What happens when an Ultra Massive Black Hole cannibalize another?

From this article:

“Bigger black hole masses are in principle possible – for example, a
hole near the maximum mass could merge with another black hole, and
the result would be bigger still. But no light would be produced in
this merger, and the bigger merged black hole could not have a disc of
gas that would make light.”

The theoretical size limit has to do with the ability to form an accretion disk, not the hole itself. While it's probably an extremely rare occurrence due to space expansion, if a 50 billion solar mass black hole was to merge with another 50 billion solar mass black hole, you'd have a 100 billion solar mass black hole. There might be some interesting spiraling in towards each other and some gravitational waves made in the process. It would be cool to study, but in such a scenario, there's no force that would prevent the two black holes from merging into a bigger black hole.

asteroids - How long does it take Dawn to orbit Ceres?

As of March 6, 2015, Dawn has entered orbit around Ceres. But it's really so far only "captured by Ceres' gravitational pull". Then, it was still 61,000 km from Ceres.

It's slowly spiraling down into an orbit that JPL's Dawn Journal calls "RC3", which will be 13,500 km above Ceres. This orbit will last 15 days per revolution.

It will take about 15 days to complete a single orbital revolution at this altitude.

The first news link above indicates that Dawn will reach RC3 on April 23, 2015.

This picture from the Dawn Journal gives us a picture of Dawn's path: it's not completing any revolutions around Ceres yet; it's still approaching the RC3 orbit as of this writing (March 10, 2015).

So as of now, March 10, 2015, there is no orbital period yet. But when RC3 is achieved, it will be a slow orbit -- 15 days.

There are other orbital altitudes planned past RC3.

According to Dawn's schedule, the "survey orbit" will only last 22 days before it spirals in to the "high altitude mapping orbit" in August 2015, although it's unclear exactly when the "survey orbit" will take place. The "low altitude mapping orbit" will be in November 2015. In between orbits, when spiraling down to the next scheduled orbit, Dawn's orbital period will slowly decrease. At the last orbit, Dawn will be left in this orbit after its batteries and hydrazine fuel are exhausted.

---

Update (Oct 23, 2015)

Excerpt from latest news update from the Dawn mission (Sept 30, 2015):

Dawn is currently orbiting Ceres at an altitude of 915 miles (1,470
kilometers), and the spacecraft will image the entire surface of the
dwarf planet up to six times in this phase of the mission. Each
imaging cycle takes 11 days.

Starting in October and continuing into December, Dawn will descend to
its lowest and final orbit, an altitude of 230 miles (375 kilometers).
The spacecraft will continue imaging Ceres and taking other data at
higher resolutions than ever before at this last orbit. It will remain
operational at least through mid-2016.

radio astronomy - Which frequency should be used to communicate with a cube satellite?

I am trying to build a cube satellite using a raspberry Pi. I am trying to figure out which radio frequency could be used to communicate with the cube sat to transfer data like images, videos, different observations etc.

How to gain access to that frequency band? Is it a global organisation or you need you do that in your local divisions?

gravity - A camera and time dilation?

For simplicity, let's say that the black hole is isolated and non-rotating (and uncharged), so that the situation is described by the comparatively simple Schwarzschild spacetime. Let's also suppose that the camera free-falls radially into the black hole.

What is the camera looking at? Suppose it is looking at some stationary object that does something with a known frequency. Your question is basically how at what frequency it will be observed on the video feed emitted by the camera.

Without loss of generality, we can suppose that the camera is looking at us, and that we're shining a laser beam at it: the 'doing something at a known frequency' would be the oscillations in the electromagnetic wave of the laser beam. We can do this because time dilation affects every physical process, so we might as well pick one that is more convenient to think about.

