Chapter 8. Experiments
The Scientific Community
The scientific community involved in JPL's experiments is worldwide and typically is composed of PhD-level scientific professionals tenured in academia, and their graduate students, as well as similar-level professional scientists and their staff from industry, scientific institutions, and professional societies.
By way of illustration, here are a few random examples of facets of the world's scientific community, who might typically become involved in JPL robotic missions:
Gathering Scientific Data
Some experiments have a dedicated instrument aboard the spacecraft to measure a particular physical phenomenon, and some do not. A designated principal investigator (PI) or in many cases, a team, determines or negotiates the experiment's operation and decides who will analyze its data and publish the scientific results. Members of these teams may have been involved in the design of the instrument. Some examples of this kind of experiment are...
Other experiments are undertaken as opportunities arise to take advantage of a spacecraft's special capabilities or unique location or other circumstance. Some examples of this kind of experiment are...
Science and Engineering Data
Data acquired by the spacecraft's scientific instruments and telemetered to Earth, or acquired by ground measurements of the spacecraft's radio signal in the case of Radio Science, in support of scientific experiments, is referred to as science data. (Please see the editorial page for discussion of collective vs. plural "data.") Science data is the reason for flying a spacecraft.
The other category of data telemetered from a spacecraft, its health and status data such as temperatures, pressures, voltages, and computer states, is referred to as engineering data. This is normally of a more repetitive nature, and if some is lost, the same measurement of pressure or temperature can be seen again in a short time.
Except in cases of spacecraft anomalies or critical tests, science data is always given a higher priority than engineering data, because it is a mission's end product. Engineering data is used in carrying out spacecraft operations involved in obtaining the science data.
The Science Data Pipeline
Science data from on-board instruments, once received at the antennas of the DSN, flows through a string of computers and communications links known collectively as the Deep Space Mission System (DSMS), formerly called the Ground Data System (GDS). The functions of the DSMS can be viewed as generally divided into two high-level segments: front-end and back-end.
Front-end processing consists of frame-synchronizing the data stream (discussed further in Chapter 18), restoring the data formats that were created by the spacecraft computers, and providing real-time visibility of engineering and tracking data for engineering analysts and science instrument teams. Back-end processing consists of data management to provide complete and catalogued data sets, production of data products such as images, and use of tools to access the data storage and cataloguing systems.
While there is typically some front-end visibility into some of the science data in real time, it is mainly through the back end systems that science teams (for whom the missions are flown) formally have access to complete sets of their science data.
Final science data products usually consist of time-ordered, gap-controlled sets of instrument-specific data records known as Experiment Data Records (EDRs). Other products that support analysis of the science data include collections of DSN monitor data which indicates the performance of DSN receivers, tracking and telemetry equipment, selected spacecraft engineering data, spacecraft ephemeris and pointing data. These are known as Supplementary Experiment Data Records (SEDRs) or the equivalent. SEDRS track the history of instrument pointing (discussed in Chapter 12), detailing the instrument's "footprint" on the object being imaged.
Spacecraft, Planet, Instruments, C-matrix, and Events (SPICE) kernels (files) provide spacecraft and planetary ephemerides, instrument mounting alignments, spacecraft orientation, spacecraft sequences of events, data needed for certain time conversions, etc. SPICE kernels are produced by the JPL Navigation and Ancillary Information Facility (NAIF) and are archived by the PDS NAIF Node. Related files include the Spacecraft and Planet ephemeris data Kernel (SpK), and the Planet Physical and Cartographic Constraints PcK Kernel.
Science data products have historically been produced within the Data Management Systems of flight projects. Cassini uses an example of a new, more distributed plan that calls for its diverse science teams to produce most or all the science data products after compilation and analysis. Cassini's small Data Management Team performs only those data management functions needed to deliver complete data sets to the science teams.
Radio science (RS) experiments use the spacecraft radio and the DSN together as their instrument, rather than using only an instrument aboard the spacecraft. They record the attenuation, scintillation, refraction, rotation, Doppler shifts, and other direct modifications of the radio signal as it is affected by the atmosphere of a planet, moons, or by structures such as planetary rings or gravitational fields. From these kinds of data, radio scientists are able to derive a great deal of information such as the structure and composition of an atmosphere and particle sizes in rings.
Gravity Field Surveys
Another science experiment, like radio science but not strictly classified as such, does not use an instrument aboard the spacecraft. Gravity field surveys (not to be confused with gravitational wave searches) use the spacecraft's radio and the DSN to measure minute Doppler shifts of a vehicle in planetary orbit. After subtracting out the Doppler shifts induced by planetary movement, the spacecraft's primary orbital motion, and small force factors such as the solar wind and atmospheric friction, the residual Doppler shifts are indicative of small spacecraft accelerations and decelerations. These are evidence for variations in the planet's gravity field strength associated with high and low concentrations of mass at and below the planet's surface. Mapping the planet's mass distribution in this way yields information that complements other data sets such as imaging or altimetry in the effort to understand geologic structure and processes at work on the planet. A prime example of Gravity Field Surveys was the 2012 GRAIL mission, which yielded high-resolution data for the Moon. Gravity field surveying is further described in Chapter 16.
Dissemination of Results
Publication of the results of the experiments takes place in the literature of the scientific community, notably the journals Science (American Association for the Advancement of Science, AAAS), Nature, the international weekly journal of science, JGR (Journal of Geophysical Research, a publication of the American Geophysical Union), and Icarus, an official publication of the Division for Planetary Sciences of the American Astronomical Society.
Experimenters whose instruments ride aboard JPL spacecraft make presentations at virtually every periodic convention of the various scientific organizations, such as those mentioned above. If an operations person has an opportunity to attend one or more, it would be very worthwhile. The news media and several magazines keep a close eye on all of these journals and proceedings and report items of discovery from them. The thin weekly magazine Science News is a notable example, as is the amateur astronomers' monthly Sky & Telescope magazine. Splendid photography from JPL's missions occasionally appears in National Geographic magazine, and many a JPL mission has enjoyed very good treatment in public television's science series Nova and documentaries on cable TV's Discovery channel.
Regional Planetary Imaging Data Facilities (RPIF) are operated by NASA's Planetary Data System (PDS) at over a dozen sites around the United States and overseas. Each maintains a complete photographic library of images from NASA's lunar and planetary missions. They are open to members of the public by appointment for browsing, and their staff can assist individuals in selecting and ordering materials. All of NASA's planetary imaging data is made available for researchers who are funded by NASA, in photographic format and digital data format, via the PDS.
Educators may obtain a wide variety of materials and information from NASA's flight projects through the network of Educator Resource Centers (ERC) in cooperation with educational institutions around the country. ERCs also support a center for distribution of multimedia resource materials for the classroom, called Central Operation of Resources for Educators (CORE). JPL's Education website offers more information about various avenues for information dissemination.
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1 The Solar System
2 Reference Systems
3 Gravity & Mechanics
5 Planetary Orbits
7 Mission Inception
9 S/C Classification
11 Onboard Systems
12 Science Instruments
17 Extended Operations
18 Deep Space Network