sufficiently large to avoid it being hit by NEO debris from the impact.
The figure above shows the NEOShield reconnaissance spacecraft mated with the impactor and the Fregat under the (transparent) Soyuz fairing in launch configuration. Fregat is protruding the fairing at the bottom. The green cones shown above represent fields of view of instruments and sensors.
A combined launch of reconnaissance and impactor spacecraft on the same rocket has been selected for the NEOShield kinetic impactor test mission.
This requires complicated trajectories for the two spacecraft including several swing-by manoeuvres by planets to give the crafts the correct speed, and allow them to arrive at the target NEO at different times.
In the terminal phase when the impactor spacecraft finally approaches the selected target NEO it has to operate under adverse illumination conditions.
Due to the large Sun phase angle of the approach trajectory, 2001 QC34 is nearly dark from the perspective of the impactor, i.e. the impactor hits the asteroid on the dark side, not on the sunlit side (illustrated in the figure below).
The figure illustrates in addition the velocities of impactor and NEO and, most importantly, the relative velocity of the impactor with respect to 2001 QC34 (about 9 km/s).
Note that the shape shown for this NEO does not claim to be realistic. Its precise shape is not known to date except for its length-to-width ratio of 1.5. The impactor spacecraft needs to be guided such that it hits the NEO preferably along a line passing through the NEO center of mass.
Therefore its position offset relative to the NEO’s center of brightness, which is captured by the on-board navigation camera, must be known to the impactor.
When the impactor spacecraft approaches the target NEO, the reconnaissance spacecraft has to acquire an observation position at a safe distance from and in a safe orientation towards the impact site.
The position must be such that material ejected by the impact are unlikely to hit and damage the reconnaissance spacecraft.
A distance of several thousand kilometers is deemed to be safe.
At the time of the impact the NEO is at a distance from the Earth of 1.8 times the Earth-Sun distance. This means that the transmission of a signal down to ground takes 15 minutes roughly. The consequence of this high latency is the need for a high degree of on-board autonomy, in particular on the impactor spacecraft.
The impactor spacecraft is supposed to crash into the selected target NEO with high precision and velocity. In order to maximize the momentum transfer during the impact, the impactor spacecraft remains mated to Fregat, the Soyuz upper stage, throughout the mission. This composite is shown in the following figure.
The body-mounted octagonal solar array on top of the composite is permanently oriented towards the Sun.
During the terminal approach to 2001 QC34 the two hot-redundant navigation cameras NAC1, NAC2 are oriented towards the NEO and provide guidance information for trajectory control.
The green cones represent the fields of view of the three star trackers used for attitude control.
The dry mass of the impactor composite amounts to 1336 kg, whereof Fregat (essentially consisting of six connected spheres of equal size) contributes 902 kg.
A suite of antennas (low-gain (LGA), medium-gain (MGA) and high-gain (HGA)) provides for communication with ground and with the reconnaissance spacecraft. For orbit and attitude control the spacecraft features a chemical propulsion system and four reaction wheels.
The reconnaissance spacecraft is supposed to:
- precisely determine the target NEO orbit before and after the kinetic impact
- determine the material composition of the target NEO
- determine the surface geometry and rotation state of the target NEO
- observe the kinetic impact of the impactor spacecraft (cratering, ejecta)
To meet these objectives, two cameras, a LIDAR and a spectrometer for the visual and the near infrared band form part of this spacecraft.
The cameras will also be used for navigation purposes.
Radio science for precise orbit determination is possible by using suitable RF transponders.
A suite of antennas (low-gain (LGA), medium-gain (MGA) and high-gain (HGA)) provides for communication with ground and with the impactor spacecraft.
The dry mass of the reconnaissance spacecraft amounts to 574 kg. In order to minimise this mass a combination of an electrical with a chemical propulsion system has been selected.