10 things you don’t know about black holes

Ah, black holes. The ultimate shiver-inducer of the cosmos, out-jawing sharks, out-ooking spiders, out-scaring… um, something scary. But we’re fascinated by ‘em, have no doubt - even if we don’t understand a whole lot about them.

1) It’s not their mass, it’s their size that makes them so strong.

OK, first, a really quick primer on black holes. Bear with me!

The most common way for a black hole to form is in the core of a massive star. The core runs out of fuel, and collapses. This sets off a shockwave, blowing up outer layers of the star, causing a supernova. So the star’s heart collapses while the rest of it explodes outwards

As the core collapses, its gravity increases. At some point, if the core is massive enough (about 3 times the mass of the Sun), the gravity gets so strong that right at the surface of the collapsing core the escape velocity increases to the speed of light. That means that nothing can escape the gravity of this object, not even light. So it’s black. And since nothing can escape, well, read the quotation at the top of the page.

The region around the black hole itself where the escape velocity equals the speed of light is called the event horizon. Any event that happens inside it is forever invisible.

OK, so now you know what one is, and how they form. Now, the video showing why they have such strong gravity.

Sure, the mass is important, but sometimes it’s the little things that count.





2) They’re not infinitely small.

So OK, they’re small, but how small are they?

I was writing about black holes in my previous job, and we got in a fun discussion over just what we meant by black hole: did we mean the object itself that collapses down to a mathematical point, or the event horizon surrounding it? I said the event horizon, but my boss said it was the object. I decided she had a point (HAHAHAHAHA! A “point”! Man, I kill me), and made sure that when I wrote about the event horizon versus the black hole itself I was making myself clear.

Like I said above, to the collapsing core, its clock keeps ticking, so it sees itself collapsing all the way down to a point, even if the event horizon has some finite size.

What happens to the core? The actual mass that collapsed?

Out here, we’ll never know for sure. We can’t see in, and it sure enough isn’t gonna send any info out. But our math in these situations is pretty good, and we can at least apply them to the collapsing core, even when it’s smaller than the event horizon.

It will continue to collapse, and the gravity increases. Smaller, smaller… and when I was a kid I always read that it collapses all the way down to a geometric dot, an object with no dimensions at all. That really bugged me, as you can imagine… as well it should. Because it’s wrong.

At some point, the collapsing core will be smaller than an atom, smaller than a nucleus, smaller than an electron. It’ll eventually reach a size called the Planck Length, a unit so small that quantum mechanics rules it with an iron fist. A Planck Length is a kind of quantum size limit: if an object gets smaller than this, we literally cannot know much about it with any certainty. The actual physics is complicated, but pretty much when the collapsing core hits this size, even if we could somehow pierce the event horizon, we couldn’t measure its real size. In fact, the term “real size” doesn’t really mean anything at this kind of scale. If the Universe itself prevents you from measuring it, you might as well say the term has no meaning.

And how small is a Planck Length? Teeny tiny: about 10-35 meters. That’s one one-hundred quintillionth the size of a proton.

So if someone says a black hole has zero size, you can be all geeky and technical and say, not really, but meh. Close enough.

3) They’re spheres. And they’re definitely not funnel shaped.

The gravity you feel from an object depends on two things: the object’s mass, and your distance from that object. This means that anyone at a given distance from a massive object - say, a million kilometers - would feel the same force of gravity from it. That distance defines a sphere around an object: anyone on that sphere’s surface would feel the same gravity from the object at the center.

The size of an event horizon of a black hole depends on the gravity, so really the event horizon is a sphere surrounding the black hole. From the outside, if you could figure out how to see the event horizon in the first place, it would look like a pitch black sphere.

Some people think of black holes as being circles, or worse, funnel-shaped. The funnel thing is a misconception from people trying to explain gravity as a bending in space, and they simplify things by collapsing 3D space into 2D; they say the space is like a bed sheet, and objects with mass bend space the same way that a massive object (a bowling ball, say) will warp a bed sheet. But space is not 2D, it’s 3D (even 4D if you include time) and so this explanation can confuse people about the actual shape of a black hole event horizon.

4) Black holes spin!

It’s kind of an odd thought, but black holes can spin. Stars rotate, and when the core collapses the rotation speeds way, way up (the usual analogy is that of an ice skater who brings in his arms, increasing his rotation rate). As the core of the star gets smaller it rotates more rapidly. If it doesn’t quite have enough mass to become a black hole, the matter gets squeezed together to form a neutron star, a ball of neutrons a few kilometers across. We have detected hundreds of these objects, and they tend to spin very rapidly, sometimes hundreds of times a second!

The same is true for a black hole. Even as the matter shrinks down smaller than the event horizon and is lost to the outside Universe forever, the matter is still spinning. It’s not entirely clear what this means if you’re trying to calculate what happens to the matter once it’s inside the event horizon. Does centrifugal force keep it from collapsing all the way down to the Planck length? The math is fiendish, but do-able, and implies that matter falling in will hit matter inside the event horizon trying to fall further but unable to due to rotation, This causes a massive pile up and some pretty spectacular fireworks… that we’ll never see, because its on the other side of infinity.





5) Near a black hole, things get weird

The spin of the black hole throws a monkey in the wrench of the event horizon. Black holes distort the fabric of space itself, and if they spin that distortion itself gets distorted. Space can get wrapped around a black hole - kind of like the fabric of a sheet getting caught up in a rotating drill bit.

This creates a region of space outside the event horizon called the ergosphere. It’s an oblate spheroid, a flattened ball shape, and if you’re outside the event horizon but inside the ergosphere, you’ll find you can’t sit still. Literally. Space is being dragged past you, and carries you along with it. You can easily move in the direction of the rotation of the black hole, but if you try to hover, you can’t. In fact, inside the ergosphere space is moving faster than light! Matter cannot move that fast, but it turns out, according to Einstein, space itself can. So if you want to hover over a black hole, you’d have to move faster than light in the direction opposite the spin. You can’t do that, so you have to move with the spin, fly away, or fall in. Those are your choices.

6) Approaching a black hole can kill you in fun ways. And by fun, I mean gruesome, horrifying, and really really ookie.

