Suisei (spacecraft)

Suisei (すいせい, lit. "Comet"), originally known as Planet-A, was an unmanned space probe developed by the Institute of Space and Astronautical Science (now part of the Japanese Aerospace Exploration Agency, or JAXA).

Spacecraft Suisei
NamesPlanet-A (before launch)
OperatorISAS (now part of Japanese Aerospace Exploration Agency)
COSPAR ID1985-073A Edit this at Wikidata
SATCAT no.15967
Mission duration5 years and 5 months (launch to fuel depletion)
Spacecraft properties
Launch mass139.5 kg (308 lb)[1]
Start of mission
Launch date23:33, August 18, 1985 (UTC) (1985-08-18T23:33Z)
Launch siteUchinoura Space Center
End of mission
DisposalRan out of fuel by February 22, 1991
Orbital parameters
Reference systemHeliocentric
Sun orbiter
Flyby of Comet Halley
Closest approachMarch 8, 1986
Distance151,000 km (94,000 mi)
Flyby of Earth
Closest approachAugust 20, 1992
Distance~900,000 km (560,000 mi)

It constituted a part of the Halley Armada together with Sakigake, the Soviet Vega probes, the ESA Giotto and the NASA International Cometary Explorer, to explore Halley's Comet during its 1986 sojourn through the inner solar system.


Suisei was identical in construction and shape to Sakigake, but carried a different payload: a CCD UV imaging system and a solar wind instrument.

The main objective of the mission was to take UV images of the hydrogen corona for about 30 days before and after Comet Halley's descending crossing of the ecliptic plane. Solar wind parameters were measured for a much longer time period.

The spacecraft is spin-stabilized at two different rates (5 and 0.2 rpm). Hydrazine thrusters are used for attitude and velocity control; star and sun sensors are for attitude control; and a mechanically despun off-set parabolic dish is used for long range communication.


Suisei was launched on August 18, 1985 by M-3SII launch vehicle from Kagoshima Space Center on M-3SII-2 mission. It was sent on an intercept course with Comet Halley, after which it would remain in a heliocentric orbit for later use as long as it was viable.

Halley encounterEdit

Suisei began UV observations in November 1985, generating up to 6 images/day.

The spacecraft encountered Comet Halley at 151,000 km on sunward side during March 8, 1986, suffering only 2 dust impacts.

Earth flybyEdit

Fifteen burns of Suisei's 3 N motors during the period of April 5–10, 1987, yielded a 65 m/s velocity increase for a 60,000 km Earth gravity assist swingby on August 20, 1992, although the craft was then lost behind the Sun for the summer.

The hydrazine fuel was depleted on February 22, 1991. Preliminary tracking indicated a 900,000-km flyby had been achieved.

Other planned encountersEdit

ISAS had decided during 1987 to guide Suisei to a November 24, 1998, encounter with 21P/Giacobini-Zinner, but due to depletion of the hydrazine, this, as well as plans to fly within several million kilometers of Comet 55P/Tempel-Tuttle on February 28, 1998, were cancelled.


  1. ^ "Suisei". NASA Space Science Data Coordinated Archive. Retrieved December 2, 2022.

External linksEdit