- The paper reanalyzes 96 Nozomi dust impacts recorded from 1998 to 2002, finding particle speeds of 3–70 km/s and masses from 10⁻¹⁹ to 10⁻¹² kg.
- The paper shows that Earth-orbit detections were unbound interplanetary or interstellar particles, while interplanetary flux varied by about a factor of two and broadly matched the IMEM model.
- The paper estimates an interstellar dust flux near 10⁻⁵ m⁻² s⁻¹ for particles above roughly 10⁻¹⁷ kg, while emphasizing uncertainties from limited statistics, wide viewing angles, and incomplete noise records.
The Mars Dust Counter (MDC) aboard the Japanese Nozomi (PLANET-B) spacecraft provided one of the few in situ dust measurements obtained in the interplanetary space between Earth and Mars. Although the mission failed to achieve its primary objectives at Mars, the dust instrument operated for approximately 3 years and 8 months, from launch on 3 July 1998 UT until a power supply failure on 24 April 2002. The present work by Krüger et al. makes the complete MDC dust data set available electronically for the first time and presents a re-analysis of the measurements in comparison with the Ulysses dust detector and with modern interplanetary dust modelling.
Instrumentation and data set
MDC was a lightweight (730 g) impact ionisation dust detector developed at the Technical University of Munich and ESA-ESTEC, with heritage from instruments flown on BREMSAT, Hiten, Galileo, and Ulysses. Impacts on gold targets above roughly 1kms−1 generate plasma whose electron and ion charges are measured by charge-sensitive amplifiers; impact speed is derived from signal rise time and mass from charge amplitude, with stated uncertainties of a factor of 2 in speed and a factor of 5 in mass. Mounted at 135° from the antenna axis on a spin-stabilised spacecraft rotating at 7–10 rpm, the sensor scanned most of the anti-Earth hemisphere each revolution.
A total of 96 dust impacts were identified: 20 during the Earth-orbiting phase (July–December 1998) and 76 in interplanetary space, where signals were classified using a neural network pattern recognition scheme reaching better than 99% classification accuracy. For 79 impacts, both impact speed and mass could be derived; speeds span 3 to 70kms−1 and masses more than seven orders of magnitude, from about 10−19 to 10−12kg. Noise was a significant operational concern — more than 20,000 noise events were recorded over the mission — but the resulting detector dead time exceeded 1% only during brief periods, peaking once at 14.5% for six hours on 8 November 1999. A caveat remains: the raw data and onboard-discarded noise events are no longer available, so the number of genuine impacts erroneously discarded cannot be quantified, though the agreement with model fluxes suggests it is small.
Earth-orbit phase: no bound or terrestrial dust
All particles detected during the Earth-orbiting phase were recorded near apogee of the highly elliptical geocentric trajectory. Deriving heliocentric particle speeds from the measured impact velocities and spacecraft attitude, the authors find values between 17 and 67kms−1, far exceeding the local escape speed from Earth's gravity. Consequently, every particle detected in Earth orbit must have been unbound to the Earth system, of interplanetary or interstellar origin. This is a strong negative result: MDC detected neither natural circumterrestrial dust nor man-made space debris, consistent with earlier findings from the nearly identical sensor on Hiten. It also rules out an apparent concentration of detections near lunar orbit as evidence for an Earth dust ring at the Moon's distance.
Interplanetary flux and comparison with IMEM
Simulations of the MDC measurements using the Interplanetary Meteoroid Engineering Model (IMEM), calibrated against COBE/DIRBE infrared observations, Galileo and Ulysses in situ fluxes, and Apollo lunar crater statistics, reproduce the measured impact rate well when a detection threshold of 1.5×10−16kg is assumed. The measured rate varied by approximately a factor of 2 over the interplanetary mission, driven mainly by spatial density and speed variations between 1 and 1.44 AU and by changing sensor orientation. The authors note that the model predicts somewhat less variation than observed, which they attribute partly to low counting statistics and partly to the fact that radiation pressure is not fully included in IMEM; specifically, MDC's effective sensitive area for slow "apex" particles dropped by about an order of magnitude between mid-1999 and mid-2000, plausibly contributing to the observed flux minimum in that interval.