Sure, if you get too close, plop! You fall in. But even if you keep your distance you’re still in trouble…



Gravity depends on distance. The farther you are from an object, the weaker its gravity. So if you have a long object near a massive one, the long object will feel a stronger gravitational force on the near end versus a weaker force on the far end! This change in gravity over distance is called the tidal force (which is a bit of a misnomer, it’s not really a force, it’s a differential force, and yes, it’s related to why we have ocean tides on Earth from the Moon).

The thing is, black holes can be small - a BH with a mass of about three times the Sun has an event horizon just a few kilometers across - and that means you can get close to them. And that in turn means that the tidal force you feel from one can get distressingly big.



Let’s say you fall feet first into a stellar-mass BH. It turns out that as you approach, the difference in gravity between your head and your feet can get huge. HUGE. The force can be so strong that your feet get yanked away from your head with hundreds of millions of times the force of Earth’s gravity. You’d be stretched into a long, thin strand and then shredded.

Astronomers call this spaghettification. Ewwww.

So getting near a black hole is dangerous even if you don’t fall in. Evidently, there really is a tide in the affairs of men.



7) Black holes aren’t always dark

The thing is, black holes can kill from a long way off.


Matter falling into a black hole would rarely if ever just fall straight in and disappear. If it has a little bit of sideways motion it’ll go around the black hole. As more matter falls in, all this junk can pile up around the hole. Because of the way rotating objects behave, this matter will create a disk of material whirling madly around the hole, and because the gravity of the hole changes so rapidly with distance, matter close in will be orbiting much faster than stuff farther out. This matter literally rubs together, generating heat through friction. This stuff can get really hot, like millions of degrees hot. Matter that hot glows with intense brightness… which means that near the black hole, this matter can be seriously luminous.

Worse, magnetic and other forces can focus two beams of energy that go plowing out of the poles of the disk. The beams start just outside the black hole, but can be seen for millions or even billions of light years distant.

They’re bright.

In fact, black holes that are eating matter in this way can glow so brightly that they become the brightest continuously-emitting objects in the Universe! We call these active black holes.

And as if black holes aren’t dangerous enough, the matter gets so hot right before it makes the final plunge that it can furiously emit X-rays, high-energy forms of light (and the beams can emit even higher energy light than that). So even if you park your spaceship well outside the event horizon of a black hole, if something else falls in and gets shredded, you get rewarded by being fried by the equivalent of a gazillion dental exams.

Black holes are dangerous. Best to stay away from them.



8) Black holes aren’t always dangerous.



Having said that, let me ask you a question: if I were to take the Sun and replace it with Folgers crystals a black hole of the exact same mass, what would happen? Would the Earth fall in, be flung away, or just orbit like it always does?

Most people think the Earth would fall in, sucked inexorably down by the black hole’s powerful gravity. But remember, the gravity you feel from an object depends on the mass of the object and your distance from it. I said the black hole has the same mass as the Sun, remember? And the Earth’s distance hasn’t changed. So the gravity we’d feel from here, 150 million kilometers away, would be exactly the same! So the Earth would orbit the solar black hole just as nicely as it orbits the Sun now.

Of course, we’d freeze to death. You can’t have everything.




9) Black holes can get big.

Q: What happens if two stellar-mass black holes collide?

A: You get one bigger black hole.

You can extrapolate from there. Black holes can eat other objects, including other black holes, so they can grow. We think that early on in the Universe, when galaxies were just forming, matter collecting in the center of the nascent galaxy can collapse to form a very massive black hole. As more matter falls in, the hole greedily consumes it, and grows. Eventually you get a supermassive black hole, one with millions or even billions of times the mass of the Sun.

However, remember that as matter falls in it can get hot. It can be so hot that the pressure from light itself can blow off material that’s farther out, a bit like the solar wind but on a much grander scale. The strength of the wind depends on many things, including the mass of the black hole; the heftier the hole, the windier the, uh, wind. This wind prevents more matter from falling in, so it acts like a cutoff valve for the ever-increasingly girthy hole.

Not only that, but over time the gas and dust around the black hole (well, pretty far out, but still near the center of the galaxy) gets turned into stars. Gas can fall into a black hole more easily than stars (if gas clouds collide head-on their motion relative to the black hole can stop, allowing them to fall in; stars are too small and too far apart for this to happen). So eventually the black hole stops consuming matter because nothing more is falling into it. It stops growing, the galaxy becomes stable, and everyone is happy.


In fact, when we look into the Universe today, we see that pretty much every large galaxy has a supermassive black hole in its heart. Even the Milky Way has a black hole at its core with a mass of four millions times that of the Sun. Before you start running around in circles and screaming, remember this: 1) it’s a long way off, 26,000 light years (260 quadrillion kilometers), 2) its mass is still very small compared to the 200 billion solar masses of our galaxy, and therefore 3) it can’t really harm us. Unless it starts actively feeding. Which it isn’t. But it might start sometime, if something falls into it. Though we don’t know of anything that can fall into it soon. But we might miss cold gas.



Anyway, remember this as well: even though black holes can cause death and destruction on a major scale, they also help galaxies themselves form! So we owe our existence to them.


10) Black holes can be low density.

Of all the weirdnesses about black holes, this one is the weirdest to me.

As you might expect, the event horizon of a black hole gets bigger as the mass gets bigger. That’s because if you add mass, the gravity gets stronger, which means the event horizon will grow.

If you do the math carefully, you find that the event horizon grows linearly with the mass. In other words, if you double the black hole’s mass, the event horizon radius doubles as well.

That’s weird! Why?

The volume of a sphere depends on the cube of the radius (think way back to high school: volume = 4/3 x ? x radius3). Double the radius, and the volume goes up by 2 x 2 x 2 = 8 times. Make the radius of a sphere 10 times bigger and the volume goes up by a factor of 10 x 10 x 10 = 1000.

So volume goes up really quickly as you increase the size of a sphere.

Now imagine you have two spheres of clay that are the same size. Lump them together. Is the resulting sphere twice as big?

No! You’ve doubled the mass, but the radius only increases a little bit. Because volume goes as radius cubed, to double the radius of your final clay ball, you’d need to lump together eight of them.

