# Small-bodies landing strategies

Looking at past landing missions on small bodies such as Eros, Itokawa and 67P I found different landing strategies

• Eros: NEAR Shoemaker probe performed 4 impulsive pre-planned open-loop maneuvers to land and slow down the vehicle.
• Itokawa: Hayabusa probe performed optical based navigation using impulsive feedback to a previously released target marker on the surface.
• 67P: Philae was released from Rosetta and followed a ballistic trajectory till it makes touchdown with the comet.

Seeing this type of approaches I wonder why two of them have propulsive effort (Near, Hayabusa) and the other (Philae) followed a ballistic trajectory? I think it has to be related with the small-bodies masses (escapes velocities), however, Itokawa mass ($10^{10}$ kg) is lower than 67P mass ($10^{13}$ kg). Are there also some safety issues such as fear of thruster malfunctioning?.

EXTRA: for what conditions continuous thrust (I have not seen this approach although is the one employed for planetary descents) or impulsive burns or ballistic descent trajectories is chosen amongst the others?

• Philae failed to land, the others succeeded. Maybe the reason why Philae used a different strategy is simply that its designers didn't think it through. – Rainer P. Mar 12 '18 at 21:44
• @RainerP. Why? It did land actually. Though without harpoons and thrusters working it resulted in some bounces and in the shadowed final location. – OON Mar 13 '18 at 4:46
• Continous thrust after landing to hold the probe down on the small body is very limited in time. But screws or harpoons to fix the probe might fail if the material is different to the assumptions used for the design of the fixing methods. – Uwe Mar 13 '18 at 10:18
• Continous thrust for decleration during descent is a problem. Due to the very small gravity of those bodies a very small thrust is needed. An escape velocity around 1 m/s allows a passive soft landing without damage. – Uwe Mar 13 '18 at 10:29
• @Uwe, what about using ion-thrusters? As I understand they provided a low quantity of thrust. On the other hand, for asteroid Eros, its escape velocity is of 10 m/s, the problem depends on the considered asteroid. – Julio Mar 13 '18 at 17:54