SPACE SCIENCE

Deep Space One: Preparing for Space Exploration in the 21st Century

The Deep Space One mission will test new technologies designed to expand the limits of space exploration. by Robert M. Nelson, NASA, Jet Propulsion Laboratory, Pasadena, Calif., USA

by Robert M. Nelson, NASA, Jet Propulsion Laboratory, Pasadena, Calif., USA

At 8:08 a.m. on October 24, NASA took a revolutionary step with the launch of the New Millennium Program's Deep Space One (DS1) mission. DS1 will fly by asteroid 1992KD in July 1999 and will then be on a trajectory toward comet 19P/Borrelly (see Figure 1). The target bodies are not what makes DS1 revolutionary, however. NASA has flown missions past comets and asteroids before. Rather, the pioneering role of DS1 is in paving the way for future, even more exciting, science missions by testing a host of new, advanced technologies that are unproven in deep space.

Fig. 1. Artist's conception of the DS1 spacecraft. The color of the xenon gas rocket plume in the drawing reasonably approximates the true color of the plume as observed in laboratory test facilities.

The primary objective of the New Millennium Program is to identify and validate in flight the technologies that hold great promise for revolutionizing observations both in deep space and in Earth orbit. However, because these technologies have not been demonstrated in space they are perceived to have a fairly high risk to missions that use them for the first time. So to help reduce the costs and risks to future missions that might use them, the program will send a series of dedicated "technology demonstration" missions into deep space and around Earth.

The program's solicitation of advanced technologies for space flight demonstration also stimulates their development and will strengthen the nation's technological infrastructure, making it more competitive in the global market. Many of these technologies also will have commercial spinoffs that will benefit the public in its daily life. The innovations to be demonstrated on DS1 are among the foundation technologies expected to support the next generation of deep space missions. Foremost among these is solar electric propulsion (SEP). In the next century, it is expected that SEP will make possible a whole class of ambitious missions that are simply impractical or unaffordable with standard chemical propulsion systems. DS1 will also test 11 other technologies including a multispectral imager and an integrated space physics ensemble.

The ion drive was started first on November 10, and after performing successfully for 4 « minutes, it autonomously shut down. Engineers familiar with ion drives had seen this behavior 20 times before when starting similar systems in space. It was also seen in laboratory experiments. In all 20 cases, the drive successfully resumed operations after remedial actions were taken. In most cases, these start-up problems involve minute particles that become jammed between the electrodes which make up the grid. On November 23, the ion drive started again and has been running continuously ever since. The DS1 spacecraft is slowly accelerating on its trajectory to its next target Asteroid 1992KD.

Solar Electric Propulsion

The principal of SEP design (Figure 2) is similar to that of a basic rocket motor where gases are generated in a contained area and directed out one opening in the containment. This creates an uncompensated force on the opposite side of the combustion chamber which thrusts the motor forward. However, SEP design does not use an oxidation-reduction chemical reaction between propellant and oxidizer to create the expanding hot gas. Instead, a steady stream of the element xenon is ionized in the rocket motor chamber. The ions are then accelerated by an electric field towards and through a grid maintained at 1280 volts. The accelerated ions pass through the grid and leave the rocket motor chamber. At a peak thruster operation power of 2300 watts, the thrust produced is about 90 millinewtons. At minimum thruster power of 500 watts, the thrust is 20 millinewtons. Although the thrust of SEP is small, a significant advantage is accrued because the 100,000 km/hour exhaust velocity of the ionized xenon is many times greater than the exhaust velocity of a conventional chemical propulsion system. These high-exhaust velocities cause the specific impulse (a measure of efficiency-thrust divided by the mass ejected per unit of time) of the DS1 ion propulsion system to be 3300 s. This exceeds the specific impulse of a typical chemical propulsion rocket motor by 1 order of magnitude. SEP therefore needs far less propellant than a chemical rocket to deliver the same payload mass to a target.

Fig. 2. Conceptual drawing of the DS1 ion propulsion system.

The low thrust of SEP means new approaches for evaluating mission trajectories. Patience is required for the small, nearly continuous, acceleration to produce high spacecraft velocity. SEP is particularly appropriate for missions needing high energy such as those intended to explore the inner solar system. For the same payload size, SEP can shorten interplanetary cruise times and require fewer planetary swingby gravity assists than a chemical propulsion system.

