Dust measurements with the Mars Dust Counter on board Nozomi (PLANET-B)

Abstract

Nozomi was Japan's first space mission to Mars, launched on 3 July 1998 UT. It was equipped with the Mars Dust Counter (MDC) which was an impact ionisation dust detector. MDC detected 96 dust particle impacts when the spacecraft was in Earth orbit and later in interplanetary space, before its operation ended in April 2002 due to a technical failure on board. We compare the Nozomi dust measurements with the dust measurements obtained with the dust detector on board the Ulysses spacecraft. Impact speeds and masses of dust particles measured by Nozomi MDC are overall consistent with the measurements obtained by Ulysses in the same region of interplanetary space. Based on the impact speeds measured while Nozomi was in Earth orbit, MDC detected neither dust particles of natural origin that were bound to the Earth nor space debris. The dust impact rate measured in interplanetary space varied by approximately a factor of 2, consistent with theoretical predictions by the Interplanetary Meteoroid Engineering Model. The particle impact direction was concentrated towards the ecliptic plane, in agreement with an interplanetary origin of the majority of the measured dust particles. No impacts of cometary trail particles could positively be identified during known cometary trail crossings of Nozomi. The Nozomi dust data may become a valuable reference for the dust measurements to be obtained in the same region of interplanetary space with future space missions like, for example, MMX and DESTINY$^+$.

Figures (12)

• Figure 1: Nozomi trajectory when the spacecraft was in Earth orbit between launch on 3 July 1998 and 25 December 1998 (red), projected onto the ecliptic plane (vernal equinox is to the right). The orbit of the Moon is shown in black, and the measured dust impacts are superimposed (asterisks). The units are mean Earth radii ($\mathrm{R_E=6,371~km}$). The Nozomi trajectory data were provided by ISAS/JAXA.
• Figure 2: Nozomi trajectory projected onto the ecliptic plane (vernal equinox is to the right). The interplanetary trajectory of the spacecraft is shown in red. The measured particle impacts are superimposed for the Earth orbiting phase (asterisks) and for the interplanetary trajectory (squares). The locations of the spacecraft at specific times are indicated in blue. The Nozomi trajectory data were provided by ISAS/JAXA.
• Figure 3: Measurement principle of MDC. Impacting dust particles vaporised when hitting the target and produced plasma around the impact location. The charges of this plasma (negative free electrons and positive ions) were separated by the electric field inside the sensor box. Positive ions were collected by the negatively biased ion channel collector and electrons were collected by the positively biased electron channel collector. Three charge sensitive logarithmic amplifiers converted the measured charges into voltage signals that were transformed into digital data and transmitted to Earth.
• Figure 4: Orientation of the Nozomi spacecraft and the MDC. The antenna (top) pointed towards Earth most of the time during the interplanetary mission phase, and MDC largely faced the anti-Earth hemisphere. The sensor axis had an angle of $135^{\circ}$ from the antenna pointing direction. During one spin revolution of the spacecraft the sensor axis scanned an angle of $90^{\circ}$. MDC itself had almost $180^{\circ}$ field of view (FOV).
• Figure 5: Spacecraft attitude: deviation of the antenna pointing direction (i.e. positive spin axis) from the nominal Earth direction during the interplanetary mission of Nozomi. The deviations are given in ecliptic longitude $\Delta\lambda_{\mathrm{ecl}}$ (top) and latitude $\Delta\beta_{\mathrm{ecl}}$ (bottom, equinox J2000).