
Japan’s Hayabusa2 spacecraft grabs close-up thirty feet above asteroid - tomrod
https://www.cnet.com/news/japans-hayabusa2-spacecraft-snaps-close-up-just-30-feet-above-asteroid/
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_Microft
_The above image comes from a 600-kilogram, refrigerator-sized robot
travelling at about 15 miles per second, around 170 million miles from Earth._
They can be glad that the asteroid is also around 170 million miles from
earth, travelling at 15 miles per second and this even in the same direction.

Relative motion is smaller though: The gravitational force between the probe
(660kg) and asteroid (4.5 * 10^11kg) is just 0.1 N. The probe has therefore to
be circling the asteroid with about 0.25m/s or just 1km/h (0.57 mph for
american readers).

And how else could it be: the asteroid was assumed to be a point mass and the
probe in a circular orbit of 450m around it.

~~~
TylerE
I'm kinda surprised an orbit that low as been stable for a year. Is this an
unusually circular asteroid or is there a fair bit of eccentricity?

~~~
pdonis
The hayabusa spacecraft is not in orbit about the asteroid. It's in orbit
about the Sun, in an orbit very, very close to the orbit the asteroid is in,
so they stay close to each other for a long time (about a year and a half
IIRC).

~~~
_Microft
I couldn't find any information on the trajectory. The sphere of influence in
which the gravitational attraction of the asteroid dominates over that of the
sun has a radius of about 6km though. So as long as the probe moves inside
this area, it has to orbit or spend fuel of the reaction control system (the
ion thrusters are inactive in the vicinity of the asteroid, they'd also have
only enough thrust to allow hovering in a distance of hundreds of meters (in
the tens of milli-newtons)).

~~~
pdonis
_> The sphere of influence in which the gravitational attraction of the
asteroid dominates over that of the sun has a radius of about 6km though._

It depends on what you consider "dominates" to mean. For example, the
corresponding value for the Earth and the Sun is 924,000 km, and the Moon is
only 400,000 km from the Earth, but the Moon's orbit is still always concave
towards the Sun.

~~~
_Microft
I'm not sure what you mean?

If Earth wouldn't dominate the space in which the Moon moves, the Moon
wouldn't be in orbit around Earth at all and the Sun couldn't perturbe this
orbit (as you even stated yourself)?

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pdonis
_> If Earth wouldn't dominate the space in which the Moon moves_

My point is that "dominate" here could mean different things, and the Earth
does not dominate all of them.

Specifically, the fact that the Moon's orbit is always concave to the Sun
means that the "acceleration due to gravity" of the Sun on the Moon is
_larger_ than the "acceleration due to gravity" of the Earth on the Moon. In
other words, the net "acceleration due to gravity" of the Moon is always
towards the Sun, _not_ towards the Earth. So the Earth does _not_ dominate in
this way.

The "sphere of influence" you mention is based on _tidal_ forces: the _tidal_
effect of the Earth on the Moon is larger than the tidal effect of the Sun on
the Moon. So the Earth does dominate in this way.

 _> the Moon wouldn't be in orbit around Earth at all and the Sun couldn't
perturbe this orbit (as you even stated yourself)?_

No, that's not what I said. Whether you think of the Moon as orbiting the
Earth or the Sun depends on how you define "orbit" and what you are trying to
do. If you define "orbit" according to which body the Moon is accelerating
towards, on net, then the Moon is orbiting the Sun, not the Earth (see above);
in this sense, the Moon and the Earth are in two closely matched orbits about
the Sun.

And my point about the Hayabusa spacecraft and the asteroid is that, if you
calculate the "acceleration due to gravity", you find that it's similar to the
Moon's--the spacecraft's net acceleration is towards the Sun, not towards the
asteroid.

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tectonic
Hayabusa2 is such a cool mission! A veritable space Swiss Army Knife with
tantalum bullets, deployable free-floating cameras, four small rovers that
hopped around the asteroid in super low-G, and a 1.5kg kinetic impactor that
blew a crater.

~~~
zaroth
> _and a 1.5kg kinetic impactor_

TFA called it a cannonball, which is really quite amusing.

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lugg
> It took a 180 million mile trip to deliver this image.

Yup, and you break my ability to zoom in or right click and view image on
mobile.

Welcome to the future.

