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Comment by wonderwonder | Hacker News Reader

Good article and very interesting content but stuff like: "The impact caused the asteroid’s orbit around another space rock to shrink — Dimorphos now completes an orbit 33 minutes faster than before the impact"

drives me nuts. How long did the orbit originally take? Without this information it very much obscures the point.

Edit: I looked it up

"Before the impact, the orbital period was 11 hours, 55 minutes. It now is 11 hours, 22 minutes. NASA's previous estimate, announced in October, was an orbital change of 32 minutes. The benchmark for success had been set as a change of at least 1 minute, 13 seconds."

So it reduced it by about 1%. Pretty cool.

Edit2: as pointed out correctly by @statusgraph it's actually close to 5%. I fumbled the math on that.

https://www.voanews.com/a/asteroid-bashing-spacecraft-phenom....

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By almost 5% actually! https://www.google.com/search?q=11hr+22min+%2F+11hr+55min&oq...

You're right! Not sure how I did that.

If you run Linux and have the `units` program installed, you can even do it without reaching for a web page:

  $ units '1-(11hr+22min)/(11hr+55min)'                                                                                                 
          Definition: 0.046153846

Or you can just convert it into minutes...

Just tried this on macOS but it must be a different 'units' command as it won't accept that input. :-(

However I just discovered that if you hit command spacebar and paste that into the spotlight search it gives the right answer! Spotlight has always done simple unit conversions but it didn't used to be able to do calculations with units in them. I wonder when that was introduced...

MacOS's default units(1) is the BSD version.

GNU Units is far more featureful (and useful IMO).

You can install that, as "gunits", using Homebrew.

Most of the time I can get away with pretending MacOS is just sitting on top of Linux, but occasionally the BSD differences pop up and bite me.

GNU/Linux, or as I've recently been taking to calling it, GNU plus Linux

There is no gunits formula on homebrew, the formula is called gnu-units.

The command to use it is `gunits` though.

Given that I invoke GNU Units far more often than I install it, yes, I referred to the command name.

It's trivial to find the actual package name using 'brew list | grep units'.

See note 2: <https://news.ycombinator.com/item?id=31449413>

I suspect a lot more people here will have access to a browser than to a Linux shell with a specific tool installed.

A lot of us always have a shell open on the side (we are dozens!!), so it's nice to learn about such tricks.

the Math is trivial - it's a bit more of a half an hour out of ~12h. That's bit more than 1/24 = ~4.5%.

Another way to quantity the effect on Dimorphos is the speed change. It was about 2 centimeter/second.

So if Dimorphos would be an object orbiting the sun by itself, then its location in its orbit would be shifted by about an Earth radius over 10 years.

(In reality, Dimorphosis is paired with the 100x heavier Didimos, so their orbit around the sun was affected much less.)

After the impact and velocity change, it would no longer follow its original orbit, but instead enter enter a new one, just as it did around Didymos.

Yes, the ellipse of the orbit would change a bit, while still passing through the point where the impact happened. Most of the change would be on the opposite side of the ellipse.

However, I think that for such a small velocity change, it is a reasonable approximation to assume that the ellipsis around the sun stays pretty much the same, and only the phase of the object (i.e. where it is on the ellipsis at which time) is affected.

Yeah, that is a quite substantial change! I was kind of expecting something much smaller - eq. measurable but not really significant.

in astronomical terms, measurable but not really significant, can become significant at cosmological times scales/distances. adjust the trajectory of something at the distance of Jupiter by a "measurable but not really significant" amount, and it could be the difference of a direct impact with earth or a near miss by the time that not really significant amount is accumulated for that distance.

What about the other way? How much did the impact craft weigh? How much delta-v did it have?

It was about the size of a golf cart so I presume it's mass would be in an equally bizarre measurement, like "mass equivalent to 175 gallons of milk".

Size of a golf cart and mass of a golf cart made of milk?

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