Dawn

Dawn was NASA's first truly interplanetary spaceship: the only spacecraft to have orbited two destinations beyond Earth, with extended stays at two very different bodies, the giant asteroid Vesta and the dwarf planet Ceres.^[1][2] Both lie in the asteroid belt between Mars and Jupiter, and both are survivors from the earliest days of the solar system.^[1]

Ion propulsion

What made two orbits possible was ion propulsion, which Dawn carried into regular planetary service after its test on Deep Space 1.^[2] Three thrusters, each 30 cm across, accelerated xenon gas to 7 to 10 times the speed a chemical engine gives its exhaust; the thrust from each was tiny, at most 91 millinewtons, but it could run for years on the 425 kg of xenon loaded at launch, with more than 2,000 days of thrusting over the whole mission.^[2] Two engines were needed to complete the mission and the third was a spare.^[2] Two solar arrays, each 8.3 m long, powered them.^[2]

The journey

Dawn launched on 27 September 2007 on a Delta II rocket and swung past Mars on 17 February 2009 for a gravity assist.^[1][2][3] It arrived at Vesta on 16 July 2011 and spent its orbital residence there in 2011 and 2012 before departing for Ceres.^[3][4] It went into orbit around Ceres on 6 March 2015, about four months before New Horizons flew past Pluto, and Ceres became the first dwarf planet to be orbited by a spacecraft.^[5]

What it found at Vesta

Vesta, discovered on 29 March 1807 by Heinrich Wilhelm Olbers, is the parent body of a whole class of meteorites found on Earth.^[3] Dawn found two colossal impact basins in its southern hemisphere, the 500 km wide Rheasilvia basin and the older, 400 km wide Veneneia, and a gravity field consistent with an iron core.^[3] Pitted terrains and gullies in several young craters came as a surprise.^[3]

What it found at Ceres

Dawn's images revealed a heavily cratered but globally uniform surface punctuated by bright features, deposits of carbonates and other salts, brightest of all in Occator Crater.^[6] Gravity and topography data showed that Ceres's density increases with depth, and the spacecraft returned evidence of recent and potentially ongoing activity in several places.^[6] The discovery of ammonia on Ceres suggests that its materials, or Ceres itself, came from the outer solar system.^[6] Dawn had acquired all the data needed for its original objectives by the middle of 2016, and its extended missions took it ever closer: in its final orbit it swooped to only 35 km above the surface every 27 hours.^[6][4]

The end

Dawn depleted the hydrazine that kept it pointed on 31 October 2018, and the loss of its radio signal at the Deep Space Network marked the end of the mission on 1 November 2018.^[1] It remains in orbit around Ceres. Like NEAR Shoemaker at Eros, it stayed at its target to the very end.^[1]