By Larry Klaes
“Suddenly in the north sky… the sky was split in two, and high above the forest the whole northern part of the sky appeared covered with fire… At that moment there was a bang in the sky and a mighty crash… The crash was followed by a noise like stones falling from the sky, or of guns firing. The earth trembled.” So wrote a witness — fully forty miles away from the event — of the Tunguska impact of 1908, whose 100th anniversary is today. As Larry Klaes notes, small bodies still undetected by astronomers could pose the threat of another Tunguska, making the hunt for Earth-crossing objects a matter of high importance not just for science but planetary security.
Across the many billions of miles of space that our Solar System occupies in its small piece of the vaster Milky Way galaxy, the most numerous members of our celestial neighborhood by far are the comets, planetoids, and meteoroids.
Although much smaller than the Sun, the major planets, and many of the attending moons of the largest worlds, comets and planetoids can be found by the billions from inside the orbit of Mercury to halfway to the next star system, Alpha Centauri. The vast majority of comets occupy the utterly dark and cold outer realms of the Solar System past Neptune, the last and smallest of the giant planets. Comets are the icy remains of the birth of our Solar System roughly five billion years ago. Most planetoids can be found scattered between the orbits of Mars and Jupiter. These rocky bodies represent worlds that could never form into anything grander due to the massive, disrupting presence of Jupiter from the earliest days of our star system. Meteoroids are the scattered remnants of planetoid collisions, ranging in size from boulders to dust; they can be found in virtually every corner of the Solar System.
Like most things in reality, though, not everything in the Solar System stays in one neat area, especially over the vast stretches of time that our system has existed. Many comets have been perturbed from their distant orbits into new ones which cause them to venture into the inner Solar System. Some comets make briefs visits and then head back out into the blackness, not to be seen again for many centuries. Others plow directly into the Sun, while some get “caught” just right and end up in new, smaller orbits that cause these icy worldlets to return to our region of space in mere decades. Halley’s Comet is perhaps the most famous example of what astronomers call a short-period comet: Last visiting Earth in 1986, Halley’s Comet will come around again in 2061. Planetoids have also been knocked about through the ages, being whipped around by each other and larger worlds, or smashing together and subsequently finding themselves in different shapes and sizes along new circuits about the Sun.
Though aware of comets and planetoids for centuries, it has only been in the last few decades that astronomers and others have begun to take seriously the remote but definite possibility that some of these “rogue” objects could one day collide with our planet Earth. While our actual world could survive an impact with these much smaller celestial bodies, it would be another story for most of the life existing on the crust of this planet. The 160 million-year reign of the dinosaurs came to an abrupt end 65 million years ago when a chunk of ancient space debris a mere five miles across slammed into Earth. Earlier epochs may have seen even more organisms rendered extinct by similar celestial impacts.
Image: Space debris is by no means a thing of the past. This artist’s conception shows the moment of explosion of a huge space rock that shattered trees throughout a large section of Siberia in 1908. Events of this magnitude are thought to occur approximately every 300 years. Credit: William Hartmann.
For those who might think that destructive collisions between our planet and a comet or planetoid happen too seldom to be of much concern for humanity and the rest of Earth’s biosphere, one should take note of an important anniversary in astronomical history which took place one century ago.
In the early morning of June 30, 1908, a huge ball of light almost as bright as the Sun was seen moving across the sky over a remote region of Siberia. The strange object was a piece of a comet or planetoid perhaps the size of a ten-story building streaking through Earth’s atmosphere at tremendous speed.
Suddenly the space visitor violently broke apart several miles above the ground near the Podkamennaya Tunguska River with a force estimated at one thousand times greater than the explosion of the atomic bomb dropped on Hiroshima, Japan in 1945. Eighty million trees for 830 square miles around the detonation zone were flattened. The ground shook like an earthquake and the resulting shock wave caused people living miles from ground zero to be knocked off their feet.
This cosmic impact, which became known as the Tunguska Event, is one of the earliest instances which made people aware of the fact that not everything plunging in from space is either a harmless and pretty meteor shower of particles no bigger than dust or a larger yet still “safe” rock or boulder known as a meteorite. Had the object which created the Tunguska Event landed on a major city instead of a remote forested region of Earth, the unfortunate urban center would have suffered major damage and a high loss of life. If a Tunguska Event happened today in either a rural or urban area, the explosion might be interpreted as a nuclear attack, with potentially deadly consequences for all humanity from the response to such a “message” from the Universe.