Solar Array

Because of the ion propulsion system power requirements, DS1 needs a high-power solar array and this is another one of the technologies to be tested. It is known as SCARLET II, which stands for solar concentrator arrays with refractive linear element technology. This solar concentrator pair uses cylindrical lenses to concentrate sunlight onto a strip of photovoltaic cells. A relatively small solar array area is actually covered by solar cells. The lens system provides a thick glass cover that greatly reduces susceptibility of the photovoltaic cells to radiation damage. Each array is composed of four panels measuring approximately 160 cm wide by 113 cm long. Multijunction GaInP₂/GaAs/Ge photovoltaic cell modules are interconnected in series to produce 100 volts and about 2300 watts at the beginning of the mission, declining over the life of the mission as the arrays age and the spacecraft recedes from the Sun. The spacecraft was launched from Cape Canaveral, Florida, on the first Delta 7326 rocket, a low-cost member of the Delta II family. Unlike other recent planetary spacecraft, DS1 is so small that another spacecraft, SEDSAT-1, built by students at the University of Alabama in Huntsville, will be sent into Earth orbit from the same small launch vehicle.

After DS1's initial checkout and certification by the mission operations team, the SEP system began to thrust. Instead of burning a strong short pulse of chemical propellant followed by a long interplanetary cruise, the ion drive emitted a very high-velocity, tenuous stream of ionized xenon. This created a very gentle but continuous period of thrust which propelled the spacecraft almost continuously during its interplanetary cruise.

Within about a month after launch, DS1 completed most performance assessments of its payload. Even if a failure of a technology causes termination of the mission, DS1 will still be considered a success if this failure can be diagnosed and the risk to future missions can be reduced. It is in these future missions that the real science return of DS1 will be found. However, this high-risk project also will attempt to return science data during its test flight. Although returning such data is not the primary goal of DS1, it is an important part of the overall demonstration that all technologies are consistent with a mission that conducts science. Two scientific instruments, the miniature integrated camera spectrometer (MICAS) and the plasma experiment for planetary exploration (PEPE), also will be tested for the first time in space by DS1 as well as an onboard autonomous navigation, other autonomy technologies, and a variety of telecommunications and microelectronics devices.

Camera Spectrometer

MICAS (Figure 3) includes a camera, two visible wavelength imaging channels, UV and IR imaging spectrometers, and thermal and electronic controls, all within one 12-kg package. It is derived from designs for fast flybys of the outer planets.

Fig. 3. a) Isometric drawing of the miniature integrated camera spectrometer (MICAS); b) Layout of the MICAS optical design showing the separate UV and IR spectrometers, which share a common telescope.

All sensors share a single 10-cm diameter telescope. The two visible wavelength detectors operate between about 500 and 1000 nm; one is a charged coupled device (CCD) with 13-microrad pixels and the other is an 18-microrad-per-pixel, metal-on-silicon active pixel sensor (APS). Unlike the CCD, the APS includes the timing and control electronics on the chip along with the detector. The two imaging spectrometers operate in push-broom mode. The UV spectrometer spans 80 to 185 nm with 0.64-nm spectral resolution and 316-microrad pixels. The IR spectrometer covers the range from 1200 to 2400 nm with 6.6-nm spectral resolution and 54-microrad pixels.

MICAS is designed to collect valuable science data during the mission, particularly during the asteroid flyby. It also is used to gather images for DS1's primary navigation system, known as the autonomous optical navigation subsystem—another technology to be tested. This system uses data already resident on the spacecraft or data acquired and processed onboard. Each image includes an object known as a "beacon" (a selected asteroid certain to be visible from the spacecraft) and known background stars. The plan is for four or five beacons to be imaged about three times per week during the initial checkout mission phase and then, with the exception of the encounter periods, for about 12 beacons to be imaged once per week during most of the remainder of the mission. Onboard image processing techniques allow accurate determination of the apparent positions of the asteroids with respect to the background stars. Asteroid ephemerides and star catalogs are stored in the autonomous subsystem software, and the spacecraft's three-dimensional position is estimated. The heliocentric orbit is computed with a sequence of these position determinations. The trajectory then is propagated to the next encounter target and course changes are generated by the maneuver design element.

Plasma Experiment

PEPE (Figure 4) combines multiple instruments into one compact 6-kg package designed to determine the three-dimensional distribution of plasma over its field of view. PEPE includes a very low-power, low-mass microcalorimeter to help understand plasma-surface interactions, and a plasma analyzer to identify the individual molecules and atoms in the immediate vicinity of the spacecraft that have been eroded off the surface of asteroid 1992KD. It is a response to desires from within the space physics community to use common apertures with separate electrostatic energy analyzers to conserve power and mass in an integrated instrument.

