Read the changing mission record

CASSIOPE’s reaction-wheel failures altered its pointing capability. After the third wheel failed on December 17, 2021, the spacecraft entered a Sun-pointing, spin-stabilized configuration using magnetic torque rods. Its routine science phase ended on December 31, 2021. Those are distinct events in the dated operations record.

The June 2024 final report states that reduced operations continued as of January 2024, outside the ESA Third Party Mission program. This does not establish unchanged full capability or a current service guarantee. Equally, the end of the routine phase should not be rewritten as an immediate loss of every signal or every possible measurement.

Pointing changes affect instruments differently. The report describes how the spinning configuration prevented the GPS occultation receiver from holding its earlier view of setting GPS satellites; it also records a later receiver failure. A geometric limitation and a hardware failure are separate explanations, even when both limit a product.

The attitude handbook distinguishes estimates based on star sensors from less precise coarse-sensor solutions. It also defines missing values and source flags. A daily file with a regular time grid does not mean every row contains an equally precise independent measurement of orientation.

Imagine a graph containing an angle of 999.0 degrees where the product documentation defines 999.0 as a missing-data marker. That row does not describe a spectacular rotation. A reader must recognize the marker and exclude or explicitly display the missing value according to the analysis. Connecting it with an ordinary line can manufacture a dramatic event.

Interpolation needs similar care. A smooth curve spanning a gap is a mathematical construction between available observations. It does not recover a measurement that never arrived. Whether the interpolation is useful depends on gap length, expected motion, the method and the stated uncertainty.

Keep a missing value visibly missing

The e-POP data site offers quicklook products, processed files and technical guidance. It directs users seeking publication-quality products to the relevant instrument team. A useful preview can help locate an event without establishing that all calibration and quality decisions needed for a scientific claim have been completed.

For an original archive exercise, version A has a sensor offset that version B corrects. The corrected result can change while the original exposure time remains the same. Keep the source file, software or calibration version, exclusions, coordinate convention and relevant mission configuration together. A later improvement should be traceable rather than silently replacing the history.

CASSIOPE’s record teaches a broader way to read space missions: identify the question, the measuring system and the conditions under which a result is valid. Its communications demonstrator, polar science and Swarm partnership are connected accomplishments, but none removes the need to date the evidence and name its limitations.