Image: A Russian expedition to the Tunguska site showed what the scene looked like fully twenty years after the event. The expedition, led by Leonard Kulik, discovered that the pattern of fallen trees pointed to an explosion caused by a meteorite, but Kulik found no fragments of the object. It is now believed that the Tunguska asteroid, about 36 meters across, exploded some 28,000 feet up in the atmosphere.
Of greatest concern to scientists, political leaders, and others is the random space object miles across with an orbit that may one day intersect our planet on its path around the Sun. Thanks to the diligence of various professional groups over the last decade, thousands of Near Earth Objects (NEOs) have been found and their orbits calculated.
The majority of these bodies pose no threat to us and our planet for the foreseeable future. However, the orbits of a few NEOs are still not known with enough certainty to remove them from the list of potentially deadly space rocks just yet. Additionally, there are quite a few smaller bodies roaming the Solar System that remain undetected by astronomers, any of which could one day cause destruction to our world on a Tunguska level or worse.
Facilities like the Cornell-run Arecibo Radio Observatory in Puerto Rico remain at the forefront in helping scientists to understand the makeup of these NEOs and how to deal with them in case one does have Earth in its sights some day. The continued existence of Arecibo and other science facilities may become the difference between a thriving future for humanity or extinction.
The 60th Carnival of Space is now up at Slacker Astronomy, and if you want to see some fine science writing, I’ll point you this week to the host, whose essay on Regulus shows what can be done when a scientist with serious writing skills takes apart an interesting scientific paper. Doug Welch knows what he’s talking about — he’s a professor of physics and astronomy at McMaster University (Hamilton, ONT), deeply involved in dark matter studies, supernovae and variable stars. So it’s no surprise that the interesting story of Regulus and its apparent companion comes alive in Slacker Astronomy‘s pages.
What about Regulus? A team led by Doug Gies (Georgia State) has studied this bright, ecliptic-hugging star for evidence of a hitherto unknown companion. The result:
They found that Regulus was indeed a spectroscopic binary. Once every 40.11 days, the system completes one orbit. Regulus itself has a mass of about 3.4 times that of the Sun. The companion of Regulus is much less massive – only about 0.30 solar masses. Such a small mass object is either a low-mass star or a white dwarf. The latter possibility provides an explanation for Regulus’ rapid rotation! The idea is that the companion was once the more massive member of the pair and when it finished hydrogen burning in its core, it expanded dramatically and started losing mass to Regulus in a manner which “spun it up”. A mass of 0.30 solar masses is very low for a white dwarf – such objects are found only in systems where it is clear that much mass has been transferred.
Intriguingly, spectra taken by the International Ultraviolet Explorer satellite showed results consistent with a white dwarf but not a cool, low-mass star. The case for a white dwarf seems confirmed, a reminder that objects we thought we knew well often yield up their secrets only slowly, and with the necessary improvements in our instrumentation. And if you have a fascination, as I do, with how these studies work, be sure to read Welch’s comments on the instruments involved, including the Kitt Peak National Observatory Coude Feed Telescope, about which this:
A Coude room is very high-resolution spectrograph capable of tearing the light from a telescope into very fine shreds of color. It was designed to be fed by the 2.1m telescope at Kitt Peak. However, observatories tend to do deep imaging around the time of New Moon (i.e. when the sky is dark) and the 2.1m served a variety of such needs. It was realized that the a smaller telescope could “feed” the spectrograph during these periods and that brighter stars could be observed with that smaller telescope plus Coude spectrograph while the big telescope was busy imaging!
This is solid science writing. Taking state of the art instrumentation and breaking it down into not just comprehensible but readable terms is hard work, and Welch does the same with GSU’s Multi-Telescope Telescope. Slacker Astronomy quickly goes into my essential RSS list. I also want to point you to a site already firmly ensconced in that list, Brian Wang’s NextBigFuture, which (among many other things) this week discussed the Space Elevator power beaming competition. Finding the cheapest route to low-Earth orbit is an essential for making a true space-based infrastructure a reality, and few concepts are as visionary as the Space Elevator. Add commercial competition to the mix and watch the ideas fly.