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strikelaserclaw
Wonder what material that greyish rock looking stuff on the surface is.

~~~
ambicapter
Here's what it looks like when shot at with a "bullet"[0].

[0][https://www.cnet.com/videos/japans-hayabusa2-space-probe-
fir...](https://www.cnet.com/videos/japans-hayabusa2-space-probe-fires-bullet-
into-asteroid/)

~~~
mzs
details:
[http://www.hayabusa2.jaxa.jp/en/topics/20190214e_Experiment/](http://www.hayabusa2.jaxa.jp/en/topics/20190214e_Experiment/)

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cowwithbeef
I would guess even operating the RCS system could launch some of those loose
looking rocks to escape velocity. Imagine blowing on a rock that's been in
place for thousands of years and watching it drift into space.

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nerdponx
It's hard to get a sense of how big these features are. 30 feet is damn close,
but it's still hard to say. How big is that big chunk of rock (?) near the
bottom-right, casting a blocky shadow?

~~~
dls2016
Given the distance to the sun and proximity to the asteroid, shouldn’t the
vehicle and its shadow be essentially the same size?

~~~
merpnderp
I'd assume without an atmosphere to diffuse light, the sun is basically a
point light for 95% of the Solar System.

~~~
Sharlin
It's a point light infinitely far away. So light rays are parallel, and
shadows are exactly the size of the object.

~~~
tzakrajs
> infinitely

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Sharlin
Yes, for all intents and purposes.

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laacz
Upon opening cnet, I had to block notifications request, click through cookies
agreement and close video window, which started autoplaying.

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Havoc
Why are the pictures so fuzzy?

~~~
ben7799
Insane Contrast ratio - ultra bright sunlight when the sun gets in the frame
(no atmosphere to diffuse light) plus ultra dark hard shadows as the light
source is millions of miles away, and again no atmosphere to diffuse the
light.

Again ultra dark, so the camera needs to have an incredibly sensitive sensor.

And incredible motion, the spacecraft is whipping by the asteroid quickly.. so
you'd need a fast shutter speed to get sharp detail. Which is made difficult
by the lighting conditions.

No human photographer on the shutter button. Way too far away for that.

Incredibly challenging photographic environment.

~~~
derefr
> no atmosphere to diffuse light

Could the spacecraft—just in theory— _introduce_ an atmosphere? Maybe not
_this_ spacecraft, but if we’re riding around in the starship Enterprise and
want to get better snaps of asteroids, would it be a good idea to just send
out a big pair of glass hemispheres, encapsulate the asteroid with them, pump
them full of air, and then take a picture of the asteroid-capsule system?

Alternately, could you stick something really good at diffusing light—like a
big block of aerogel—between the sun and the asteroid?

~~~
jandrese
This shot was taken from 9m away. A powerful camera flash would have been
sufficient. It fact it seems like a bit of a missed opportunity not to have a
handful of powerful but tiny LEDs on the spacecraft and take shots with
different lighting to try to get the most detail out of this as possible. Yes
you need to be quick, but this can be designed.

~~~
maxxxxx
As far as I know these things are designed for scientific value and not for
producing cool pictures for viewers. I am sure they are getting the best
possible value for their budget with whatever they designed.

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tomrod
> not for producing cool pictures for view

Science funding often needs some one's buyin.

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tbassetto
30 feet = ~9.14 meters

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Sharlin
30 feet = 10 meters at this level of accuracy. 9 if you want to be a pedant.
Significant figures matter.

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vvvv
It bothers me (too much) that we are going to space and still using 'feet' as
a unit of measurement.

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magduf
Who's "we"? This article is about Japan. The Japanese do not ever use "feet"
as a unit of measurement.

It's only Americans who still use feet and other imperial units, which is why
they lost a Mars probe.

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SmellyGeekBoy
You mean the British-built one?

I think us Brits are the worst of the worst when it comes to mixing units.
Fuel is sold in litres but consumption measured in miles per gallon. Milk is
sold in 1 pint or 2/4/6 litre bottles. Road signs to towns are in miles but in
roadworks are in metres. And so on.

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GordonS
To add to the mix, some signs are in _yards_ \- who even knows what one of
those equals these days?!

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merpnderp
Yards are the easiest to translate. A yard is roughly a meter (~9/10's).

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WaitWaitWha
I just want to go 15 miles per second (54000 mph/86904.6 Kph/46925 knt).

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dclowd9901
Does knots even make sense outside of the context of being located on a
sphere?

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asib
According to
[https://en.wikipedia.org/wiki/Nautical_mile](https://en.wikipedia.org/wiki/Nautical_mile),
nautical mile used to be defined as 1/60 of a degree of latitude, but is now
defined as exactly 1852 metres. So feasible to use even in space, although
unusual.

