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The BBC News article Hayabusa-2: Japanese probe set to 'bomb' an asteroid says:

The charge is carried on the Small Carry-On Impactor (SCI) device.

This is a 14kg conical container attached to Hayabusa-2 and packed with plastic explosive.

The SCI is due to separate from Hayabusa-2 at 01:56 GMT on Friday at an altitude of 500m above the surface of Ryugu. That will kick off a 40-minute countdown to detonation. When the device explodes, it is expected to punch a 10m-wide hole in the asteroid.

In the meantime, Hayabusa-2 should have manoeuvred itself to hide away on the other side of the asteroid, shielding the spacecraft from any flying debris.

The answer to How far will the debris cloud from Hayabusa-2's touch-and-go sample recovery travel around Ryugu? Will any have escaped to heliocentric orbit? makes it clear that even low-velocity projectiles produced near Ryugu's surface could travel in Ryugu-centric as well as heliocentric orbits. Of course since the Ryugu-centric (elliptical) orbits intersect the surface at one point, they'll intersect somewhere else as well, and either come gently to rest or scatter secondary additional projectiles.

So just because Hayabusa-2 will move to the other side doesn't mean it can't get hit by something.

I'm curious what kinds of debris Hayabusa-2 mission-planners at JAXA might have thought about ahead of time, so I'd like to ask: How far did Hayabusa-2 back off before the "bomb" went off?

Lower volume before playing video because it contains an Earth-shattering kaboom!!

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  • $\begingroup$ "So just because Hayabusa-2 will move to the other side doesn't mean it can't get hit by something." Are you sure about this? Escape velocity is like 30cm/s and orbit is 26cm/s and just intuitively, I'd wager that debris thrown up by the explosion is either 1. Fast enough to escape Ryugu gravity. 2. Not fast enough to escape but thrown so elliptically that it won't return for a long time. 3. Slow enough to enter orbit but not cause spacecraft damage if it hit. $\endgroup$
    – Dragongeek
    Commented Jun 13, 2020 at 13:23
  • $\begingroup$ @Dragongeek see answer(s) to Could ejecta from asteroid Bennu enter into temporary orbit around it? If so, how? I can't speak to the damage as a function of velocity though... $\endgroup$
    – uhoh
    Commented Jun 13, 2020 at 15:07

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The answer is that the Hayabusa-2 was 4.61 km from the SCI when it detonated.

The link provides a mission status briefing from JAXA which describes the entire sequence of events, including positions. Slide 10 has the relevant mission data laid out on a grid.

http://www.hayabusa2.jaxa.jp/en/enjoy/material/press/Hayabusa2_Press20190411_ver10_en.pdf

enter image description here

Before the bomb went off, I’m interpreting to mean explosive release of the copper impactor.

enter image description here

In a sense, another bomb went off again when the copper impactor struck Ryugu 500 M below the SCI. The answer for that is 4.12 km.

enter image description here

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    $\begingroup$ Updated. Having a blast, thanks for keeping me sharp. $\endgroup$
    – Anthony Stevens
    Commented Jun 15, 2020 at 13:20
  • $\begingroup$ "Before the bomb went off, I’m interpreting to mean explosive release of the copper impactor" well I'm interpreting it as the moment when the "14kg conical container attached to Hayabusa-2 and packed with plastic explosive" which exploded after the "40-minute countdown to detonation" which was "expected to punch a 10m-wide hole in the asteroid." I'm not convinced that your answer talk about this exact explosion. $\endgroup$
    – uhoh
    Commented Jun 16, 2020 at 9:07
  • $\begingroup$ Well, you go on to talk about debris. There’s no chance of the impactor accidentally hitting Hayabusa-2. The real danger to the craft was something getting ejected from the surface, damaging the craft and ending the sample return mission. That would have been devastating. $\endgroup$
    – Anthony Stevens
    Commented Jun 16, 2020 at 10:43
  • $\begingroup$ I don't see an answer to my question yet then. When the impactor detonated, and presumably punched a hole in the asteroid, how far was Hayabusa-2 from Ryugu? Can you address that and clearly indicate your answer? Thank you! $\endgroup$
    – uhoh
    Commented Jun 17, 2020 at 0:34
  • $\begingroup$ The Hayabusa2 spacecraft was (hiding) on the other side of Ryugu and out of the line-of-sight to the SCI when it was detonated. Distance was approx 4.6km; see slide 32 in the presentation. $\endgroup$
    – astrosnapper
    Commented Nov 17, 2020 at 22:56
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I obtained the chart from data I logged and reverse-engineered for Hayabusa OnBoardCamera and LIDAR; unfortunately telemetries feed broke right after SCI operation, for several days:

Hayabusa 2 SCI operation - 2019/04/04

Planned timeline:

Planned timeline for SCI

Interactive chart: http://programmi.hostingerapp.com/hayabusa2/simulator/haya2-dates-5000.html

Schedule was:

  • 04/05 01:44 - 500 meters - Estimated target altitude arrival time
  • 04/05 01:56 - SCI separated
  • 04/05 02:14 - DCAM3 separated
  • 04/05 02:36 - SCI explosion

At the scheduled time for the SCI separation (01:56 GMT), the LIDAR reading was around 469 meters from surface; explosion was planned at 02:36 GMT , but all readings after 2:07:10 GMT stated "482 m" for several days, so it looks like this information is not available in my amateur log.

Logged LIDAR data

Raw data (amateur log):

http://programmi.hostingerapp.com/hayabusa2/simulator/ONC_A-log-2019-04-04_2019-04-19.txt

Data from JAXA site; although they don't cover SCI explosion timeslot, maybe other similar files are availble on the site:

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  • $\begingroup$ Thanks! But I'm having trouble understanding this. At what distance was Hayabusa-2 from the asteroid when the device detonated? I'm asking for a number, written as text in an answer, can one be found or at least estimated from these images or other date? $\endgroup$
    – uhoh
    Commented Nov 19, 2020 at 11:00
  • $\begingroup$ answer updated with data anlysis and data sources $\endgroup$
    – jumpjack
    Commented Nov 19, 2020 at 14:40
  • $\begingroup$ I appreciate your posting answer, but I think it can be currently summarized as "I tried looking in my archived LIDAR data but it doesn't cover the time frame in question". Have I got that right? $\endgroup$
    – uhoh
    Commented Nov 19, 2020 at 23:51
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    $\begingroup$ yes, no data on my PC, but possibly they are hidden on hayabusa2.jaxa.jp site , they're just hard to find. $\endgroup$
    – jumpjack
    Commented Nov 20, 2020 at 8:45

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