With the 100th anniversary of the Tunguska impact in Siberia coming up on Monday (and we’ll look at it closely then), several items seem germane to the topic of asteroid deflection. Yesterday, a technical briefing at the University of Calgary outlined the Canadian NEOSSat (Near Earth Orbit Surveillance Satellite) mission, a space telescope designed to track small objects near Earth, some of which may pose a collision threat. The suitcase-sized NEOSSat (launch date 2010) capitalizes on technology developed for Canada’s MOST (Microvariability and Oscillation of STars) satellite, which was designed to measure stellar ages in our galaxy.
While NEOSSat’s asteroid-hunting capabilities draw most media attention, the satellite is also going to act as a monitor on other satellites orbiting the Earth, contributing to the worldwide Space Surveillance Network. Satellite-tracking tests using the MOST instrument have proven that a microsatellite can track other satellites, but tuning the technology for asteroids takes another leap. David Cooper, general manager of Ontario-based Dynacon, the prime contractor for NEOSSat and the manufacturer and operator of MOST, puts the issue this way:
“NEOSSat requires remarkable agility and pointing stability that has never before been achieved by a microsatellite. It must rapidly spin to point at new locations hundreds of times per day, each time screeching to a halt to hold rock steady on a distant target, or precisely track a satellite along its orbit, and image-on-the-run.”
‘Rock steady’ is an exquisitely apt phrase for an asteroid hunter. And while the mission’s telescope, at 15-centimeters, is an instrument smaller than that used by many amateurs, placing it 700 kilometers about the atmosphere should offer great advantages. In a fifty minute polar orbit, the satellite will, according to team members, be able to detect asteroids delivering as few as 50 photons in a 100-second explosure. The more asteroid cataloging we can do, the better, and NEOSSat’s unique vantage point also offers the possibility of identifying non-threatening, slow-moving asteroids close to Earth for possible rendezvous missions.
Although it demands a sunshade for the satellite, NEOSSat’s polar orbit will allow it to search the sky to within 45 degrees of the Sun, a region hard to observe from the ground but the place where near-Earth asteroids are concentrated, and the only part of the sky where asteroids that orbit entirely inside Earth’s orbit can be discovered. With current estimates of 100,000 asteroids greater than 140 meters in diameter in near-Earth space, finding those on Earth-crossing orbits should be a high priority for a space-capable civilization, one to which even a small aperture telescope can make a contribution.
Add to this the new analyses of Martian terrain published this week in Nature, three letters that examine the Borealis basin, which covers about forty percent of the planet’s surface, and find it to be the remains of a huge impact early in the Solar System’s formation. Mars is home to abundant testimony of the effect of collisions, but the 5300-mile width of the Borealis basin is four times that of the Hellas basin, also on Mars, that had been the largest impact crater identified in the Solar System before now. Every sign points to the Borealis event having been caused by an object larger than Pluto, with an impact at least 3.9 billion years ago.
The early Solar System was rife with such impacts, but we have seen in our own time that much smaller objects (think Comet Shoemaker-Levy on Jupiter in 1994, or Tunguska in 1908) continue to pose a threat to our planet. Every battered landscape we see on the moons of the outer planets, not to mention our own Moon, bears witness to the violent history of our surroundings. Ensuring planetary security will lead us not only to improved asteroid cataloging but, inevitably, to missions to nearby asteroids as we develop the technology needed to adjust a threatening trajectory. Defense alone pushes us deep into the Solar System, from which point we should have the infrastructure needed to stay.
By Larry Klaes
Just how dangerous a place is our universe? As Larry Klaes notes, the apparent calm of a quiet summer sky masks events that can dwarf the imagination. New instruments, particularly those in space, are now giving us an unprecedented look at stellar flares and exploding stars, allowing us to observe the earliest phases of their activity. The implications for life are also striking, as flaring red dwarfs and titanic supernova can attest.