But that’s different than a black hole. Double the mass, double the size of the event horizon. That has an odd implication…

Density is how much mass is packed into a given volume. Keep the size the same and add mass, and the density goes up. Increase the volume, but keep the mass the same, and the density goes down. Got it?

So now let’s look at the average density of matter inside the event horizon of the black hole. If I take two identical black holes and collide them, the event horizon size doubles, and the mass doubles too. But volume has gone up by eight times! So the density actually decreases, and is 1/4 what I started with (twice the mass and eight times the volume gives you 1/4 the density). Keep doing that, and the density decreases.

A regular black hole - that is, one with three times the Sun’s mass - with have an event horizon radius of about 9 km. That means it has a huge density, about two quadrillion grams per cubic cm (2 x 1015). But double the mass, and the density drops by a factor of four. Put in 10 times the mass and the density drops by a factor of 100. A billion solar mass black hole (big, but we see them this big in galaxy centers) would drop that density by a factor of 1 x 1018. That would give it a density of roughly 1/1000 of a gram per cc… and that’s the density of air!

A billion solar mass black hole would have an event horizon 3 billion km in radius - roughly the distance of Neptune to the Sun.

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9 Global Devastation Hotspots, Before and After

The world is changing. It has gone beyond the perceptions of the skeptics who say it’s “a cycle” and demonstrated beyond any reasonable doubt that humans are having a dramatic and negative impact on the world.

Here are the images that portray the greatest human impact on the environment:



Almeria, Spain

“Agriculture Development”

This pair of satellite images shows the impact of massive and rapid agricultural development in Almeria Province along Spain’s southern coast.

In the earlier image, the landscape reflects rather typical rural agricultural land use. In the 2000 image, much of the same region-an area covering roughly 20 000 hectares (49 421 acres) - has been converted to intensive greenhouse agriculture for the mass production of market produce.

Greenhouse-dominated land appears as whitish gray patches.

In order to address increasingly complex water needs throughout Spain, the government adopted the Spanish National Hydrological Plan (SNHP) in 2001.

Initially, this water redistribution plan involved the construction of 118 dams and 22 water transfer projects that would move water from parts of the country where it was relatively abundant to more arid regions.

In 2004, the Spanish government announced it would begin exploring more environmentally friendly water-saving technologies, such as wastewater recycling and seawater desalinization.




Beira Fire Scars, Mozambique

“Arson”

During Mozambique’s dry season—May to October—fires leave burn scars on the landscape. Over a third of the country is affected by fire each year. NASA’s Earth Observatory recorded an especially large number of fires in August 2006.

The widespread nature of the fires suggests that they may have been intentionally set. Population growth in Mozambique has drastically intensified the need for agricultural land as well as for forestry and wildlife products, thus putting increased pressure on limited resources. Fires have become a primary means of clearing land for cultivation.

The 21 May 2006 satellite image was acquired at the beginning of the 2006 dry season, before many fires had left their mark.

The 9 August 2006 image shows the same area roughly 2.5 months later. Pink, dark red, and black fire scars cover much of the landscape.

Many plants in Mozambique are adapted to periodic fire. However, the increasing frequency of fires affects the natural regeneration of vegetation and is believed to be reducing species diversity in Mozambique’s forests.

Frequent fires can also increase soil erosion and negatively impact hydrology.




Aral Sea, Kazakhstan, Uzbekistan

“Diverting Rivers for Cotton Production”

The name “Aral Sea” comes from the word “aral” meaning island. The sea’s name reflects the fact that it is a vast basin that lies as an island among waterless deserts.

The Aral Sea was once the world’s fourth largest inland sea. Its problems began in the 1960s and 1970s with the diversion of the main rivers that feed it to provide for cotton cultivation in arid Soviet Central Asia.

The surface of the Aral Sea once measured 66 100 km² (25 521 square miles).

By 1987, about 60 per cent of the Aral Sea’s volume had been lost, its depth had declined by 14 m (45 feet), and its salt concentration had doubled, killing the commercial fishing trade.

Wind storms became toxic, carrying fine grains of clay and salts deposited on exposed sea floor.

“Re-engineering will leave the South Aral Sea completely dry, perhaps within 15 years.”

Life expectancies in the districts near the sea are significantly lower than in the surrounding areas.

The sea is now a quarter of the size it was 50 years ago and has broken into two parts, the North Aral Sea and the South Aral Sea.

Re-engineering along the Syr Darya River delta in the north will retain water in the North Aral Sea, thereby drying the South Aral Sea completely, perhaps within 15 years.




Santa Cruz, Bolivia

“Where People Go, Nature Dies”

Santa Cruz is situated in Bolivia’s rich, fertile lowlands, a region highly suitable for agriculture.

In the 1975 satellite image, the region’s forested landscape appears as a dense, essentially unbroken expanse of deep green that extends to the Rio Grande (Guapay) River. It was beautiful from the sky and on the ground.

By 1986 roads had been built that linked the region to other population centers.

As a result, large numbers of people migrated to the area.

A large agricultural development effort (the Tierras Baja project) led to widespread deforestation as forests were clear-cut and converted to pastures and cropland.

By 2003, almost the entire region had been converted to agricultural lands, including the area east of La Esperanza across the river.

In the area north and west of Los Cafes (upper left), notice the grid of squares on the landscape, each with an internal star-shaped pattern.

At the center of each square is a small community.



Nangbeto Reservoir, Togo

“First Law: Do No Harm”

A feasibility study in the 1960s identified the Nangbéto region as the best location for hydroelectric power development in Togo.

The site - 160 km upstream from the coast – is the only place where a dam of sufficient volume to regulate the flow of the Mono River was possible.

As demand for electricity grew, the decision was made in the 1980s to proceed with the Nangbéto Hydroelectric Dam.

Satellite images from 1986 and 2001 show the region before and after the dam’s construction.

The completed dam created a reservoir with a surface area of approximately 180 km2 and a volume of 1,465 million m3.

In addition to generating electricity for domestic and commercial use, the dam also provides water for agricultural irrigation and is a source of commercial fishing and tourism. However, these benefits have been offset by environmental costs.

