210,000 kilometers per hour.
Almost 60km/40mi per second. Amazing speed when you think about it. Wonder if there would be any time dilation noticable if the craft ever gets captured and brought back to earth.
Relativistic effects are measurable with accurate enough clocks in earth satellites (or even just taking a clock to a mountain) and correction is necessary in systems with finicky time requirements like GPS.
Incidentally, the relative speed you can napkin math estimate - an object in orbit flying that close to the planet is going to have escape-velocity-ish speed. It's around 60km/sec for Jupiter.
> We would come back about 20 ns older compared to her [the wife who stayed behind].
≥ Or, the other way to look at it (since this is relativity after all), is that she would become 20 ns younger than us upon our return. Note to husbands: this could be a useful gift idea for your wife.
By now I think they managed to measure relativistic effects of clocks that are a few milimeters apart in terms of distance from earths gravitational center.
It's about 0.01% the speed of light, which is definitely fast enough to have a measurable impact. Even ISS astronauts experience a tiny amount of time dilation (on a millisecond level) and they move at much slower speeds.
As a point of reference, our solar system orbits our galaxy at approximately 4 times that speed at 828 megameters per hour, so 210 megameters per second isn't that big a deal in the grand scheme of things...
Kilometer is the largest SI-prefixed variation of meter that is commonly used (among physicists that I know). I would rather suggest that you use scientific notation if you do not want to deal with large numbers of kilometers.
Given how hard it is to convince most Americans to use prefixed metre measurements instead of inches, feet, yards and miles, I think it would be even harder to convince the world that a megametre, picometre, etc is the way to go.
Not that I don't agree (I fully do!), the ease of using multiples of 10 is way easier. But it is very difficult to introduce change to the masses, no matter how sensible it is.
The US is one of the only countries on the planet that still fully stick to the old system. And they're also one of the only countries that spell 'meter' instead of 'metre.' For everyone else in the world, 'meter' is a measuring gauge or tool, and so is everything that rhymes with 'thermometer' except for the American pronunciation of 'kilometre' that too many have adopted up here.
Likewise, adult Canadians still use pounds at the gym and their body weight, and feet/inches for their height. Young people are far more reasonable about measurements these days.
Change is hard, and seems to get really messy when everyone goes in different directions from the start.
As a European born and raised in metric units, all I can do is shake my head and sigh...
... But as we speak about metric measurements I doubt that the US insistence on weird units apply. The average US citizen would presumably be confronted with "mega" and "giga" in their day-to-day lives, and millimeter, micrometer and nanometer are commonly used. Even femtosecond is a common, practical unit that anyone receiving refractive surgery will hear.
Picometer is unpopular because it only makes sense for sub-atomic lengths giving it few practical uses at the current time, not because "pico" is hard to grasp.
Also note that German, Dutch, Danish (my native tongue), Swedish, Norwegian, even Hindi and presumably many others also refer to the unit as "meter", so that particular disagreement is not "the US against the rest of the world".
(All languages have quirks - in Danish, we call a folding ruler in meters an "inch stick", and say the number "53" as "3 and half three's twenties" (skipping the "twenties" part in modern speech). Learn to enjoy the differences rather than hate on them.)
I think you're right, but with a tiny handful of notable exceptions in astrophysics, where gigametres can be a characteristic or upper length scale.
LISA is practically always described as a gigametre-scale observatory.
One example: "LISA, a gigameter-scale space-based gravitational wave observatory, will explore the gravitational wave universe in the band from below 0.1 mHz to above 0.1 Hz." <https://about.cern/news/announcement/physics/cern-colloquium...>, and a trawl through arxiv will show a common association of Gm and LISA.
Assuming LISA is successfully deployed, gigametre may be seen more commonly.
The only other place I've seen Gm scales in common use is in galactic physics, particularly with respect to turbulence and star and planetary nebula formation (stars have ~ Gm diameters; very large stars like VY Canis Majoris have ~ Tm diameters; star systems like ours have gravitationally bound rocky and icy objects at ~ Tm diameters).
1 petameter ~ 0.1 lightyear; 30 petameter ~ 1 parsec, so those are obvious cutoffs for the SI unit of length in astronomy and astrophysics.
Penultimately, truly long lengths are typically measured in cosmological redshift z, which is unitless (being a ratio \frac{\delta\lambda}{\lambda}, or 1+z = \frac{a_{now}}{a_{then}} where a is the scale factor), leading to such things as a comoving volume (1 + z)^3. For z > 0.2 one would be using Gpc lengths, or in SI units Ym; or when working in these sorts of comological volumes in 2023's Britain, trevigintillions of acre-feet.
Finally, cf. the excellent printable table at <https://arxiv.org/abs/1303.5961>, the leftmost column (redshift) and the r_comov column (megaparsecs) being the most directly relevant.
Not sure about the composition of atmosphere at those exact altitudes on Jupiter, but aerodynamic heating was a serious concern during Cassini's close Titan flybys, and that's "just" at ~6.3km/s max. IIRC the closest one was the T-70 at 880km; a lot of work has been done to make sure it won't tumble or overheat. (Titan's low gravity makes its dense atmosphere reach very far, so 880km is really low)
I've read that time moves a tiny bit slower on Earth than it does in space and that it affects satellites. I wonder how much difference there is for something like the Voyagers, which has been in space since the 70's, and us on the ground.
Incidentally, the relative speed you can napkin math estimate - an object in orbit flying that close to the planet is going to have escape-velocity-ish speed. It's around 60km/sec for Jupiter.