The Young Astronomers
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NASA’s twin lunar orbiters, GRAIL-A and GAIL-B (Gravity Recovery And Interior Laboratory) have successfully entered lunar orbit.
Artist's impression of the twin GRAIL orbiters in lunar orbit Credit: NASA/JPL
The two probes which are designed to precisely map the moon’s gravity field, entered orbit on New Years Eve and New Years Day, with GRAIL-A arriving just ahead of her sister probe.
The twin orbiters were launched on September the 10th 2011, and are expected to commence an 82 day primary science phase in March 2012. The data from the orbiters should allow scientists to gain a far deeper understanding of the moon’s internal structure by detecting tiny variations in the strength of the moon’s gravity indicating regions of higher and lower density.
The mission is predicted to increase the accuracy of our lunar gravity field maps by somewhere between 100 and 1000 times, exposing features that have currently escaped detection.
The mission is part of NASA’s Discovery program which also includes, Dawn, MESSENGER and Kepler.
Astronomers at the ESO have identified the next meal of our galaxy’s central black hole.
A massive hydrogen gas cloud weighing about three times the mass of the Earth is hurtling towards its doom at a speed of more than 8 million kilometres per hour.
The cloud was discovered as part of the ESO’s twenty year project monitoring the orbit’s of the star’s close to our galaxy’s dark heart to learn more about the black hole’s mass and the structure of the regions close to it.
The central black hole - Sagittarius A* (Sgr A*) – weighs in at 4.31 million 
and is at most 6.25 light hours across – 4.2 billion miles (which is marginally smaller than the diameter of Uranus’ orbit).
The cloud is currently cooler than its surroundings at 280 degrees Celsius (as you can see the conditions in the galactic core are less than inviting), but is already beginning to glow as it is bombarded by the harsh ultraviolet radiation from nearby stars however, its current temperature is balmy compared to what is to come for this poor dusty cloud.
Its orbit is highly elliptical, with one point in particular taking it very close to the black hole, too close. At the start of June 2013 the cloud will have its closest approach to Sgr A*at just 36 light hours – 3.885×1010 km. Whilst this may seem an exceptionally large distance, we must remember that we are dealing with an approach to an object that has 4.3 million times the mass of the sun, nothing gets that close to a supermassive black hole and leaves looking pretty.
The position of the cloud over the last 10 years - notice the large shifts in position Credit: ESO/MPE
As the cloud nears the black hole it will heat up to the million of degrees and will start to emit x-rays, which should be detecable here on Earth and allow astronomers to gain a better understanding of our galaxy’s ravenous heart.
Whilst this will be a fascinating event for astronomers, the poor cloud isn’t going to enjoy the experience. As well as being superheated, it will have the pleasure of being ripped apart – the gravitational stresses produced by the black hole’s gravitational pull are already beginning to shred the cloud, a process that will accelerate as the cloud moves closer.
Artist's impression of the approach of the gas cloud Credit: ESO
Now lets explore for a moment how much force the gas cloud will experience as a result of being so close to Sag A* – yes there is going to be maths involved, and no you don’t need a pen, paper or a calculator.
Before we begin I feel obligated to point out a few assumptions and inaccuracies in the following calculation: -
- I am using not entirely accurate measurements and even if I was to use the exact measurements there would still be observational uncertainties
- I am assuming the gas cloud is a single point mass rather than a diffuse cloud – i.e. that all the clouds mass is contained in a nice neat sphere – clearly it isn’t
- I’m going to be using Newton’s Law of Universal Gravitation and thus am ignoring all relativistic effects
With these out of the way the calculation is quite simple.
We would like to know the gravitational force experience by the cloud at the point of closest approach to the black hole, to do this i’m going to use Newton’s Law of Universal Gravitation: -
This states that:
Every point mass in the universe attracts every other point mass with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.
