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The Wikipedia article NASA Solar Technology Application Readiness says:

The ions are accelerated through two fine grids with roughly a 1300 V difference between them for 2.3 kW operation, with a thrust of 20-92 mN, a specific impulse of 1950-3100 N·s/kg and a total impulse capability of 2.65 x106 Ns.

and the Wikipedia article subsection Dawn (spacecraft); Propulsion system says:

The Dawn spacecraft was propelled by three xenon ion thrusters derived from NSTAR technology used by the Deep Space 1 spacecraft, using one at a time. They have a specific impulse of 3,100 s and produce a thrust of 90 mN.

Both numerical values are about 3,000.

But Isp in seconds is obtained by dividing Isp in N·s/kg (which has units of velocity) by Earth's standard gravity of 9.80665 m/s^2, so one of those numbers is off by roughly a factor of 10.

  • Which one is wrong, or are they both wrong?
  • If so, then what's the right number?
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  • $\begingroup$ thought tempting, identify-this-error is probably a bad idea for a tag $\endgroup$
    – uhoh
    Commented May 18, 2019 at 19:07
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    $\begingroup$ It's possible that some future person will be searching for the correct Isp for Dawn. Such a person is more likely to come here if the question title asks for the correct answer, rather than disputing the wrong answer. $\endgroup$
    – DrSheldon
    Commented May 19, 2019 at 1:25
  • $\begingroup$ Maybe that's how they nearly missed their initial target... $\endgroup$
    – AtmosphericPrisonEscape
    Commented May 19, 2019 at 1:37
  • $\begingroup$ @DrSheldon I understand the concept, but in this case I think it's most appropriate to leave it as-is. This is exactly the question I needed answering, and it is exactly the question that has been addressed in the posted and accepted answer. If a future person queries a search engine, they will now get the right number in either article in Wikipedia. People don't usually come to Stack Exchange to get a specific numerical value like an engine's Isp, and this question will serve as a warning that even if they do move on to a more encyclopedic website like Wikipedia, numbers there can be wrong! $\endgroup$
    – uhoh
    Commented May 19, 2019 at 1:42
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    $\begingroup$ @uhoh validate-my-hypothesis might be a good one :). I've wanted one for validating a line of thinking for awhile now. There's a lot of open-ended questions, but I've seen a lot of people asking with an attempt at the question first (E.G. this). $\endgroup$
    – Magic Octopus Urn
    Commented May 23, 2019 at 16:41

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The 3100 s figure is the correct one. Note that a little further along in the same article it says "the engine achieves a specific impulse of one to three thousand seconds."

The N·s/kg unit is equivalent to meters/sec of exhaust velocity, and ~3000 m/s (or ~300 s) is typical for small, pressure fed, chemical bipropellant rockets. If you weren't getting 10 times better specific impulse out of the deal, there would be nothing attractive about complex, power-hungry electric thrusters with thrust levels measured in milliNewtons.

In fact, the citation on the NSTAR wikipedia article goes to a paper titled Performance of the NSTAR ion propulsion system on the Deep Space One mission, which says 1950-3100 s rather than 1950-3100 N·s/kg. The edit history of the Wikipedia article shows a well-meaning attempt to correct an error in unit conversion. The mis-corrected conversion has now been corrected, for the time being.

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    $\begingroup$ Excellent! Wikipedia is now a tiny bit better. $\endgroup$
    – uhoh
    Commented May 18, 2019 at 19:27
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    $\begingroup$ It would be even better if the article read the way it did before user 2A00:1028:8388:586E:3990:D513:9FA8:2CF defaced the article back in November, 2015. Before that, the article claimed "a specific impulse of 1950-3100 s". The 's' there is rather naked; it would have been better to use 'seconds'. But not N·s/kg. $\endgroup$
    – David Hammen
    Commented May 18, 2019 at 23:45

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