Lunar science missions: the smallsat alternative
by Jeff Foust
|LRO may be so large that it will require a launch on a powerful, and expensive, EELV.|
If these missions were small or mid-sized cars, then LRO, by comparison, is a full-sized SUV. LRO will be charged with carrying out a number of scientific and exploration objectives ranging from identifying the resources in the lunar polar regions to measuring the radiation environment around the Moon. To carry out those objectives LRO will be filled with a number of instruments, with up to 120 kilograms tentatively allocated for them.
That payload mass may prove to be a significant issue for LRO. In addition to the scientific payload, there is the rest of the spacecraft as well as a sizable amount of propellant—required for both entering lunar orbit as well as maintaining that orbit in the Moon’s irregular gravity field—to contend with. NASA has stated that they plan to launch LRO on a Delta 2, the workhorse of planetary and space science missions. However, in a talk at the Goddard Memorial Symposium in March, GSFC planetary scientist David Smith noted that the Delta 2 can place only about 800 kg into lunar orbit. (That is actually less than what the Delta 2 can put into orbit around Mars, because Martian orbiters can use aerobraking to reduce the amount of propellant they need, an option obviously not available for lunar missions.) LRO’s mass has become enough of an issue that there is serious consideration now given to switching from the Delta 2 to the more powerful—and more expensive—Atlas 5 or Delta 4. (See “Lunar Reconnaissance Orbiter: the cornerstone of the vision”, April 26, 2004)
LRO also represents a departure of sorts from how NASA handles many planetary science missions. The Discovery and Explorer programs, for example, rely on competitive selection processes to choose entire missions: proposers must provide information not just on the science they plan to achieve but how they plan to build and launch their spacecraft. For LRO, all NASA is asking for in an Announcement of Opportunity (AO) scheduled for release later this month are “investigations”—that is, instruments—for the mission. As the official notice of the impending AO put it, “The launch services and spacecraft will be NASA-provided resources.” To be fair, this approach is hardly unprecedented: NASA has used it for much of its Mars exploration program, including the Mars Exploration Rovers. However, while the rovers were technologically challenging missions that arguably could be done best, and perhaps only, by NASA, lunar orbiters have been off the leading edge of space exploration for decades.
Is there a better way to accomplish the goals of LRO than with a single large orbiter? One option worth exploring is to replace the single large spacecraft with a number of smaller spacecraft, or smallsats. The instruments needed to meet the scientific requirements of the LRO mission would be split among several smallsats, each carrying perhaps only one or two instruments. Those smallsats, built by perhaps several different companies or organizations, could be launched over a period of time using a variety of different, lower-cost launch options.
At first glance that approach may seem to be rather inefficient. If the same instruments are going to the same destination, it would seem to make sense to combine them into a single spacecraft, sharing the same power, communications, and other resources, rather than building a separate spacecraft for each instrument. Yet there are a number of advantages to the smallsat approach that go beyond science and engineering.
Smallsats offer a wider range of launch alternatives. Rather than having to choose between a Delta 2 or an EELV, a smallsat could launch on smaller vehicles, such as a Taurus or SpaceX’s low-cost Falcon 1 or 5. Alternatively, a lunar smallsat could be launched as a secondary payload on a commercial launch of a communications satellite bound for geosynchronous orbit. The smallsat would be deposited into a geosynchronous transfer orbit and use a kick stage, or electric propulsion, to make its way to the Moon. That’s the approach used by SMART-1, Europe’s first lunar mission, which was launched as a secondary payload on an Ariane 5 last September and is gradually making its way to the Moon with its ion engine.
|What could be more exciting to a prospective engineering student than the opportunity to work on a spacecraft that will fly to the Moon?|
Smallsats also lend themselves to a staged approach to exploration. Rather than lumping a number of instruments on a spacecraft, then using those results to shape the development of a second- or third-generation mission, flying a steady stream of one or two (or more) smallsat missions a year allows the results of those missions to more rapidly be incorporated into the design of follow-on missions. This is, in effect, a form of “spiral development” that has become the buzzword du jour at NASA and the Defense Department. Flying large spacecraft packed with instruments makes sense with Mars missions, for example, where the launch windows are open for only a few weeks every 26 months, but makes far less sense for lunar missions, where the launch windows are effectively continuous.
Smallsats also allow a larger number of companies to participate. Outside of NASA, there are only a handful of companies that could realistically be able to build a spacecraft the size of LRO: Boeing, Lockheed Martin, Northrop Grumman, and a few others. Smallsats, on the other hand, are within the capabilities of a larger number of companies, including those like AeroAstro and SpaceDev who specialize in smallsat development. This increased competitiveness can lead to better, less expensive designs, while the use of multiple smallsats allows several companies to participate, strengthening the industrial base in the field.
In fact, some of the best developers of smallsats in the US are not necessarily companies. A number of universities have participated in smallsat development for both educational and research purposes, giving graduate and even undergraduate students hands-on experience building flight hardware. This experience can prove useful for lunar missions, but it could have other benefits as well. During the public hearings of the President’s Commission on Moon, Mars, and Beyond (also known as the Aldridge Commission), a number of witnesses have expressed the need to find new ways to encourage students to pursue careers in relevant science and engineering fields. By requiring, or at least strongly encouraging, lunar smallsat projects to involve universities in some manner in their development, NASA can harness both the experience of universities and promote the development of a new generation of spacecraft engineers. After all, what could be more exciting to a prospective engineering student than the opportunity to work on a spacecraft that will fly to the Moon?