Construction of the dam, creation of the reservoir, and installation of transmission lines resulted in the loss of nearly 150 km2 of savannahs and gallery forests that provided habitat for rare local fauna.

The reservoir submerged 1,285 households and 5,500 hectares of agricultural land. Loss of the natural vegetation in the region has altered the climate enough to have had a negative impact on nearly 350 hectares of banana plantations. The creation of the reservoir has also increased the population of two species of aquatic snails that serve as intermediate hosts of the parasite that causes the disease bilharzia.




Shume Magamba, United Republic of Tanzania

“TIMBER!”

Shume Magamba forest reserve is located in the West Usambara Mountains. It is one of the thirteen blocks forming the Eastern Arc Mountains of Tanzania and Kenya, along the Albertine Rift.

It is comprised of 12 000 ha of moist montane forest, which is a gazetted forest reserve, with 2 500 ha under exotic plantation.

The Eastern Arc is one of the most biologically rich regions in the world, with a large number of endemic animal and plant species. It is regarded as one of the world’s top 25 global biodiversity hotspots and is increasingly being managed for biodiversity conservation.

The forest is threatened by timber harvesting (pit sawing) and agricultural encroachment.

Part of the Shume-Magamba Forest on the West Usambara Mountains was degazetted from a Forest Reserve soon after independence in 1961 and was then converted to agriculture by land-hungry residents.

Other major threats to the forests in the West Usambaras include fire spreading from surrounding farmlands and gold mining. In the former case, the enhanced burning regime is believed to have been the main cause of the replacement of Afromontane forests with grassland and scrub-grassland across large areas.

The sharp boundaries at the edges of the forest indicate areas where forest has been converted to farmland. The 2005 image shows these boundaries pushing further into the forest in several places. The high resolution image (see photos panel below) shows detail of the area highlighted by the yellow box in the above images. In addition to crops, areas of forest plantation are displacing natural forest. Areas of trees with parallel lines cut through them are generally tree farms.

Tanzania had the sixth largest annual net loss in forest area between 2000 and 2005 in the world of about 412 000 ha/yr; second largest in Africa after Zambia. In total, between 1990 and 2005, United Republic of Tanzania lost 14.9 per cent of its forest cover. Currently, 39.9 per cent of the country is forested. Apparently, a number of mountains have lost at least 80 per cent of their original forest cover, including Taita, Ukaguru, Mahenge, and West Usambara.

The energy economy in Tanzania is largely focused on collecting, distributing, and consuming wood fuels (wood and charcoal) to satisfy household demands for cooking. As much as 90 per cent of all primary energy consumed in Tanzania is biomass based.





Lake Hamoun, Afghanistan and Iran

“Competing for Water”

Iran’s Lake Hamoun is fed primarily by water catchments in neighboring Afghanistan.

In 1976, when rivers in Afghanistan were flowing regularly, the lake’s water level was relatively high.

Between 1999 and 2001, however, the lake all but dried up and disappeared, as can be seen in the 2001 satellite image above.

The “dry phase” of Lake Hamoun is a striking example of how competition for scarce water resources can transform a landscape.

When droughts occur in Afghanistan, or when water in watersheds that support Lake Hamoun are drawn down for other natural or human-induced reasons, the end result is a dry lakebed in Iran.

In addition, when the lake is dry, seasonal winds blow fine sands off the exposed lakebed.

The sand is swirled into huge dunes that may cover a hundred or more fishing villages along the former lakeshore.

Wildlife around the lake is negatively impacted and fisheries are brought to a halt. Changes in water policies and substantial rains in the region saw a return of much of the water in Lake Hamoun by 2003 .



Huang He Delta, China

“Sediment Building Up”

Sometimes, it isn’t what humans do, but rather what we don’t do that have a dramatic effect on the environment. As you can see by the images, there is a large protrusion of land that is sticking out that wasn’t there before.

The Huang He (Yellow River) is the muddiest river on Earth and is China’s second longest river, running 5 475 km (3 395 miles) from eastern Tibet to the Bohai Sea.

The Huang He’s yellow color is caused by its tremendous load of sediment, composed primarily of mica, quartz, and feldspar particles.

The sediment enters the water as the river carves its way through the highly erodable loess plateau in north-central China (Loessial soil is called huang tu, or “yellow earth,” in Chinese).

Centuries of sediment deposition and dike building along the river’s course has caused it to flow above the surrounding farmland in some places, making flooding a critically dangerous problem.

Where the Huang He flows into the ocean, sediments are continuously deposited in the river delta, where they gradually build up over time.

Between 1979 and 2000 - as these satellite images show - the delta of the Huang He river expanded dramatically. Several hundred square kilometres of newly formed land were added to China’s coast during this period.



Sakhalin, Russian Federation

“People Increase Risk of Fire”

Mixed deciduous and evergreen needle-leaf trees dominate the boreal forests of Sakhalin Island, just off the eastern coast of Russia.

The tremendous natural reserves of the boreal forests serve as “carbon sinks” that help to regulate global climate. They are among the most important natural “CO2 blockers” in the world today. Boreal forests are also home to a unique collection of plants and animals, including rare and endangered species such as the Amur Tiger.

“Roughly 300 intensely hot fires burned an area nearly the size of Luxembourg.”

Fire is a natural and often vital component in maintaining the health of boreal forests. But since the 1950s, the frequency of fires has increased on Sakhalin Island as its forests have been subjected to rapid exploitation and disturbance in the acquisition of lumber, oil, coal, and peat.

As people moved into the region in greater numbers, the risk of fires started by trains, cars, trash fires, and wood stoves increased greatly. These satellite images show the impact of forest fires on Sakhalin Island.

In 1998, roughly 300 intensely hot fires burned an area nearly the size of Luxembourg. Three people died and nearly 600 were made homeless by a very rapidly moving crown fire that consumed the town of Gorki within a few hours. The 1999 image very clearly shows the extent of the fire damage to the island’s forests near the end of that year.