When we look up at the night sky with our eyes alone, everything about it seems calm and even peaceful. Aside from a passing airplane or satellite, only the occasional meteor or twinkling star indicate any natural activities up there. Otherwise, the Universe seems almost immobile and permanent, even when we watch the stars for a long while.
Recent news by the astronomy community shows just how much of an illusion this perception actually is. On May 14, NASA announced the discovery of the youngest local supernova remnant yet known, an object unpoetically known as G1.9+0.3, located near the center of our Milky Way galaxy about 26,000 light years from Earth.
Though most stars exist for ages far longer than human minds can conceive, they are not immortal. Some last for billions of years and eventually more-or-less quietly fade away; this will be the fate of our Sun.
More massive suns do not exist for quite so long, nor do they leave the Universe peacefully. These natural fusion reactors often end up in a titanic explosion called a supernova. Some lose their nuclear fuel, causing their cores to collapse and release huge amounts of energy in the process, leaving a neutron star or black hole in their wake. Other large suns that are part of a binary system where one star is a white dwarf create their death act when too much material from the giant star is pulled onto the white dwarf companion, causing its core to heat enough to create runaway nuclear fusion and tear itself apart.
Image: This artist’s impression shows what the supernova explosion that resulted in the formation of the supernova remnant G1.9+0.3 might have looked like. The expanding debris from the supernova explosion is shown in white, including some interaction with the surrounding gas (green). The crowded environment near the center is shown by diffuse gas (red) and dust (brown) as well as large numbers of stars with different masses and colors. Credit: NASA/CXC/M.Weiss.
While these explosions mean certain destruction and death for these suns and anything near them in space, the event also means life for any new systems that form in their wake. The debris expelled from the destroyed star contains many heavy elements that become part of other nebulae of gas and dust that collapse into new solar systems triggered by the shockwave of the supernova. This is recycling on a cosmic scale.
Such an important process for the creation of stars, worlds, and life has naturally led scientists to want to know more about the complete cycle of supernovae. Finding that G1.9+0.3 is only about 140 years old, several centuries younger than the previously youngest known supernova, is a major boon to this field.
Astronomers did not know about this particular supernova before the 1980s due to the heavy amounts of dark interstellar dust and gas that lie between Earth and that stellar remnant. The gas and dust being spewed into space from the stellar explosion did heat up the surrounding environment, which allowed the supernova to be detected by the Chandra X-Ray satellite and the Very Large Array (VLA) group of radio telescopes in the desert of New Mexico. The scientists were able to witness the rapid expansion of the supernova debris cloud moving at five percent of light speed (186,000 miles per second) over the last two decades, enabling them to determine the relatively young age of this celestial phenomenon.
Meanwhile, a supernova in the spiral galaxy NGC 2770 labeled SN2008D was recently caught by the X-ray telescope of NASA’s Swift Observatory satellite while the space sentinel was gathering data on a different supernova in that same galaxy. This fortuitous situation allowed astronomers to witness a supernova in the act of first forming, though that galaxy and SN2008D are over 90 million light years from our Milky Way.
Regarding one of the results of a major stellar explosion, in particular a pulsar (a rapidly rotating neutron star), a sky survey named PALFA (for Pulsar Arecibo L-band Feed Array), conducted with the Arecibo Radio Observatory in Puerto Rico managed by Cornell University, came across an unusual type of pulsar named PSR J1903+0327 situated 21,000 light years from Earth.
This particular pulsar has a companion star, which in itself is not terribly unusual: The first binary pulsar was discovered in 1974 using the giant Arecibo radio telescope. Even though PSR J1903+0327 spins on its axis 465 times each second (2.15 milliseconds), which makes it among the fastest known such rapidly rotating neutron stars, this feature also does not make that pulsar so terribly unusual among its kind.
What does set PSR J1903+0327 apart from its fellow supernova remnants is the fact that it has a very elongated 95-day orbit around its companion sun, which happens to be a fairly “normal” star similar to our Sun. All other known millisecond binary pulsars orbit in nearly perfect circles around other neutron stars.
Astronomers have several theories as to why this pulsar system is so different from the rest. One idea involves the binary pulsar having formed in a globular star cluster and becoming disrupted and ejected from that large collection of suns by a close encounter with another star in that cluster. The other theory postulates a third companion to that system, perhaps another neutron star or even a white dwarf. If the latter idea turns out to be true, then PSR J1903+0327 would become the first known triple pulsar system.
