The aim of this project, as stated in the overview at Section 2, was to see if it would be feasible to carry out a small spacecraft mission within the following constraints:
It is not difficult to achieve at least part of the first objective. Existing small satellite launchers combined with available boost stages can send small satellites of a size regularly operated in Earth orbit on a trajectory that intersects that of asteroids passing close to the Earth. The more challenging parts of this objective as compared with LEO small satellite operations involve accurate navigation and trajectory control to within a very small margin of error over several million km and communicating large quantities of data over the same distance. This study showed though that a small on-board propulsion system combined with existing standards of asteroid and spacecraft tracking should allow sufficiently accurate navigation. It was also found that a combination of a small transmitter and a moderately-sized high-gain antenna allowed high-speed data transfer via the Deep Space Network normally used for supporting space probes.
The second objective was more difficult and arguably remains the biggest obstacle to implementing a rapid-reaction mission. Storage of the probe in a parking orbit was rejected for a number of reasons, leading to the requirement for it to be launched at short notice. Such a launch is difficult to procure even with the growing number of small satellite launch systems on offer. However, the prospect of low-cost access to space via the various reusable launch vehicle projects currently under development raises the possibility of being able to book small satellite launches at short notice by the middle of the next decade. Even if such a rapid launch is available though, managing the operations support of a fast-paced space mission carried out at such short notice will be problematic.
The third objective, by contrast, can be met without excessive difficulty. Tracking a fast-moving target at a range of several hundred km will require an accurate and responsive attitude control system, but again this study has shown that it is not in principle impossible to do this on a small spacecraft. Assuming such tracking, the performance available from a combination of a CCD sensor and a moderately-sized optical assembly is more than adequate to produce high-resolution multispectral images of the target. Use of modern hardware-based image compression allows a large quantity of such image data to be stored during the encounter for subsequent transmission.
The final objective, cost, was just about met according to the terms specified. The estimated probe cost of $13.65 million, combined with the launch cost of $5 to 15 million, gives a total mission cost of about $18.5 to 28.5 million. The lower total mission cost is dependent on development of low-cost reusable launchers, as arguably is the short-notice launch requirement.
As mentioned in Section 19 though, another factor to consider is the return for this investment. On the face of it, this mission provides only a few minutes data return for its cost. However, the data returned in those few minutes will dramatically increase the knowledge of the specific target body and will substantially add to the small amount of 'ground truth' about asteroids in general. Successive missions to a number of such bodies would add progressively less new knowledge in overall terms, but it is reasonable to assume that such a production run of probes would result in reduced mission cost. Similar cost benefit questions can be raised regarding other planned missions; in particular, the proposed Pluto Express mission will provide just a few hours data on Pluto for a cost of $250 million and after a mission of 10 to 13 years [Wertz96]. Again, the justification is in terms of the relative amount of knowledge gained.
Further Work. This project was more of a feasibility study than a design exercise. As such any effort to implement it would have to carry out detailed design work in all areas. In particular though I feel that further development work would need to be carried out on the structural and thermal design of the probe. These areas were modelled in a very simplistic way in this project and would be the subject of much more detailed analysis in a formal design. The communications and on-board data handling system were specified in very broad terms and again a more detailed design study would be in order for further development. It was noted at the end of Section 11 that further study of the attitude control system should be carried out to investigate its dynamics and assess whether additional sensors or more precise actuators would be required. Finally, as suggested in Section 20, the question of how to manage mission support for a spacecraft that is to be launched at short notice should be studied carefully, as should the availability of deep-space tracking support under such circumstances.
Moving beyond this particular design study though, this project highlighted a number of potentially fruitful areas of study. In particular it seems clear that some form of small spacecraft mission to a nearby asteroid is eminently feasible in the near term. As shown in Section 6 there are a number of forthcoming close approaches by NEOs that offer attractive opportunities for such a mission. Without the requirement to be launched and operated at short notice, or to carry out the mission with such speed, the probe design would be considerably simplified from that presented here. Indeed, for some of the target NEOs considered (e.g. Nereus) the encounter velocity for a 'chase' rather than 'direct intercept' is potentially low enough that a rendezvous may even be possible. It is thus suggested that an NEO be seriously considered as the target for a mission by advanced small spacecraft such as the minisatellite series currently being developed at the University of Surrey.
| Previous | Contents | Next |