The first space probes - i.e. spacecraft targeted at other bodies in the solar system - were, like the first Earth satellites, small and simple. This was mainly a function of the limited payload that early launch vehicles could send on Earth-escape trajectories and the lack of experience in building such spacecraft. As with Earth satellites though, successive generations of space probes became larger and heavier, and concomitantly more expensive and prolonged in their development. The culmination of this trend is arguably the Galileo Jupiter Orbiter, launched in 1989 after a development programme lasting 15 years*. Galileo is so large that to reach Jupiter it had to use a multiple gravity assist via Venus and Earth, taking over 6 years to make the journey.
[*Although much of this delay was due to extensive redesigns caused by cost cutbacks, changes in available upper stages and the Challenger disaster. Nonetheless a cheaper and faster programme would probably not have become caught in the cycle of delays and redesigns that Galileo did.]
The recognition of the growth of mass, cost and development time of Earth satellites led to efforts to reverse these trends via the use of low-cost small satellites [Larson92, Wertz96]. Such satellites make use of modern developments in microelectronics to pack highly-capable payloads into spacecraft of only a few hundreds or even a few tens of kg mass. Smaller spacecraft cost less to produce, which means that the penalty for failure is reduced and thus less effort needs to be made to ensure maximum reliability and long mission life. This itself reduced cost in what, if managed properly, becomes a 'virtuous circle' of progressively reduced spacecraft costs. In the last 15 years many such small satellites have been flown, often developed at low cost by research organizations and universities (e.g. the University of Surrey's UoSAT programme). Recently NASA has also adopted the so-called 'smaller, faster, cheaper' approach by instituting the 'Discovery' series of missions. Individually limited to $100-150M cost, they are designed to be small enough to use lower-cost Delta launchers rather than the heavier Atlas, Titan or Shuttle previously used for space probes. Perhaps best known of the low-cost space probes though is Clementine, a US Dept of Defense sensor testbed that was used in 1994 to carry out the first US lunar mission in 22 years [Wertz96].
To date though, even these 'smaller, faster, cheaper' missions have typically cost $100M and involved development times of 2 to 4 years. They are thus considerably larger and more expensive than many of the recently-flown Earth-orbit small satellites, which have typically had costs in the region of $2-20M and development times of a year or so. As such, whilst suitable for pre-planned missions they are not appropriate for missions to a type of body yet to be visited by a space probe: the near-Earth object 'target of opportunity'**. Near Earth Objects (NEOs) are the hundreds of small asteroids whose orbits regularly bring them to within a few million km of the Earth. New NEOs are regularly discovered by sky surveys, some on orbits that will soon bring them far closer to Earth than any other deep space body except the Moon. Yet at present there is no way that a mission to such a newly-discovered object could be carried out, as the time available from target discovery to launch and encounter would be a matter of months or even weeks.
[**The Near Earth Asteroid Rendezvous (NEAR) mission is en route to a near-Earth body (the asteroid Eros) but this is a mission to a long-known target.]
It was therefore decided to carry out a feasibility study to determine if a genuinely small, low-cost space probe could carry out a reconnaissance of such an object. This project considered a mission with the following broad constraints:
This project did not set out to design such a probe in detail. Rather it sought to determine if such a mission was possible within the above constraints, to identify a suitable mission profile and to sketch out a general design concept for such a probe to verify its capabilities.
One of the most important design drivers for spacecraft has traditionally been weight. In order to take the best advantage of expensive launchers, spacecraft designers usually try to make the best use of every gram of available payload capacity. This tendency is particularly true for space probes, where weight is at an added premium owing to the reduced payload that a launcher can place into escape trajectories. This constraint is so important that probes are normally sent to their targets via the minimum-energy available trajectory, usually a Hohmann cotangential transfer orbit [Fortescue95]. The use of such trajectories constrains launches to narrow windows that recur at intervals of months to years. Such windows add a further constraint to the availability of space probe launchers and act as a further motivation to make the most efficient use of such launches as are available. As a second issue, such low-energy trajectories are almost always the longest direct ones to the target body, with flight times of many months or even years being common. Such prolonged missions lead to the requirement for long-lived and highly reliable spacecraft, a factor that is itself a common driver towards higher cost and weight.
Space probe designers are thus traditionally under pressure to make best possible use of the available launch mass so as to allow as large and diverse a payload as possible. To achieve this both bus and payload design are normally subjected to stringent procedures for weight control and, where necessary, reduction. Such a design approach adds to the spacecraft's cost, both directly, through the use of lightweight materials and complex fabrication procedures, and indirectly, though the cost of the time and expertise needed to design and build a low-mass but strong and mechanically reliable system. This latter factor is particularly important for custom-built, 'one-off' space probes, where there is no scope for reducing costs through large-scale production or the repeated application of the same development work.
The same approach affects the design of all aspects of a space probe's systems. In order to keep the fuel mass requirements low, the delta-V budget is minimized by use of accurate navigation and careful advance trajectory planning. Mechanisms must be carefully designed so as to achieve reliable operation after having been subjected to launch loads, all with the lowest possible weight.
We can contrast the design of a space probe with that of a missile - for instance and air-to-air or surface-to-air missile. Such a missile is not designed for any particular engagement, although it will have a target envelope within which it can be successfully employed. Rather, it is provided with the range and manoeuvring capability to engage any target within this envelope. Guidance is not by flight along a predetermined trajectory (even against a static or non-manoeuvring target) but by continuously or regularly updated 'homing' towards the target's optical, IR or radar signature. Such a missile is undoubtedly heavier than one optimized to attack a particular target position, carrying as it does excess fuel to allow it to use a less-than-optimum flight path. However, it can be used at short notice against any target within its envelope rather than having to be built or configured for a specific target situation.
Indeed, it would not be practical to design such a missile for the absolute minimum weight, as other design requirements tell against this. A missile is typically built for high speed and manoeuvrability, and as such must be of strong and durable construction. A very lightweight design is thus impractical; however, this means that there is no need to expend time and effort in producing a light design.
This comparison suggests an alternative paradigm for space probe design. Hitherto all missions have been to pre-selected targets and have been to a great extent - if not entirely - built specifically for them. They have thus generally conformed to the design methodologies described earlier, with maximum capability at minimum weight an important objective. A general-purpose probe though must meet a number of requirements that are similar to those described for a missile:
It thus appears that a design methodology closer to that of a missile rather than a conventional space probe may be appropriate for a rapid-response general-purpose probe. Rather than optimize a probe for one specific mission it is more effective to specify it to be able to cope with the full range of targets of interest. Although this may result in the probe being over-specified for a particular mission, if this avoids excessive efforts to pare mass to the minimum or extract the maximum possible performance from a design it may reduce overall cost and development time.
Space missions - even when at the level of proposals - usually have some distinctive name or designation. After some thought*** this mission was christened Nereus, the Near Earth Reconnaissance Experiment - University of Surrey. As well as describing its mission and origin, this is the name of a small asteroid that is a candidate for exploration by a small probe such as this.
[***An earlier proposal was Earth Grazing Opportunity Mission to Analyse Newly Identified Asteroids and Comets. Much as my supervisor liked the resulting acronym I could not quite bring myself to use it as the formal project name.]
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