The details on this strange pulsar were published in the May 15 issue of Science Express, the online version of Science magazine (the paper is also available at the arXiv site). As if to bring home the fact that our Universe is anything but a quiet, unchanging realm, astronomers reported late last month that the Swift satellite, which observed the supernova “birth” in galaxy NGC 2770, also detected a massive flare from EV Lacertae, a faint red dwarf star just 16 light years from our planet.
Image : An artist’s depiction of the red dwarf EV Lacertae and its enormous flare. Credit: Casey Reed/NASA.
How big was the flare? The huge amount of gas and other particles that erupted from this young star were so bright that they caused Swift to automatically shut down its observing telescope for safety reasons. Astronomers noted that had the EV Lacertae flare occurred on our Sun, it would have stripped away Earth’s atmosphere and sterilized our planet’s surface.
Figuring out what makes up 74 percent of the universe is no small matter. But the late 20th Century discovery that the rate of expansion of the universe is not slowing but accelerating makes the research all but imperative. The Dark Energy Survey is behind the construction of an extraordinarily sensitive camera that will be installed on the Cerro Tololo Inter-American Observatory (CTIA) 4-meter telescope in Chile, with the aim of creating an unprecedented sky survey to probe these questions.
I’m looking at the original proposal for the DES survey as submitted to the National Optical Astronomy Observatory office (NOAO controls the Cerro Tololo site). The document calls the discovery of accelerated expansion ‘arguably the most important discovery in cosmology since the serendipitous detection of the cosmic microwave background radiation by Penzias and Wilson in 1965’ (it’s hard to argue with that!). And it goes on to state the challenge posed by dark energy in stark terms:
According to General Relativity, if the Universe is filled with ordinary matter, the expansion should be slowing down due to gravity. Since the expansion is speeding up, we are faced with two logical possibilities, either of which would have profound implications for our understanding of the fundamental laws of physics: (i) the Universe is filled with a completely new kind of stress-energy with bizarre properties (in particular, negative effective pressure), or (ii) General Relativity breaks down on cosmological scales and must be replaced with a new theory, perhaps associated with extra dimensions.
You can see why dark energy would be of interest not only to astrophysicists but to those looking into advanced propulsion. Are there clues here that, properly understood, could lead us to new technologies? Accounting for how dark energy works is currently the greatest challenge in astrophysics. We’re a long way from untangling it, but the implication that energies of a new order could become available to a sufficiently advanced civilization will keep dark energy research on our watchlist.
Image: The CTIO 4-meter Blanco telescope, which will be used in the Dark Energy Survey. In this photograph, the instrument is silhouetted against the Magellanic Clouds (at left) and the Milky Way, as seen from Cerro Tololo in Chile. This is not a composite image. It was taken by Roger Smith using a 2048×2048 scientific CCD which has much higher sensitivity than photographic film, revealing greater detail in exposures short enough (20 seconds) to eliminate star trails. The CCD, normally used on the telescope pictured, was temporarily mated to a Zeiss Distagon 40 mm f/4 lens by a “camera body” made in house. The only source of illumination is starlight. Credit line: Roger Smith/NOAO/AURA/NSF.
As for DES, the plan is to map 300 million galaxies through multiple filters in a field of 5000 square degrees, creating a galaxy map that should bring higher precision to current dark energy studies. We’re still early in the game here, but it’s encouraging to see that the pieces of glass for the camera’s five lenses have now been shipped from the US to France, where they will be shaped and polished. The largest lens is a meter in diameter, to be installed in a camera with 500 megapixel capability and a fast data acquisition system that can take images in 17 seconds.
So the long process of building the DECam (Dark Energy Camera) has begun, the lenses to be polished to the smoothness of a millionth of a centimeter. Using multiple, complementary methods of studying redshift distribution, gravitational lensing, the evolution of galactic clusters and type 1a supernova distances, the plan is to improve our measurement of dark energy through a catalog of galaxies far deeper than what is available in the Sloan Digital Sky Survey. Observations in Chile are scheduled to begin in 2011 and continue until 2016.