Fig. 4. Line drawing of the plasma experiment for planetary exploration instrument. Ions and electrons enter the common entry path shown as light green. The electrons are deflected upward to the analyzer where they strike a microchannel plate following the path shown as dark green in the drawing. The ions are deflected downward along the red path where they strike foils that emit secondary electrons which provide information on the energy and mass of the incident ions.

Thus, PEPE combines several plasma instruments into one package. It measures electron and ion energies spanning a range of 3 eV to 30 keV, with 5% resolution. It also measures ion mass from 1 to 135 atomic mass units with 5% resolution. Using no moving parts, it electrostatically sweeps its field of view both in elevation and azimuth.

PEPE serves three functions. First, it tests the design of a suite of space physics instruments in one package. Second, it assists in determining the effects of the ion propulsion system on spacecraft surfaces and instruments and on the space environment, including interactions with the solar wind. Third, it conducts scientifically interesting measurements during the cruise and the encounter with asteroid 1992KD. Analysis of PEPE data also will assure future users that there are no incompatibilities with space physics measurements and a spacecraft operating with solar electric propulsion.

Both MICAS and PEPE represent a new direction for the evolution of science instruments for interplanetary spacecraft. These two instruments are meant to take over a large part of the functions of five other instruments that had typically flown on NASA's deep space missions.

The NASA peer-review process has selected 15 scientists to conduct investigations on DS1 using both MICAS and PEPE, and also using measurements from the spacecraft engineering diagnostics systems for serendipitous data.

Additional Technologies to be Tested

The other technologies to be tested by DS1 are all designed to improve the engineering environment in deep space. All can be expected to reduce costs and increase frequency of space exploration missions. Several are related to telecommunications and three others involve electronics experiments.

The small deep-space transponder is designed to facilitate communication back and forth between mission control and the spacecraft. It combines the spacecraft's receiver, command detector, telemetry modulation, exciters, beacon tone generation, and control functions into one small, 3-kg package.

The transponder also can generate tones, in a beacon monitor operations technology, that could prove a big help in reducing the large demand on NASA's deep space network. The spacecraft determines its own health and need for human assistance. It selects one of four tones indicating its diagnostic findings, transmitting the tone to Earth so that mission control can decide what, if any, action is needed.

The other telecommunications technology to be tested is a Ka-band solid state power amplifier. This band uses a smaller antenna and less power to transmit data than has been the case. The first of the electronics experiments, lower power electronics, tests the performance of a variety of electrical devices (including a ring oscillator, multipliers, and discrete transistors) with very low voltage and capacitance. It requires significantly less power than comparable devices using conventional technology.

The second experiment involves a multifunctional structure packaging technology combining load-bearing elements with electronic housings and thermal control, greatly reducing the mass of spacecraft cabling and traditional chassis. A power activation and switching module device, the last of the electronics experiments, contains two sets of four power switches that feature quadruple the packing density of current switches.

One of the criticisms that has been levied against NASA in past years has been that the agency is unwilling to take the prudent risks necessary to incorporate bold, innovative technologies into deep space missions. The NASA administrator, Dan Goldin, has encouraged his agency to entertain greater risk in developing new missions; however, many members of the agency's panels that approve mission proposals are fearful of being identified with selecting a mission that subsequently fails. NASA managers are rewarded for mission success, not for taking risks. Thus, they are most comfortable with missions that use components with a long heritage and systems configured to ensure the greatest possible redundancy in order to maximize the possibility of a "work around" should a component fail. This is sometimes called the "both belt and suspenders" approach to project management.

In the absence of a system that rewards risk, NASA needed to establish a method to demonstrate unvalidated technologies in deep space, thus reducing the associated risk. This led Goldin's associate administrator for space science, Wes Huntress, to spearhead the development of the New Millennium Program, managed for NASA at its Jet Propulsion Laboratory (JPL). New Millennium was to be the final step in validating selected technologies so that they could be proposed for future missions without the stigma of risk.

At JPL, the Deep Space One project manager is David Lehman and the management team includes Leslie Livesay, Phillip Varghese, and Curtis Cleven. The science team, led by Robert Nelson, includes Frances Bagenal, Daniel Boice, Daniel Britt, Robert H. Brown, Bonnie Buratti, Wing Ip, Jurgen Oberst, Tobias Owen, Bill Sandel, Laurence A. Soderblom, Alan Stern, Nicholas Thomas, Joseph J. Wang, Roger Yelle, and David T. Young. The NASA headquarters science leader is Tom Morgan.

Source: Eos, October 13, 1998, p. 493.