Speeds, masses, and impact directions
The average impact speed of particles detected in interplanetary space was 19±15kms−1, versus 23±11kms−1 in Earth orbit — qualitatively consistent with gravitational focusing by Earth, though not statistically significant. Ulysses, traversing the same heliocentric range in 1990 at much higher spacecraft velocity, measured a substantially higher average of 36±17kms−1, explaining much of the apparent discrepancy. Similarly, the Ulysses mass distribution peaks two orders of magnitude below the Nozomi distribution, largely accounted for by the strong dependence of the detection threshold on impact speed (mthr∝vimp−3.5) together with a somewhat more sensitive low-speed threshold of the Ulysses sensor. Several MDC impacts exceed the local solar escape speed of 35–70kms−10, making them candidates for hyperbolic, likely interstellar, particles.
Impact directions concentrate towards the ecliptic plane, supporting an interplanetary origin for the majority (~95%) of detections. However, trajectory reconstruction is severely limited by the wide field of view: Senger's analysis gives a sensitive solid angle of 0.77–0.83 70kms−11 sr, and by analogy with the similar Hiten instrument, individual trajectories are accurate only to about 70kms−12. This limitation motivates the far superior angular resolution of upcoming instrumentation.
Interstellar dust
Sasaki et al. and Senger independently identified 3 and 5 interstellar particle candidates, respectively. Assuming four detections over 1999–2002 and accounting for the detection geometry, the authors derive an interstellar dust flux of approximately 70kms−13 (factor-of-5 uncertainty) for masses above roughly 70kms−14kg, consistent with measurements by Hiten, Helios, Ulysses, Galileo, Cassini, and with the Stardust aerogel collection, which yielded 70kms−15 for micron-sized grains despite using entirely different detection techniques. A detailed dynamical re-analysis of the interstellar candidates is explicitly deferred to future work.
During Nozomi's crossing of the Leonid stream of comet 55P/Tempel-Tuttle around 18 November 1998, four impacts were detected even though the high voltage had been switched off part of the time. None of the derived speeds or approach directions match the Leonid stream — the particles arrived from roughly the opposite direction — and an explanation via ejecta from Leonid impacts on the lunar surface is ruled out by travel-time arguments, since Nozomi was about 250 Earth radii away at the time. The origin of these four particles remains elusive. A statistical search of the full 96-event data set for other cometary trail crossings, applying the technique used successfully with Ulysses, yielded no statistically significant concentrations owing to the small number statistics.
Limitations and open questions
Several limitations qualify the results. The mass-velocity calibration differences between MDC and Ulysses cannot be investigated further because details of the decades-old electronics are unavailable. Trajectory determination carries uncertainties of order 70kms−16, precluding individual source attribution. The raw noise records are lost, leaving a small unquantified possibility of misclassification. Whether any of the several hyperbolic-speed candidates are genuinely interstellar, rather than fast bound particles within measurement uncertainty, remains unresolved pending dedicated dynamical modelling.
Conclusion
This work consolidates and re-evaluates the complete Nozomi MDC dust record, confirming that the measurements are broadly consistent with Ulysses observations in the same region of space and with the IMEM meteoroid model. The principal scientific outcomes are the absence of detectable bound circumterrestrial or debris populations, a factor-of-2 flux variation consistent with model predictions, ecliptic-concentrated impact directions indicative of interplanetary origin, and a tentative interstellar dust flux agreeing with independent techniques. With the MMX Circum-Martian Dust Monitor (70kms−17 sensitive area, launch 2026) and the DESTINY70kms−18 Dust Analyzer (trajectory accuracy of ~10° and mass resolution 70kms−19–150, launch 2028), missions traversing the same Earth–Mars corridor will be able to test these results with far larger statistics and much finer trajectory and composition diagnostics.