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Ten Things You Don’t Know About Hubble

On April 24, 1990, the Space Shuttle Discovery roared into space, carrying on board a revolution: The Hubble Space Telescope. It was the largest and most sensitive optical-light telescope ever launched into space, and while it suffered initially from a focusing problem, it would soon return some of the most amazing and beautiful astronomical images anyone had ever seen.

Hubble was designed to be periodically upgraded, and even as I write this, astronauts are in the Space Shuttle Atlantis installing two new cameras, fixing two others, and replacing a whole slew of Hubble's parts. This is the last planned mission, ever, to service the venerable 'scope, so what better time to talk about it?

Plus, it's arguably the world's most famous telescope (it's probably the only one people know by name), and yet I suspect that there are lots of things about it that might surprise you. So I present to you Ten Things You Don't Know About the Hubble Space Telescope.



Hubble has observed every planet in the solar system but one: Mercury.

So Hubble has observed the Sun, but it did so literally bass-ackwards. That was to protect its mirror; raw UV from the Sun can photochemically damage sensitive parts inside the 'scope, and of course can heat them up to dangerous levels. Also, as I pointed out before, some of the cameras would in fact be damaged by direct sunlight.

Because of that, Hubble is not allowed to point anywhere near the Sun, just to make sure no stray light seeps in. This "solar avoidance zone" is a circle 50 degrees in radius around our star. Anything closer than that is forbidden.

This directive has been broken by Hubble precisely once: to observe Venus, which gets about 45 degrees from the Sun at maximum. These observations were made using WFPC2 (shown in the image above; it was taken in the near-UV to see structure in the Venusian clouds) and the Goddard High Resolution Spectrograph. Astronomers were looking for sulfur dioxide in the atmosphere of Venus, a chemical which had been detected by an earlier probe and might be emitted by volcanism on the planet. All kinds of overrides had to be sent to the telescope to allow these observations, and it was so difficult that it hasn't been and probably won't ever be repeated.

But Mercury never gets even that far from the Sun; at most it is a mere 28 degrees from the Sun, far to close to ever be seen by Hubble. But that's OK: we have the MESSENGER spacecraft. It's zipped past Mercury twice already, and will fly by Mercury one more time in the coming months before falling into orbit around the innermost planet in 2011, where it will map the planet with far higher detail than Hubble ever could.

Not everything it sees is on purpose.

This one's a bit personal, so allow me to expound a bit here.

Hubble has several cameras on board. They sit in the very bottom of Hubble, in the wider portion below the mirror (unlike a normal telescope, the mirror for Hubble is located a third of the way up from the aft end). Each sees a slightly different region of the sky, separated by a few arcminutes (the Moon is 30 arcminutes across for comparison). So if one camera is being used to look at, say, the heart of the Andromeda galaxy, then the others are looking near the galaxy's center but not right at it.

Enter the Parallels Program. When a new solid state recorder was placed on board in 1997, it greatly enhanced Hubble's capability to record data (which was done using tape drives before then). The pipeline was fat enough to record data from three cameras at the same time, so when one was observing as the primary camera, the other two could take data as well.

Sometimes while observing some primary target Hubble would be rotated to point the other cameras at something interesting (like was done with the lunar observations I mentioned earlier), but sometimes they were simply allowed to record whatever the heck they saw. This procedure was called the Parallels Program, because the other cameras were used in parallel with the primary one.

In October of 1997, Hubble was pointed at the Large Magellanic Cloud, a small galaxy that orbits the Milky Way. WFPC2 was the primary instrument, but the camera I worked on, the Space Telescope Imaging Spectrograph, or STIS, was being used as a parallel instrument. That happened a lot, and at my office the first thing I would do every morning was go through the previous day's parallels using STIS and see if there was anything interesting in them.

Yes, part of my job was to look at Hubble images of regions of space no one had ever seen before and check them out. And yes, it was pretty damn cool.

The majority of the time there wasn't much to see: faint fuzzy galaxies, or a wisp of nebulosity. Sometimes the primary camera would observe a nearby galaxy many times over the course of months, and after a while just by glancing at the STIS image I could tell you what galaxy it was from the brightness and density of stars. Not a terribly marketable skill, but still. Cool.

Anyway, one day we got that LMC observation -- the one shown above -- and I noticed the fuzzy circle at the top. I knew right away it was a small planetary nebula, a blast of gas emitted from a dying star. You can see it in the image, and it's zoomed at the bottom left. To my disappointment it had been discovered before, so this wasn't new and I couldn't name it. But we did get good spectra, which allowed me to take some basic diagnostics of the nebula that hadn't been done before.

I was able to publish my results in a paper, which also was nice. My work on STIS was awesomely fun sometimes, but I rarely got to publish anything; my name was always way down the list of people who contributed to the work. So this was a nice perq.

The Parallels Program still continues. I don't know what it's found since I left the project. Maybe someday I'll poke around the archives and find out.

Introduction

On April 24, 1990, the Space Shuttle Discovery roared into space, carrying on board a revolution: The Hubble Space Telescope. It was the largest and most sensitive optical-light telescope ever launched into space, and while it suffered initially from a focusing problem, it would soon return some of the most amazing and beautiful astronomical images anyone had ever seen.

Hubble was designed to be periodically upgraded, and even as I write this, astronauts are in the Space Shuttle Atlantis installing two new cameras, fixing two others, and replacing a whole slew of Hubble's parts. This is the last planned mission, ever, to service the venerable 'scope, so what better time to talk about it?

Plus, it's arguably the world's most famous telescope (it's probably the only one people know by name), and yet I suspect that there are lots of things about it that might surprise you. So I present to you Ten Things You Don't Know About the Hubble Space Telescope.



Hubble took the deepest visible light image yet made.

In 2003, an astronomer (and friend with whom I worked on a Hubble project) named Tom Brown pointed Hubble at the outer fringes of the Andromeda Galaxy, a nearby large spiral like our own Milky Way. Using the Advanced Camera for Surveys, he commanded the space telescope to basically sit and stare at one spot for a total of three and a half days. His goal to was to be able to get good data on very faint stars in Andromeda, to characterize the way stars form in the galaxy.