Mathematically this can be written as: -
Where:
F is the force experienced
m1 is the mass of the larger body
m2 is the mass of the smaller body
and r2 is the distance between the gravitational centres of the two body’s squared
That funny looking symbol in the middle is the ‘is proportional to’ sign – we can change this to the more familiar equals sign by adding a constant to the left hand side of the equation, so we are now dealing with:
Every thing else in the equation remains the same except now we have added G – the Universal Gravitational Constant.
G pops up in a lot of places in astrophysics so if you are going to be reading our mathematics in astrophysics series you are going to become very familiar with it.
As G is a constant it takes a fixed value of 6.67×10-11m3kg-1s-2 – this is a very small value (with the rather odd units there to make the equation mathematically correct).
So now we have everything we need to work out the gravitational force experienced by the cloud all we have to do is substitute in the values – once they have been appropriately converted into the correct units – which (take my word for it) makes our calculation look like this:
Which might look horrendous but it isn’t actually that bad. All we are doing is multiplying and dividing.
So after number crunching we get that:
Now I think we all understand what is going to happen to this poor dusty cloud, though is case this number is too abstract for you lets have a word based description.
Firstly this force is so large the cloud is being torn apart, indeed astronomers have noted that between 2008 and 2011 the cloud has become markedly more disrupted and distorted. As the cloud approaches the black hole it will become increasingly stretched this is due to different parts of the cloud experiencing a differing forces. This comes about because as the force of gravity is affected by distance, with closer objects feeling a stronger force, the front end of the cloud is accelerating away from the back end as a result the whole cloud elongates.
This will be particularly noticeable after the cloud has spun round the black hole with it being drawn into a long streamer over the majority of its orbit.
A simulation of the cloud after its encounter with SagA* - notice it is now very extended Credit: ESO/MPE/Marc Schartmann
I would also strongly advise watching this video simulation of the cloud’s approach and the resulting chaos - http://www.eso.org/public/videos/eso1151c/
You can read more about the cloud here or from the paper here
This post is part of the Young Astronomers’ Databank Project and our series on the Mathematics of Astronomy and Astrophysics
The planets are some of the most well known objects in the universe, after all you are living on one! Our own Solar System has nine eight planets (after poor Pluto was demoted), and there are currently another 555 confirmed to orbit other stars at varying distances from Earth, with more being discovered all the time.
The ancient societies of Earth have long since known about 5 planets apart from our own, the inner Mercury, Venus and Mars and the giants Jupiter and Saturn (Uranus and Neptune were relatively recent additions to the picture as they are much fainter and so harder to detect from Earth). In fact the planets get there name from the Greek – planētēs astēr – which if you aren’t fluent in ancient Greek translates as ‘Wandering Star’, a fitting term as viewed from the Earth they seem to move ‘quickly’ across the background of the stars, which has recently taken on a whole new meaning.
Artist's Impression of One Such Free Planet Credit: NASA/JPL-Caltech
Up to now all the planets detected have been found around stars through a ground-breaking piece of research NASA scientists have detected free floating planets drifting through space alone, without a gravitationally linked star (I should add now that these objects are not by definition planets as they are not found within Solar Systems and are so more correctly termed Planetary Mass Bodies).
Such objects have been predicted for sometime but this is the first time such objects have been directly detected, and as typical in science we move from one problem to another. We have switched from a situation where too few such objects had been detected for current theories to a situation where we have too many than can be produced using current knowledge – typical!
This new data comes from a survey a central region of the Milky Way during 2006 and 2007. It contains 10 detections of Jupiter mass objects and distances between 10,000 and 20,000 light years from Earth. While 10 detections over two years seems like a tiny amount for two years work, it is several times more detections than the team was expecting, and so the models for the numbers of such orphaned worlds have had to be revised, significantly. The research team has calculated that there are at least as many of these objects floating around in space as there are more conventional planets with the number perhaps in excess of the 300 quadrillion stars in the universe, indeed they suggest a figure of twice the number of stars!
So in our own Milky Way alone there may be upwards of several hundred billion of these floating rocks. The survey was only sensitive to planets around the size of Jupiter and Saturn and larger, with any smaller bodies being too difficult to detect though calculated to be more common. The planets are thought to come from gravitationally disrupted solar systems in which one or more of the young planets have been thrown clear of there original home, exiled to wonder the universe alone by the harsh rule of gravity.
