The Young Astronomers
From the monthly archives: October 2011
Last month NASA’s Upper Atmosphere Research Satellite (UARS) came crashing down somewhere in the Pacific (no doubt much to the disappointment of the media who seemed obsessed about the chance that it might hit somebody), and as I write this there are reports that the Roentgen Satellite (ROSAT) satellite belonging to DLR, the German space agency, has also fallen out of orbit. Both of these craft had reached fulfilled their objectives and become redundant, and frightening though it may be for us on the ground, re-entry is greatly preferable to leaving the disused craft in orbit.
In 1957 Sputnik became Earth’s first artificial satellite. From that point on we’ve send thousands of craft into orbit between 6,500 (Low Earth) and 42,000 (Geostaitionary) kilometres up.
But like UARS and ROSAT, a great many of those have become redundant and now form part of a blanket of ‘space junk’ which, according to a recent report commissioned by NASA, has now reached critical levels and is posing a very serious threat to functioning satellites as well as the International Space Station.
Space Debris Credit: Wikimedia
This is not a new phenomenon either, in 1995 a US committee on space debris wrote that “The threat that orbital debris poses to international space activities is presently not large, but it may be on the verge of becoming significant. If and when it does, the consequences could be very costly – and extremely difficult to reverse.”
Since all of the craft in orbit were designed to withstand the extreme forces of launch as well as the exotic conditions of space, it may seem odd that a slight bump can do so much damage. Orbiting by its very definition involves travelling so fast that the Earth falls away faster than the satellite falls to Earth (so that it just keeps going round), somewhere in the region of 8km/s. At these immense speeds, even the smallest specks of material can do huge amounts of damage, breaking delicate components or knocking a satellite out of alignment. When inspected after their return to Earth, all of the Space Shuttles showed evidence of tiny impacts which had the potential to damage their ceramic heat-shielding.
Though most craft have some facility for manoeuvring in orbit, the problem is that every movement requires the use of precious fuel, and as the amount of debris increases, more and more movement are required. Obviously there comes a point when the craft no longer has enough can no longer avoid this debris, and for many satellites that point is not far away.
In early 2009 a private American Iridium Communications satellite collided with a defunct Russian Kosmos military satellite at a speed of about 12km/s (42,000 km/h, 26,000mph). Not only did the collision destroy both satellites, but in the process thousands of pieces of debris were created. Though this was the first collision of two intact satellites, it is unlikely to be the last.
Space Fence, a group of very high frequency radar stations operated by the US Space Surveillance Network tracks orbital debris, and similar programs are operated by other agencies across the world. There are estimated to be about half a million objects larger than a centimetre and tens of millions larger than a millimetre, ranging from discarded rocket components to blobs of frozen liquid. Though this tracking data exists, national military agencies that collect the data are rarely keen to share it, and two years after the Iridium/Kosmos collision, the US Air Force is still unsure about how much of its database to make available to other groups.
As the risks posed by this debris have increased, so has the call to do something about it. Part of the problem lies in the fact that each country is currently only allowed to salvage its own objects, since many space projects involve technology that national agencies would prefer not to be public knowledge. In an effort to mitigate the growth of the debris field, space agencies are working to create functionality that allows satellites to de-orbit themselves at the end of their lifespan, either by directly attempting re-entry or by moving into a decaying orbit that will cause the satellite to gradually burn up in the atmosphere. To deal with the existing debris, many proposals have been put forward – catching the debris in nets, collecting it with giant magnets, and blasting it with a ‘laser broom’ – but there is currently no unified clean-up strategy.
At the moment space debris, though prolific, does not pose enough of a threat to significantly affect our current satellites or our plans for the future – but as the 1995 committee said, unless we deal with the problem in the near future then the consequences will not only be very costly but extremely difficult to reverse.
This fantastic image has been created using X-ray data from the Chandra observatory (shown in shades of purple) with infra-red data from the Spitzer observatory (with varying wavelengths shown in red, green and blue).
It shows the star forming region NGC 281 focusing on the open star cluster IC 1590 affectionately called the Pacman Nebula by observers.
NGC 281 Credit: X-ray: NASA/CXC/CfA/S.Wolk; IR: NASA/JPL/CfA/S.Wolk
The nebula is located just 9200 light years from Earth it provides an important nearby laboratory for studies into high mass stars – those stars that are at least eight times the mass of the Sun.
As well as its proximity, its location nearly 1000 light years above the galactic plane from our view allows us to gaze at it almost uninterrupted and achieve precise measurements of what lies within.
Amateur astronomers with a good sized telescope and fortunate enough to have clear, dark skies.
NGC 281
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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