He certainly was able to do that (and found many stars younger than expected; in Andromeda's halo the stars were several billion years younger than in our own halo), but what he also got was the deepest optical image of the Universe ever taken. Stars down to 31st magnitude can be seen in the data -- those are stars one ten-billionth as bright as what you can see with your unaided eye!

The image here shows different regions in that deep image. You can see faint background galaxies, stars in both Andromeda and the Milky Way, a densely-packed globular cluster, and much more.

The Moon is not too bright to see with Hubble.

A lot of people claim that some objects are simply too bright to observe with Hubble. For some limited cases this is true -- there's a camera on board Hubble sensitive to ultraviolet light, and at a 2500 Volt potential too many UV photons can fry the instrument.

But that's not true for most of Hubble's cameras. Actually, some of the brightest objects in the sky have been observed... including the Moon! The image shown here is of Copernicus, a 90 kilometer wide impact crater on the Moon. It wasn't actually Hubble's primary target; another camera (the Space Telescope Imaging Spectrograph, or STIS, a camera I worked on for many years) was observing reflected sunlight off the Moon's limb, and Hubble was rotated so that Wide Field/Planetary Camera 2 (WFPC2) would be able to take snapshots of the crater.

So while the Moon is not too bright to observe with Hubble, it is moving too rapidly across the sky for the 'scope to track it. So the observations were made in what's called "ambush mode": Hubble is pointed at a spot in the sky where the Moon is going to be, and when the right moment arrives the images are taken. It's a very difficult operation, which is one the reasons why there are so few observations of our nearest neighbor.

Back in 1999 I took part in a set of lunar observations using Hubble; we were hoping to get spectra of water ice splashed up from the Moon's south pole when the Lunar Prospector probe impacted there at high speed. Unfortunately, the spectra were screwed up; the pointing was off by a bit and we didn't see anything (it turns out no one saw anything using any telescope, so we didn't really miss much). Although it failed, that observation run was incredibly exciting, some of the most fun I've had using Hubble.

One problem with using digital detectors is knowing exactly what you're seeing. If a star looks brighter than another, is the star really brighter, or is the electronic chip just a little too sensitive right there? You have to calibrate the chip to know exactly what it's doing. There are several steps in that process, but one involves using a "flat field", observing a region of the sky that is perfectly evenly illuminated. That way, if one pixel or another is too sensitive, you can see it in the observation.

With Hubble, though, every patch of sky has some object in it, which would screw up the flat field. Some telescopes have internal illumination; little LEDs or some other method, but using them is notoriously difficult to get an evenly illuminated field. So what can you do when using Hubble?

One method is to observe the Earth! As Hubble orbits at 8 km/sec, the out-of-focus Earth screams by. If you observe for a while, objects will actually leave streaks in the image, and these can be treated mathematically to produce a flat field. The image shown here is just such a "streak flat". That's a Hubble observation of our home planet, with objects flying past. It's hard to say what they are, exactly. It depends on where Hubble was when the image was taken, and where it was pointed. They might be trees, hills, valleys, mountains, or even houses!

But don't worry, it can't see people. If the Moon is too fast to track, the Earth is certainly out of the question. But y'know, the company that made Hubble's mirror had an awful lot of those same sized mirrors lying around, and there are no other astronomical telescopes (you know, telescopes that point away from the Earth) with that same mirror. So what could those mirrors have been for?

Hmmm.

It observes the Earth... quite often!

If the Moon is not too bright to see, what about the Earth? On average, it's much more reflective and therefore much brighter. Well, it turns out Hubble not only has observed the Earth... it's done it thousands of times!

One problem with using digital detectors is knowing exactly what you're seeing. If a star looks brighter than another, is the star really brighter, or is the electronic chip just a little too sensitive right there? You have to calibrate the chip to know exactly what it's doing. There are several steps in that process, but one involves using a "flat field", observing a region of the sky that is perfectly evenly illuminated. That way, if one pixel or another is too sensitive, you can see it in the observation.

With Hubble, though, every patch of sky has some object in it, which would screw up the flat field. Some telescopes have internal illumination; little LEDs or some other method, but using them is notoriously difficult to get an evenly illuminated field. So what can you do when using Hubble?

One method is to observe the Earth! As Hubble orbits at 8 km/sec, the out-of-focus Earth screams by. If you observe for a while, objects will actually leave streaks in the image, and these can be treated mathematically to produce a flat field. The image shown here is just such a "streak flat". That's a Hubble observation of our home planet, with objects flying past. It's hard to say what they are, exactly. It depends on where Hubble was when the image was taken, and where it was pointed. They might be trees, hills, valleys, mountains, or even houses!

But don't worry, it can't see people. If the Moon is too fast to track, the Earth is certainly out of the question. But y'know, the company that made Hubble's mirror had an awful lot of those same sized mirrors lying around, and there are no other astronomical telescopes (you know, telescopes that point away from the Earth) with that same mirror. So what could those mirrors have been for?


Hubble once observed... wait for it... wait for it... THE SUN.

Oh, I got you with that one, didn't I? Admit it: you had no idea that Hubble actually and for real once observed the Sun, on purpose. I didn't know about it for a long time, until my friend and fellow astronomer Glenn Schneider clued me in. Glenn is a surprising guy in many ways -- he chases solar eclipses all over the planet, for example -- but this one was a doozy.

He has the whole story on his website. The short version is that some kinds of electronic detectors get extra electrons trapped in them, kinda like plaque in your arteries. One way to flush out these extra electrons is to flood the detector with ultraviolet light. The chips used in the original Wide Field/Planetary Camera launched with Hubble suffered from this, so they needed that UV flood. And it turns out there's a fairly bright source of UV light in space...

Maybe you see where this is going.

So the engineers rigged WFPC with a little mirror that stuck outside the camera. This part of the camera was actually mounted flush against Hubble's side, so the mirror stuck out from the 'scope like a wee periscope (there's a picture on Glenn's site that'll help). It faced backwards, towards Hubble's aft end. The great observatory was then pointed in the opposite direction of the Sun so the rear-view mirror was facing the Sun, and the sunlight was channeled right into WFPC.

The result is the image above: a bona-fide 100% actual image (well, mosaic) of the Sun taken by the Hubble Space Telescope.

How freaking cool is that?

