Giotto (596 km), the first space probe to get close-up color images of the nucleus of a comet. (ESA)
Vega 2 (8,030 km), which dropped a balloon probe and lander on Venus before going on to Halley. (USSR/France Intercosmos)
Vega 1 (8,889 km), which dropped a balloon probe and lander on Venus before going on to Halley. (USSR/France Intercosmos)
Suisei (151,000 km), also known as PLANET-A. Data from Sakigake was used to improve upon Suisei for its dedicated mission to study Halley. (ISAS)
Sakigake (6.99 million km), Japan's first probe to leave the Earth system, mainly a test of interplanetary mission technology. (ISAS)
Without the measurements from the other space probes, Giotto's closest distance would have been 4,000 km instead of the 596 km achieved.
Other space probes had their instruments examining Halley's Comet:
Pioneer 7 was launched on August 17, 1966. It was put into heliocentric orbit with a mean distance of 1.1 AU to study the solar magnetic field, the solar wind, and cosmic rays at widely separated points in solar orbit. On 20 March 1986, the spacecraft flew within 12.3 million kilometers of Halley's Comet and monitored the interaction between the cometary hydrogen tail and the solar wind.
Pioneer Venus Orbiter in orbit of Venus, was positioned perfectly to take measurements of Halley's Comet during its perihelion February 9, 1986. Its UV-spectrometer observed the water loss when Halley's Comet was difficult to observe from the Earth.
The Space Shuttle Challenger, on its launch on January 28th, 1986, was carrying SPARTAN-203 with the mission to make observations of Halley's Comet. STS-51Lfailed to reach orbit, resulting in the total loss of crew and vehicle. That launch failure resulted in the cancellation of dozens of subsequent shuttle missions, including the next scheduled launch, STS-61-E, planned for March 6, 1986, with a payload including the ASTRO-1 observatory, which was intended to make astronomical observations of Halley's Comet.