Here there be Dragons.....

Kessler eh? I didn't realize it had a name, thanks.

I encountered a fascinating fictional account of it in Neal Stephenson's novel Seven Eves.

 

But then we can have Kessel run competitions.

 

Hoped for a sub breaching photo and was not disappointed . I do believe that their centers of mass remain below surface though. Otherwise:

a) a rough landing
b) might reveal secret top speed capabilities.

==
Without doubt, largest ever mobile items beneath seas are there right now. When in photic zone I fancy that blue whales would say something like "what the heck was that?"

 

We're going to have people working debris section sooner or later.

 

Quite far away (55 million light years), in center of galaxy M87, there is a black hole. A very large one. By combining signals from radio telescopes across Earth, this thing has been imaged. Rather than possibly violating copyright law, just search "event horizon telescope" to see the image.

Doesn't look like much, right? A blurry donut? Yet, astronomers are stoked because it is only 70 micro arcseconds across. This is far better resolution than any single telescope (radio or 'vis') could attain. Now, living in times of very accurate clocks, such resolution can be attained. With petabytes of data, hundreds of astronomers, computational power like you can scarcely imagine.

But how fine is this resolution, really? My favorite way to describe it is with another (possibly copyrighted) image:


Put one of those at a distance equal to Moon's from Earth.

That tiny.

Sweet eh?

 

Falcon Heavy with 3 firesticks ascended and those 3 made their landings. No cars were sent up this time.

Israel's lunar lander was still moving 130 meters/second when it got to moon, so that will not be called a landing.

 

That would be a little over 260 mph. Even at 1/6th Earth's G, not terribly survivable. The Chinese still have the most recent Lunar landing.

Bob Wilson

 

Aw c'mon Bob, lunar gravity is not a factor in decelerating Beresheet from 130 to 0.

Mass 180 kilograms. 1/2 mv^2 Maybe a bit of a crater, maybe just aerospace rubble pile.

 

It may not a great stretch to connect exploration and navigation to time. I enjoyed thinking about how time-keeping accuracy has been improved.

Sundials, since about 1500 BC, have uncertainty of about 10,000 parts per million (ppm). This was improved with “equation of time”, about 1760, to 1000 ppm.

On a separate path, pendulum clocks circa 1650 were telling time with uncertainty of 200 ppm. But you could not travel with them, not useful for navigation. Spiral-spring chronometers were most excellently represented by Harrison chronometer in 1761 with 30 ppm. Precision navigation at sea commenced.

Mechanical watches did not (and could not) improve much on that. By 1970, quartz oscillators made consumers watches with inaccuracy of 6 ppm.

Consumer GPS circa 2005, uses atomic clocks in earth orbit, with inaccuracy of 0.006 ppm. So move to parts per billion, 6 ppb.

A stay-at-home atomic clock in 2000 achieved 0.000001 ppb. Quantum clock in 2010 was 10 times more accurate than that. Tired of typing zeroes.

Your GPS is 1000 times more accurate than cheap wristwatch, 5000 x better than Harrison’s fabulous chronometer. Beats beats of pendulum clock by 35,000 times, and a nice sundial by about 170,000 x.

 

Highly accurate clocks are also required for very-long-baseline interferometry, such as was used to produce this week's first-ever direct image of a black hole.

With regular telescopes, imaging to the theoretical limit requires that the entire optical surface be controlled to errors less than a quarter wavelength of the light or radio frequency being used. For VLBI, this means that the master clock errors or instabilities between the different telescope sites must be kept to similarly tiny values.

VLBI has been around for several decades, but keeps getting better and capturing tinier targets as clocks improve.

 

That is just the thing, and I hope it's not only we two here - else we should just go 'private"

For marine navigation, clocks better than 30 ppm would have been limited by functional resolution with star-sighting sextants. It boils down to about 1 kilometer of 'where am I?'

==
Time-aligning planet-scale VLBI signals has only now been done at this fabulous sub-100-microarcseconds scale. Which inverts to 0.000 ... many zeros ppm in time. It was by all accounts, very difficult work. I expect near-future targets will be:

Not this tiny
Still at 'radio' wavelengths, both because dust yonder and earth's atmosphere cause scatter of 'light'
Among those appealing to media attention.

==
Closely time-aligning telescopes at earth's L4 and L5 could reveal a lot more about out yonder. Please realize that this changes exploration from going out to looking out (very very well). Our current going-out speed (~20 km/sec) is ... sadly low.

 

A quick geometry check suggests that VLBI observatories placed at Earth-Moon L4-L5 could produce a resolution improvement of approximately 50X, compared to the current ground based observatory placement. Without even any clock improvements, if the same wavelength is used.

The current EHT-network telescopes operate at a wavelength of 1.3 mm, much shorter than the 21 cm hydrogen line observations being used when I first read about VLBI systems decades ago. (Minimum resolution angle is inversely proportional to wavelength).

Optical use of this interferometry would represent a 2300-fold improvement in resolution, but would require equivalent improvements in clocks and data conversion and recording. It is going to be a while before we (or our grandchildren) reach thisgoal, but there should be plenty of value to find at each incremental step along the way. (Optical interferometry is done on much shorter baselines, where the telescopes are close enough for the collected light to be piped to a central combiner in real time. But resolution is inversely proportional to baseline too.)

I hope we can see a similar picture of our own Milky Way central black hole (Sagittarius A*) soon. While it is much smaller than the M87 black hole pictured this week, it is also much closer. Comparing mass and distance estimates, it seems that its angular size should be about 30% larger than M87's BH.

 

Other things look very tiny in sky - like stars. Here is a clever way to 'size' them, using handy solar-system debris:

Asteroids help scientists to measure the diameters of faraway stars: New technique doubles resolution of angular size measurements -- ScienceDaily

Found angular dimension of 94 micro arcseconds, similar to black hole mentioned above.

 

Here is a sundial:



Where I drew a red circle around shadow of tip. That point moves daily and monthly and could teach a lot if somebody put some markers in the lawn. Science outdoors! US' unappreciated gnomon!

On cloudy days, just take kids to DC museums. Also good.