HHubble cannot see the Apollo artifacts on the Moon.

This question is sent to me roughly once a month, and sometimes even more often: why don't we shut up the people who think the Apollo Moon landings were faked by pointing Hubble at the Moon and taking pictures of the Apollo sites?

Well, one reason is that, duh, NASA and astronomers have better things to do than try to prove something blaringly obvious to people who would just claim the resulting images are faked anyway.

But also, Hubble cannot see the artifacts on the Moon! They're way too small.

This surprises a lot of folks, since they're used to seeing razor-sharp images of nebulae and galaxies. However, remember that while those objects may be far away, they are also very, very big. Light years across, maybe thousands of light years across. The remains of the lunar landers are only 4 meters across. That's a tad smaller.

Sure, you say, but the Moon's a lot closer, right? Yeah, it is, but it turns out it's not close enough.

You can calculate how small an object a telescope can resolve (that is, see as more than just a one pixel wide dot) using really basic algebra. It depends mostly on the telescope's mirror size. When you do this for Hubble, you get an angular measure of about 0.1 arcseconds, a tiny amount to be sure. The Moon is 1800 arcseconds across, so 0.1 arcseconds corresponds to about 200 meters on the Moon! In other words, something has to be bigger than a football stadium on the Moon before Hubble can see it.

It's surprising, I know, but that's how the math works out. The lunar lander is about 0.002 arcseconds in size, well beyond the capabilities of any normal telescope (go to that link above for more info on ways this still might work).

So really, the only -- and best -- way to see the Apollo artifacts is to go back to the Moon. Of course, the Moon hoax believers will still deny it's real. Their refusal to see reality is cosmic in its proportions.



Even today, 19 years after Hubble's launch, it's not all that uncommon to hear a newscaster refer to "Hubble's lens". I once heard it used by an announcer on a science show produced by the Space Telescope Science Institute, the agency that runs Hubble!

The thing is, Hubble doesn't use a lens. It has a mirror.

Galileo used a telescope with a lens, as did everyone up until Isaac Newton. He was the genius who figured out that a properly shaped mirror could focus light as well, and has advantages over a lens: mirrors need only be ground on one side (lenses have two), and mirrors can be made larger than lenses because they can be supported all across their back side, while lenses must be supported around their circumference, where the glass is thinnest and most vulnerable.

Over a certain size, lenses are simply impractical, so mirrors are used. Hubble's primary mirror is 2.4 meters across, about 8 feet. Although it's the biggest mirror for astronomy ever lofted into space, it's considered small by ground-based standards; many telescopes today have mirrors 4 or more meters across. The mammoth twin Keck 'scopes in Hawaii have mirrors made of segments that total 10 meters across each!

It turns out the cameras on board Hubble use mirrors too. Why? Glass absorbs light. Not much, maybe 2% of the incident light, but that adds up. A lens has two surfaces, each of which reflect a little bit of light, so you lose more through a lens than with a mirror. Also, mirrors can be made to reflect light of different colors about the same, but lenses bend light at different colors differently. So all in all, doing it with mirrors makes a lot more sense.

However, there are lenses on board: they are used in the Fine Guidance Sensors, small telescopes that track stars with incredible accuracy and help keep Hubble locked onto to its targets.

Hubble doesn't use lenses. Sorta.

Even today, 19 years after Hubble's launch, it's not all that uncommon to hear a newscaster refer to "Hubble's lens". I once heard it used by an announcer on a science show produced by the Space Telescope Science Institute, the agency that runs Hubble!

The thing is, Hubble doesn't use a lens. It has a mirror.

Galileo used a telescope with a lens, as did everyone up until Isaac Newton. He was the genius who figured out that a properly shaped mirror could focus light as well, and has advantages over a lens: mirrors need only be ground on one side (lenses have two), and mirrors can be made larger than lenses because they can be supported all across their back side, while lenses must be supported around their circumference, where the glass is thinnest and most vulnerable.

Over a certain size, lenses are simply impractical, so mirrors are used. Hubble's primary mirror is 2.4 meters across, about 8 feet. Although it's the biggest mirror for astronomy ever lofted into space, it's considered small by ground-based standards; many telescopes today have mirrors 4 or more meters across. The mammoth twin Keck 'scopes in Hawaii have mirrors made of segments that total 10 meters across each!

It turns out the cameras on board Hubble use mirrors too. Why? Glass absorbs light. Not much, maybe 2% of the incident light, but that adds up. A lens has two surfaces, each of which reflect a little bit of light, so you lose more through a lens than with a mirror. Also, mirrors can be made to reflect light of different colors about the same, but lenses bend light at different colors differently. So all in all, doing it with mirrors makes a lot more sense.

However, there are lenses on board: they are used in the Fine Guidance Sensors, small telescopes that track stars with incredible accuracy and help keep Hubble locked onto to its targets.



You can see one of Hubble's cameras in the National Air and Space Museum.

Like I said, Hubble was designed to be periodically updated. When new tech makes for better cameras, old ones can be taken out and replaced with new ones. When STIS and the infrared camera NICMOS were inserted into Hubble in 1997, the Goddard Spectrograph and the Faint Object Spectrograph (FOS) were removed.

While I was still at Goddard Space Flight Center, I used to take a walk around the compound after lunch. I'd sometimes slip through one building that had a massive warehouse, and usually there was something cool to see in there. I saw satellites being constructed, the upper stage of a rocket (without fuel!) on a crane, and all sorts of odd and wonderful sights.

One day, from across the warehouse, I spot what looks like a big black telephone booth sitting on a pallet. Could it be...? I walked over, and yes! It was the FOS! I couldn't believe it. It was just sitting there, this camera which cost tens of millions of dollars to build. Two sides of it had been removed, and one had been replaced with clear thick plastic. I realized it must be going to a museum; the plastic would allow people to see inside it. But one panel was still removed, so the guts of the camera were exposed. Hmmm...

So of course I reached in and poked around. I had used the FOS for my PhD, analyzing spectra it had taken of an exploding star on two different dates. We wound up not using the data because we didn't know precisely where the telescope was pointed each time, and so I couldn't compare one spectrum with another. Still, I spent months learning how the camera worked, and seeing it in front of me was too tempting. It was amazing; I could see exactly how it worked, and all those diagrams I had pored over five years earlier suddenly came alive.