At this point, it is straightforward why the camera feed will not show any time dilation: being equivalent to a reflected laser beam, the gravitational blueshift when going inward will be cancelled by the gravitational redshift going outward.

the sun - Is it possible to move a planet out of its orbit? At least a lighter planet?

yes it is possible by very few different ways.

https://www.uwgb.edu/dutchs/pseudosc/flipaxis.htm

Nothing acting solely from on or within the Earth could change its orbit or seriously alter its rotation.
One way to move an object is to throw mass in the opposite direction, the way jets or rockets do.
If we think really big and imagine blasting a chunk out of the Earth as big as North America and 100 miles thick so that its final speed, after escape, with respect to the Earth is 25,000 miles an hour, we will have expelled only 1/500 of the total mass of the Earth. The Earth would move in the opposite direction 1/500 as fast or 50 miles an hour. The speed of the Earth in its orbit is about 67,000 miles an hour. We will not change the orbit of the Earth very much--if we apply the impulse to speed up the earth in its orbit we would put the Earth into a new orbit with its most distant point about 70,000 miles further from the Sun than now--and the Earth's distance from the Sun varies now by three million miles over the course of a year! Exactly the same arguments apply to changing the orbit of the Earth through the impact of a large asteroid. The largest asteroid, Ceres, about 600 miles in diameter, is only about as massive as our hypothetical chunk of Earth above. Changing the orbit of a planet is a tall order. An impact big enough to have even a tiny effect on the Earth's orbit or rotation would almost certainly destroy all life on Earth as well.

http://usatoday30.usatoday.com/news/science/astro/2001-02-15-orbit.htm

hope this two links provides enough satisfactory explanation to you. if not do say so i will make further search to clear you the answer.

natural satellites - Why does Jupiter have so many moons?

Bigger is better.

Most moons, especially those of gas giants, are not "formed", they are just "captured" (unlike our Moon, which could have been captured, but probably was formed in a much more exciting way).

Jupiter is the most massive planet in the solar system. It stands to reason that it has a larger region of gravitational influence (where its influence outweighs the force due to the other planets and the sun). So, it's easy for it to capture rocky masses.

If you have a look at the contours on the following image (Ignore the Lagrange points marked on it, I only want the contours)

the circular area around the Earth is more or less the area (there's a velocity dependence here which I'm not getting into) in which a moon-like body can form a reasonably stable orbit. The size of the small "well" will increase as the planet moves farther from the sun, and also when the planet is more massive.

Jupiter is both pretty far away from the Sun, and is very massive. This leads to a huge sphere of influence.

The asteroid belt may have something to do with this too, but I doubt it (it's pretty far away). However, if we assume the "half-baked planet formation" theory for the formation of the belt, Jupiter may have leeched off much of the mass that would have otherwise become part of that planet during the formative period.

orbit - Are there ever any simultaneous transits of both Mercury and Venus as seen from the Earth?

EDIT: As it turns out, I'm not the first or even the second person to run calculations like this:

Meeus' work (second link) mentions the 13425 CE event in "Table 1. Simultaneous and near-simultaneous transits of Mercury and Venus, years 1 to 300,000"

Within the limits of DE431 (7 May 13201 BCE to 7 May 17091 CE), there is no
time at which both Mercury and Venus transit the Sun.

The closest we get to this:

• On 16 Sep 13425 CE at 11:57pm UTC, Venus starts transiting the Sun. This
  transit ends the next morning (17 Sep 13425 CE) at 7:30am.




• Less than 9 hours later, at 4:27pm, Mercury starts transiting the
  Sun. This transit ends at 10:26pm.

The program I used to compute this:

https://github.com/barrycarter/bcapps/blob/master/ASTRO/bc-solve-astro-13227.c

The list of transits I computed while solving this:

https://github.com/barrycarter/bcapps/blob/master/ASTRO/mercury-transits.txt.bz2
https://github.com/barrycarter/bcapps/blob/master/ASTRO/venus-transits.txt.bz2

Although I believe this answer is correct, Stellarium does not agree with
me, and HORIZONS doesn't compute positions past 9999 CE, so don't put too
much faith in this answer, since there's no good way to confirm it. I
believe that I'm correct and Stellarium is wrong this far in the future, but
it could be the other way around.

Even if my calculations are correct, the uncertainty in calculating the
relevant positions (Sun, Merucry, Venus, Earth) this far in the future is
high. On their own transit pages, NASA only computes Venus transits from
2000 BCE to 4000 CE, and Mercury transits from 1601 CE to 2300 CE, even
though they could've made the same calculations I made from 13201 BCE to
17091 CE:

This suggests NASA isn't confident enough of Mercury/Venus (and Earth/Sun)
positions to predict that far in the past or future, so my results may be fairly inaccurate.