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Cassini is to fly within 50 km of Enceladus.

What is the closest flyby a spacecraft has ever made? What object was targeted and when was it? Is 50 km a new record?

(For the purposes of this question, an ultra-close orbit is also included, but lithobraking, landing, or any procedure that ultimately resulted in contact or was designed to, is not)

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  • $\begingroup$ 50 km is not a new record.Cassini already made a closest approach of 25 km in 2008.50 km is the closest to ever fly over the plumes.Check this $\endgroup$
    – r2_d2
    Oct 28, 2015 at 9:29
  • $\begingroup$ IIRC, Apollo 10 came within about 15 nautical miles of the moon's surface. $\endgroup$
    – Michael Borgwardt
    Oct 28, 2015 at 13:56
  • $\begingroup$ @MichaelBorgwardt: Apollo 10 got below 16 KM (not miles) of the Moon's surface, that's less than 10 miles. $\endgroup$
    – dotancohen
    Oct 29, 2015 at 9:13
  • $\begingroup$ @dotancohen ah, right, looks like I misremembered that. In any case, it's certainly the closest manned flyby. $\endgroup$
    – Michael Borgwardt
    Oct 29, 2015 at 10:18

3 Answers 3

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JAXA's Hayabusa came to within 44 meters of 25143 Itokawa, which, in an unfortunate sequence of events, lead to its lander MINERVA to miss the target:

MINERVA was deployed on 12 November 2005. The lander release command was sent from Earth, but before the command could arrive, Hayabusa's altimeter measured its distance from Itokawa to be 44 m (144 ft) and thus started an automatic altitude keeping sequence. As a result, when the MINERVA release command arrived, MINERVA was released while the probe was ascending and at a higher altitude than intended, so that it escaped Itokawa's gravitational pull and tumbled into space.

It is possible that MINERVA itself came closer to Itokawa than 44 meters, but it was an unpowered mini lander, so that probably doesn't count:

                     Hayabusa's MINERVA lander missing the asteroid 25143 Itokawa

  MINERVA lander (in yellow circle) missing the asteroid 25143 Itokawa as seen by Hayabusa probe. Image: JAXA

Perhaps also interesting (tho it doesn't fit requirements of your question), a week later and as a result of a separate set of approach maneuvers, Hayabusa also landed on the asteroid, collected samples, and then took off again. Next probe to perform a similar feat of briefly touching an asteroid to collect samples will be NASA's OSIRIS-REx at 101955 Bennu.

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Chinese Chang'e 2 flew 3.2 km from the asteroid 4179 Toutatis (size: 4.7×2.4×1.9 km) in 2012 after having ended its primary Lunar mission. Maybe a record for intentional flyby's?

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Here's a list of the 21 asteroids and comets and minor planets visited by spacecrafts, with closest approach altitude. One way to define it is in number of object radii, and then Dawn wins at Ceres according to that table (but many Earth, Moon, Mercury, Venus and Mars orbiters beat that).

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The closest I know of is Rosetta, which orbited 67P at altitudes down to 6 km.

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  • $\begingroup$ I didn't realize they dared take it so low to an active comet. I thought Cassini's 50 km from Enceladus in 2008 and Mars Express 45 km from Phobos in 2013 would be near records. $\endgroup$
    – LocalFluff
    Oct 28, 2015 at 10:34
  • $\begingroup$ 6 km orbits were done early in the mission when the comet was less active. At the moment (close to perihelion) they're several hundred km away. $\endgroup$
    – Hobbes
    Oct 28, 2015 at 10:59
  • $\begingroup$ @LocalFluff That orbit turned out to be too close and Rosetta's star tracker started confusing comet's debris for stars, so they increased its orbit again. But I believe they plan to eventually "soft crash" / "hard land" the probe into the comet (I didn't follow up if it was approved, but it was proposed by Matt Taylor). $\endgroup$
    – TildalWave
    Oct 28, 2015 at 11:24
  • $\begingroup$ Rosetta's orbit is now 0 km ;-) $\endgroup$
    – gerrit
    Oct 7, 2016 at 23:10

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