Astronomy Question of the Week

[NoCry]

i still think comet was correct;)

[Da6onet]

i still think comet was correct;)

Ah right I forgot about your answer. So most comets come from the Kuiper Belt and Oort Cloud and are on elliptical orbits. Some comets come from outside the solar system and have hyperbolic trajectories. Both types can be modeled by

v^2 = G*(Msun + mobject)*(2/r - 1/a)

Where G is the universal gravitational constant, r distance of the object from the sun and a is it's orbit semi-major axis. Since the mass of the sun >> than the mass of a comet, this equation can be simplified to

v = 29.8*(2/r - 1/a)^0.5 where r and a are measured in AU

(G = ~6.67E-11 N*(m/kg)^2, Msun = ~1.99E30 kg, 1 AU = ~149.6 million km for those that wish to keep the equation in SI).

Let's assume the comet is from so far away that a --> infinity and that term drops out. Then lets estimate such a comet at perihelion for a couple ranges.

r = 0.9 v = 44 km/s (this is about Hale Bopp's distance/speed)
r = 0.1 v = 133 km/s
r = 0.01 v = 421 km/s (this was ISON's closest approach and it was utterly destroyed)

For comparison, our Voyager spacecraft plod along at 17km/s and LEO objects like the ISS are as low as 8km/s.

The escape velocity from the galaxy is right around 500km/s (from 8000 parsecs out that is), but our Sun isn't massive enough to fling anything that fast without it passing too close and getting destroyed.

Fun fact, based on this equation, and given that Earth is moving at 30km/s at 1 AU, the speed limit of asteroids and comets impacting Earth is ~70km/s.

I digress, these velocities are nice, but light in a vacuum travels at 300,000 km/s!

[NoCry]

I was focusing more on the below extract from an (admittedly Wikipedia) article and reference to superluminal.

Your question was focused on astronomy - hence my desire to turn back to comets;)

 Daily sky motion[edit]

For an earthbound observer, objects in the sky complete one revolution around the Earth in 1 day. Proxima Centauri, which is the nearest star outside the solar system, is about 4 light-years away.[6] On a geostationary view Proxima Centauri has a speed many times greater than c as the rim speed of an object moving in a circle is a product of the radius and angular speed.[6] It is also possible on a geostatic view for objects such as comets to vary their speed from subluminal to superluminal and vice versa simply because the distance from the Earth varies. Comets may have orbits which take them out to more than 1000 AU.[7] The circumference of a circle with a radius of 1000 AU is greater than one light day. In other words, a comet at such a distance is superluminal in a geostatic, and therefore non-inertial, frame.

[Da6onet]

Ah yeah, I should have specified actual information propagation rather apparent effects of us running into a stream photons or in the case of AGN blobs, a trigonometry trick of the eye. I think I'm going to post a math based question next just so Lithium can show you all what's up. <3 engineers.

[Broin]

I was focusing more on the below extract from an (admittedly Wikipedia) article and reference to superluminal....

Objection counselor, I believe the judge has already spoken on this matter. No need to argue an already decided point.   Which I believe was something like   BROIN IS AWESOME! !!!

[NoCry]

[Da6onet]

-Rise in average temperatures caused the Larsen B ice shelf to break up in early 2002. This ice shelf, which is about the size of Rhode Island, is thought to have been part of the Antarctic coast for the past 12,000 years.

Week of 5/25/14

Antarctica has an area of 13 million square kilometers and is covered by an icecap that varies in thickness from 300 meters near the coast to 1800 meters in the interior. Assuming that water and ice have roughly the same density, estimate the amount by which the water level of the world’s oceans would rise if Antarctica’s ice were to melt completely (you must show your maths!).

Bonus: Look up coastal cities like New York (think Manhattan) and compare their altitudes above mean sea level to the amount you calculated!