The team does point out that it is currently impossible to rule out that some of these detected lone objects may be on extremely long orbits of their parent stars though they also validly point out that such orbits for Jupiter mass planets are exceedingly rare.
The objects were detected using a phenomena known as gravitational microlensing, where the light from a background star is distorted by a massive foreground object in this case the free floating planets. The technique has also been used to detect conventional exoplanets.
You can read more here.
This post has been written by the Young Astronomers Editor Jansen Penido
On September 10th, NASA successfully launched – after many postponements – the Gravity Recovery and Interior Laboratory, or GRAIL: a mission composed of two satellites that will circle the Moon with a single objective: to map the Moon’s gravity to determine what’s its composed of. They’ll do so by flying above the lunar surface in the same orbit and making accurate measurements of the distance between two satellites – named GRAIL-A and GRAIL-B – while they circle the Moon.
However, GRAIL is not the first mission to use this technique. Another mission named Gravity Recovery and Climate Experiment – fondly nicknamed “GRACE” – has essentially been making the kind of scans of our own planet since March 2002. It’s a partnership between NASA and German Space Agency. It also forms part of the Earth System Science Partnership (ESSP), a program that is improving our understanding of the Earth’s geology and climate change.
Artist's impression of the twin satellites in orbit. (Credits: NASA)
Studying Gravity
Gravity is one of the fundamental forces of the universe. Newton’s law of gravitation states that a body attracts another other with strength proportional to its mass – as more massive is an object, more powerful this attraction is. In our everyday life, it is perceived as the force that gives “weight” to objects – pulling them to the ground – and keeps our feet on the floor.
But what many don’t know is that, although the Earth has an “almost-spherical” shape, its gravitational field isn’t uniform. In other words, the attraction doesn’t have the same intensity everywhere on the planet. This is due to the uneven distribution of mass within the Earth, caused by the flattening at the north and south poles, but also through other factors. For example, since ice is less dense than liquid water, the gravity in one area of the world will change subtly as an ice sheet or glacier forms melts. Such uneven distribution of mass on Earth’s surface manifests itself as “lumps” (points where the gravity stronger than normal) in the planet’s gravity field.
Through recording these differences in gravity, GRACE shows how the mass is distributed around our planet, as well as how this distribution changes over time.
A Gravity Anomaly Map from GRACE Credits: University of Texas Center for Space Research and NASA
The map above shows how the Earth’s real gravitational field – based on the data collected by GRACE– differs from the gravity field of a uniform, featureless one. Places colored yellow, orange, or red are areas where the actual gravity field is higher than planet’s average – such as the Himalayan Mountains in Central Asia (top left of the left-hand globe) – while the progressively darker shades of blue indicate places where the gravity field is less – such as the area around Hudson Bay in Canada (top center of right-hand globe).
Even these variations being very small – less than one percent – the Grace’s results are crucial for scientists of many disciplines to better understand several kinds of natural processes occurring in the Earth, such as the thinning of ice sheets, the flow of the magma inside the planet and the variation of ocean bottom pressure.
How the measurements are made?
While the twin probes fly around our planet – 16 times a day and 500 kilometers (311 miles) above the ground – they sense tiny variation in the Earth’s surface mass below and corresponding variations in the Earth’s gravitational pull. Regions of slightly stronger gravity, for example, will affect the first spacecraft, pulling it slightly away from the other one and hence enlarging the distance between them. As the second probe approaches that area, it’ll speed up as well, reducing the distance to the lead one.
To know exactly how this distance changes, both satellites has a microwave raging instrument, that send microwave signals to each other and provide precise measurements of the variation of these signals. This system is so sensitive that is able to detect a variation of one micron (the width of a human hair).
Then, by taking the distance between the probes and combining that data with precise positioning measurements from Global Positioning System (GPS) instruments, scientists are able to construct precise maps of the Earth’s gravity field.
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