SMART-1 Moon Impact Observation

September xx, 2006

SMART-1 is a spacecraft that orbits around the moon, lunched by European Space Agency (ESA). Its aim was to test the solar electric propulsion and other deep-space technologies, as well as investigating the lunar geochemistry (especially ice). SMART- 1 was lunched on September 27, 2003, and by November 15, 2004 it became the first spacecraft in Europe to enter the orbit around the moon. SMART-1 mission was extended for a further year, and was in use until July 2006.

Smart = Acronym for Small Mission for Advanced Research in Technology

Smart-1 Lunar Surface Impact
Smart-1 was decided to be crashed to the moon after its mission. It was originally on the orbit where impact will take place behind the moon, which forbids observation from earth. Later at the end of June, ESA made orbit correction to favor the observation from earth. The impact date after the correction is planed to be on September 2, 2006, Hawaii time (Sep.3; Japan); Big Island (Hawaii) being the most favorable place for observation. Impact can not be observed from Japan since the moon will be below the horizon at that time.
Spacecraft with a mass of 285kg, moving in a speed of 2km per second will collide in an angle of 1 degree over the geoid, therefore; the impact energy is estimated to be smaller than that of the Deep Impact event. However, since SMART-1 might collide on coarse surface, the incident angle of the impact could get as large as 10 degrees; moreover, the moon is closer than 9P/ Tempel 1, which makes this observation possible. In response to ESA’s call, research teams all over the world are setting up a plan to observe this incident.
Starting next year, 2007, for the subsequent 2 years, total of 4 spacecraft, SELENE from Japan, CH-3 from China, LCROSS from US and CHANDRA from India are going to be lunched toward the moon and in all cases, the spacecrafts are planed to be crashed after their missions.
Observing the impact effect on the moon by SMART-1 is a valuable opportunity to gain fudimental data for planning the effective use of lunar orbiter upon future impact event. In January1990, Japan lunched a “HITEN” spacecraft, which released its small orbiter “HAGOROMO” to the circumlunar orbit; was also crashed to the lunar surface on April 11, 1993. The Australian team has succeeded in capturing the impact flash in infrared light, emitted by fuel explosion, happened right on that moment of impact.

Impact Observation by Japanese Team
Japanese observation group consist of scientist in the field of impact experiment and solar system astronomy, underwent month and month of discussion on possible observation method to detect the impact phenomena.

  1. Thermal flash (flash in visible wavelength)
    Duration time of the flash is estimated to be approximately 3ms with magnitude of 8.8.
  2. Solar light reflection from ejecta (in visible light)
    100 seconds after the impact, high-speed ejecta goes beyond the day-night boundary and rapidly becomes bright. After another 220 seconds, the brightness reaches its peak. This duration time is in process of analysis. The magnitude during its peak is estimated to be around 10.
  3. Thermal emission from ejecta heated by solar radiation (in mid-infrared)
    The flux density of the thermal emission from the ejecta in the solar radiation is estimated to be 76 Jy, which is very bright. However, it spreads to a very wide region of about 1.3 arcmin causing its optical depth to be as small as τ= 2x10-5. Thus, thermal emission from the night side of the lunar surface (~5x104 Jy), seen transparent behind the dust plume, might be brighter.

Proposal for the observation using the Subaru Telescope was declined because of technical reasons (strong stray light from the moon) and conflict in scheduling. As a consequence of further discussion, the plan settled in a direction of observing the impact using a small telescope at the Subaru premise.

Observation results using small telescope

Observation team (※) from National Observatory led by Toshihiro Kasuga, observed the impact effect by setting up a CCD camera onto a small telescope with a diameter of 30cm and recorded the images on a video tape. As far as the observation went, no flash was detected from the monitor around the impact time (Sept. 2, 2006; 19:41 Hawaii time); although, there is still a possibility of detecting a flash by conducting data reduction on the recorded image.
The final observational results will be reported on this website when is becomes available.

*Observation team member: Toshihiro Kasuga (National Observatory), Seiji Sugita (University of Tokyo), Ken Tsukada (Tokyo Gakugei University), Taichi Kawamura (University of Tokyo), Nruhisa Takatou (National Observatory), Tetsuharu Fuse (National Observatory), Junishi Watanabe (National Observatory)

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