I convinced a friend to come with me the next day to see it, and he took the picture above of me pretending (Yes! Pretending! That's it!) to snip the wires with a wire cutter. Haha!


Years later, I was visiting DC. I went to the National Air and Space Museum, having completely forgotten the incident at Goddard. I rounded a corner, and there was my old friend. I smiled; I knew it would end up here. The second exterior panel had been replaced with plastic, and you could see into the camera. If you compare the picture above with the one here (click to embiggen) you can see it's the same beast.


It's the only piece of Hubble I ever physically touched. Well, besides the insulating blanket that flew on Hubble for years and was taken back to Earth after a servicing mission. Someone had draped the shiny silver blanket over a chair in a room we used to test STIS. When I saw it, I... hmmmm. No. That's a whole 'nuther story.

You can look at all the images it has ever taken, as long as they're over a year old.

Since its launch in 1990, Hubble has orbited the Earth over 100,000 times and taken something like a half million separate observations. Those figures alone are a bit staggering. But did you know that you (potentially) have access to those images? Well, most of them, anyway.

All the data taken by Hubble that is more than one year old is stored in an archive that the public can query. Want to know what Hubble was observing on your birthday two years ago, or at the moment your kid was born? Just ask the database! In many cases, when you search the database, you can also get a preview of the image; the above shot is of the spiral galaxy M51, also called the Whirlpool Galaxy. The preview shows the raw data right off the 'scope; it's not always particularly pretty. To beautify it you need to process it, which means subtracting a dark frame, a bias frame, dividing by the flat field, flagging bad pixels, combining multiple exposures to get rid of cosmic rays, performing a geometric correction... and if you want color, you have to do that for the other filters used in the observation, and then combining those using Photoshop or some other software.

Obviously, not everyone can do that (it's a lot harder even than it sounds). So not everyone is allowed to actually retrieve the data; that would strain the archive servers. To do that you have to justify the need and get an account. I used to have one, but I lost my password a long time ago. Probably all for the best; I'd just download gigabytes of cool images and get everyone at the archive ticked off at me.

Oh, about that "... as long as they're over a year old" thing: data is proprietary to the person who took it for the period of one year, so the scientists involved have time to look it over. It does take some time to process the data, and a lot more time to analyze it; if everyone had instant access to all the data, someone more experienced than you could scoop you on your own observations! However, it's also not fair to let people have the data forever. The compromise is the one year proprietary period; that gives scientists time to look things over, but still motivates them to get things done. I think this is a fine idea, and it even works in practice in the real world, amazingly. If a scientist wants, they can release the data early, too, so everyone wins.

In fact, I used old data quite a bit back in the day. If we found something interesting in our own data, we could go look for older observations to see if it had been seen before, or if there were related observations. And many times, even if the older data were still proprietary, the scientists involved were interested in collaborating. Funny thing about scientists: in lots of cases they are open, friendly, and interested in seeing what everyone else is doing. There were exceptions, of course, but that's what I found for the most part.

Maybe that's the thing that'll surprise you most in this article. But it's true.

Conclusion

Choosing just ten things for this article was, as usual, tough. I can think of lots more things to add: JWST won't replace Hubble, it succeeds it; Hubble isn't really a telescope, it's a whole observatory; it has flown the finest UV camera ever built, which was so sensitive that a massive and hot O-type star in the Andromeda Galaxy could have damaged it (and once I nearly blew it up); when there is a strong meteor shower, they point HST in the opposite direction.

There are tons of things about Hubble that I'm sure I don't know either; I worked on it for a decade, but in fact I haven't worked on it for nearly a decade since. It's a complicated and beautiful machine, and it changed the way we look at the Universe, maybe forever. It certainly changed the way scientists do astronomy... and I know that the best thing it did, the best thing it could do, was to let people see just how phenomenally gorgeous our Universe is.

And for that, I'm very grateful. And that's one thing I do know.

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NASA Space Shuttle Processing

NASA Space Shuttle Processing

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How Killer Electrons Form in Space

Superstrong pulses in Earth’s magnetic field can drive electrons to near light speed, physicists reported in June. These “killer” electrons can cripple satellites and they present a radiation threat to astronauts. Scientists have long wondered how they accumulate enough energy to zip around in space.

Qiugang Zong, of the University of Massachusetts Lowell, led a team of physicists who analyzed data from the European Space Agency and NASA’s Cluster spacecraft, four satellites situated at the edge of Earth’s magnetic field. The satellites observed the pulses in the wake of an October 2003 magnetic storm triggered by a coronal mass ejection—a plasma spitball shot out by the sun—that slammed into Earth’s magnetosphere. The influx of energetic particles created waves in our planet’s magnetic field, Zong’s team discovered. As the pulses approached Earth, the ultralow frequency waves made the planet’s magnetic field lines oscillate and accelerated electrons traveling along the field lines to extraordinarily high speeds.

“ULF waves are standing waves that stay in their location and vibrate like a string,” Zong says. “It’s amazing that the wave power transfers to the killer electrons.” Zong’s study represents the first time this process has been observed directly.

The storm that the Cluster spacecraft witnessed damaged several satellites and caused power outages in Sweden. Astronauts in the International Space Station were ordered into a heavily shielded module during the storm. Fortunately for surface-­dwelling ­humans, Earth’s magnetic field and atmosphere do a good job protecting us from such killer electrons

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Pictures From The Past Mission. (Space)

Pictures From The Past Mission. (Space)

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Cool Alert: Nasa Astronomy Picture of the Day

Not fashion related, but in our geeky way we really love Nasa's Astronomy Picture of the Day archive. The explanations of the pictures are somewhat complicated to say the least, and not all the images are of beautiful distant galaxies and new stars forming, but there are lots of amazing photos which are very relaxing to look at and into which you can really immerse yourself.

Click images to enlarge; on the Nasa site they're in even higher definition (think 4,000 px wide..)

See more here. Not quite sure why this section of the Nasa website looks so, um, retro in its web design.

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