What asteroid(s) do first?
Human missions to asteroids (as recently announced by US President Obama* on April 15 at the Kennedy Space Centre) are in several aspects different from those to Mars or to the Moon, among others because of the wide choice: there are millions of them in the Asteroid belt between the orbits of Jupiter and Mars and thousands in the Near Earth vicinity (NEAs). Not all of these are equally easy and safe to reach – and return from.
The team has taken up the challenge of filtering the good ones out finding possible return trajectories to all known asteroids.
For a first human mission to an asteroid after 2020, one can assume some basic requirements in order to help downselecting a list of interesting asteroids:
- the asteroid should be not too small in order to be able to eventually land and take samples;
- a minimum stay on the asteroid should be guaranteed to allow for a useful scientific return.
Furthermore, it is reasonable to assume some further human-related constraints, such as:
- the trajectory should allow for a safe way to return rapidly to Earth in case of problems up to half way to the target asteroid;
- the overall mission duration should not be too long in order to keep the journey bearable for astronauts.
As well as some technical constraints such as:
- no new revolutionary propulsion system by 2020 - thus delta V values within the range of current capabilities;
- a maximum hyberbolic excess velocity relative to Earth of 4.5 km/s for the return leg so as to allow for a safe landing of the return capsule.
The technical parameters used for the selection are:
Table 1: Baseline for asteroid selection
|Apparent magnitude (corresponding to roughly 200m)
||H ≤ 22|
|Time of flight||< 1.5 years|
|Stay time on the asteroid||> 5 days|
|Relative velocity at Reentry||4.5 km/s|
|Propulsion||Chemical, with 1 deep space manoeuvre per leg|
|Launch V infinity||6 km/s|
By adding all these elements into the team’s model to calculate trajectories to asteroids (previously used to populate a mission-to-asteroid database) and considering all the asteroids classified as Near Earth Objects in the JPL NEO database, researchers at the Advanced Concepts Team have made a preliminary selection. One deep space manoeuvre (DSM) is included on both outbound and inbound trajectory legs. The trajectory to each asteroid (more than 4000 considered) has been optimized for the minimisation of the total delta-V, using PaGMO, an open-source framework for parallel global optimisation of engineering problems developed by the team.
For each solution found, a backup mission was computed in case of system failure. The backup mission was designed for a fast return to Earth. Solutions were pruned out if the resources were not sufficient (e.g. the required delta V, resp. the time of flight, to return back to Earth in case of emergency must be less than the remaining delta-V, resp. time of flight, of the nominal mission). The emergency return trajectory is computed for the worst case scenario with respect to the date of decision to return.
For the final sorting, the best asteroids were those which required a low deltaV for the nominal mission, and a short time of flight for the backup mission.
Table 2 lists those asteroids considered as the prime candidates for such a mission, while Figure 1 shows their respective orbits.
Table 2: Preliminary list of best suited asteroids for a human mission
nal return time of flight
return time of flight (days)
roid orbit's semi major axis a
roid orbit's eccen-
|1365||207945 (1991 JW)
|209||65717 (1993 BX3)
|468||10302 (1989 ML)
Figure 1: Orbits of the best 10 asteroids for human missions.
* Quote of US President Obama on 15 April 2010: “And by 2025, we expect new spacecraft designed for long journeys to allow us to begin the first-ever crewed missions beyond the Moon into deep space. So we’ll start -- we’ll start by sending astronauts to an asteroid for the first time in history.”