Step 1 Definition of Mission Objectives

The first step in analyzing and designing a space mission is to define mission objectives: the broad goals which the system must achieve to be productive. Figure 1-4 shows sample objectives for FireSat We draw these qualitative mission objectives largely from the mission statement In contrast, the mission requirements and constraints discussed in Sec. 1.4 are quantitative expressions of how well we achieve our objectives—balancing what we want against what the budget will allow. Thus, whereas we may modify objectives slightly or not at all during concept exploration, we often trade requirements throughout the process. For FireSat to be FireSat, it must detect, identify, and monitor forest fires. As we trade and implement elements of the system during concept exploration, we must ensure that they meet this fundamental objective. An excellent example of the careful definition of broad mission objectives for space science missions is given by the National Research Council [1990].

Ordinarily, space missions have several objectives. Some are secondary objectives which can be met by the defined set of equipment, and some are additional objectives which may demand more equipment Nearly all space missions have a hidden agenda which consists of secondary, typically nontechnical, objectives. Frequently political, social, or cultural, they are equally real and equally important to satisfy. For example, a secondary objective for FireSat could be to show the public a visible response to

FireSat Mission Objectives

Primary Objective:

To detect, Identify, and monitor forest fires throughout the United States, including Alaska and Hawaii, In near real time.

Secondary Objectives:

To demonstrate to the public that positive action Is underway to contain forest fires.

To collect statistical data on the outbreak and growth of forest fires.

To monitor forest fires for other countries.

To collect other forest management data.

Fig. 1-4. FireSat Mission Objectives. Unlike requirements, which specify numerical levels of performance, the mission objectives are broad statements of what the system must do to be useful.

frequent forest fires. Third World nations produce satellites in part to show that their developing technology makes them important players in international politics. Of course, this secondary political objective for space programs has been important for many years in both the United States and the former Soviet Union. If we are to meet all of a space mission's objectives, we must identify secondary and nontechnical objectives as well as primary ones.

Multiple objectives also occur when we use a single satellite to meet different demands. For example, we may use FireSat's temperature-sensing instruments to monitor global changes in ocean temperatures. In this case, the secondary objectives could become as important as the primary ones. A second example would be adding a communications payload to FireSat to permit better communications among the distributed groups who fight forest fires. Although the primary objective usually will be quite stable, secondary objectives may shift to meet the users' needs and the redefined potential of the space mission concept

As in the case of most of the top-level trades, we recommend strongly against numerical formulas that try to "score" how well a mission meets its objectives. We can compute probabilities for achieving some technical objectives, but trying to numerically combine the coverage characteristics of different FireSat constellations with the political impact of launching FireSat is too simplistic for effective decision making. Instead, we must identify objectives separately so we can judge how to balance alternative objectives and mission concepts.

Good mission objectives incorporate user needs and, at least indirectly, the space characteristics we are exploiting to achieve them. As stated earlier, space is expensive. If our end objective does not use one of the fundamental space characteristics, it will likely cost less to do on Earth. For example, there is little reason to manufacture low-cost consumer goods or publish books in space.

What fundamental characteristics of space make space missions desirable? Table 1-4 lists some of them with their corresponding missions. Exploring and using space serves various objectives, from extremely practical telecommunications and weather, to major scientific observatories hoping to understand the universe better, to advanced military applications and exploring and exploiting the Moon and planets. Our objectives are diverse partly because we use many different space characteristics.

For example, materials processing uses the microgravity and high vacuum of space, disregarding the spacecraft's position over the Earth. Conversely, communications or observation satellites emphasize Earth coverage as the most fundamental space characteristic to achieve their objectives.

TABLE 1-4. Characteristics of Space Exploited by Various Space Missions. Note the wide variety and that many are only beginning to be used. (Spacecraft acronyms are defined in the index.)


Relevant Missions

Degree of Utilization

Sample Missions

Global Perspective

Communications Navigation Weather Surveillance

Some are mature industries; major new advances will come with increased onboard processing

IntelSat GPS

NOAA satellites DBS

Above the Atmosphere

Scientific observations at all wavelengths

Well developed; space observatories will continue to dramatically change our view of the universe

Space Telescope GRO

Chandra X-Ray Observatory IUE

Gravity-free Environment

Materials processing in space

Now in infancy, may be many future applications

Industrial Space Facility iSS


Abundant Resources

Space industrialization Asteroid exploration Solar power satellites

Essentially none

Space colonies Solar power satellites NEAP

Exploration of Space Itself

Exploration of Moon and planets, scientific probes, asteroid and comet missions

Initial flybys have been done; Some landings done or planned; limited manned exploration

Manned lunar or Martian bases Apollo Galileo

Table 1-4 reveals a second important feature: the varying levels of exploitation for different space characteristics. Many current missions use the global perspective of space—for telecommunications, weather, navigation, and other aspects of Earth monitoring. Space-based telecommunications will continue to grow, but it is already a major and mature industry. Satellite communications by telephone and television have become a part of everyday life and have helped to bring about a communications revolution largely responsible for our shrinking world. Equally dramatic changes are likely in the future as new applications for space-based communications and navigation continue to emerge.

In contrast to telecommunications, materials processing and precision manufacturing in gravity-free space is only in its infancy. Major strides appear possible in pharmaceutical and semiconductor devices that may bring about an entirely new industrial segment Exploiting space's almost limitless resources is even further removed. Unlimited continuous power and huge, accessible supplies of physical materials may, in the long run, maintain an industrialized society without destroying the Earth's fragile environment These objectives will require greater vision than those for the more fully developed areas of communications, resource mapping, and monitoring.

We see from Table 1-4 that we have either not used or only begun to use most of the major characteristics of space, so changes in future space exploration should be far larger than present development To take practical advantage of these characteristics, we must greatly reduce the costs of exploring and exploiting space. Finding ways to lower these costs is a principal objective of this book. (See Wertz and Larson [1996].)

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