PHILIPPINE HEADLINE NEWS ONLINE: Since 1997 © Copyright (PHNO) http://newsflash.org
PHNO SCIENCE & INFOTECH NEWS THIS PAST WEEK
(Mini Reads followed by Full Reports)
'PHILAE' THE LITTLE SPACE ROBOT: SPACE PROBE FINDS LOST 'PHILAE' LANDER ON COMET
SEPTEMBER 6 -Philae is wedged in a dark crack on the surface of the frozen comet. The photos show Philae wedged in a shadowy crack, with its boxy, one-meter-wide body and two of its three legs clearly visible. 'Philae', the little space robot that has captured hearts around the world, was thought to be forever lost. But delighted scientists announced Sunday that the European Space Agency's comet lander has come back from the cosmic dead. New images downloaded from the Rosetta probe in orbit around the awkwardly named Comet67P/Churyumov-Gerasimenko show the long-lost Philae wedged in a crack between some rocks. Rosetta captured the images in the nick of time, as its mission is ending in less than a month. "THE SEARCH IS OVER! I've found @Philae2014!!" announced the Rosetta Mission team on Twitter. MORE PHOTOS READ MORE....
ALSO: EARLIER REPORT (July 2015) No alien life on Philae comet
JULY 2015 -No aliens here. Comet scientists sceptical of sensational claim. Photograph: ESA/ESA via Getty Images A sensational claim that ESA’s Philae spacecraft has landed on a comet teeming with life doesn’t hold water.... The Guardian’s story “Philae comet could be home to alien life, say scientists” has been met with scepticism and outright dismissal by leading comet experts. The people behind the headline are Chandra Wickramasinghe, University of Buckingham, and Max Wallis, University of Cardiff. Today, the Daily Mirror reported Wickramasinghe as saying, “Data from the comet seems to unequivocally point to micro-organisms being involved.” However, the evidence for any life on Philae’s comet is flimsy at best. Even Wickramasinghe’s own colleague fell short of agreeing with him outright. At the academic lecture that triggered the story, Paul Sutherland of SEN reported Wallis as saying, “If there is any active biology in the comet, we’d hope to detect it”. Certainly, the vast majority of comet scientists would agree that comet 67P’s surface features are much more easily explained by non-biological mechanisms. READ MORE...
ALSO: ABOUT Rosetta (The spacecraft)
FROM WIKIPEDIA: Rosetta is a space probe built by the European Space Agency launched on 2 March 2004. Along with Philae, its lander module, Rosetta is performing a detailed study of comet 67P/Churyumov–Gerasimenko During its journey to the comet, the spacecraft flew by Mars and the asteroids 21 Lutetia and 2867 Šteins. On 6 August 2014, the spacecraft reached the comet and performed a series of manoeuvres to be captured in its orbit. On 12 November, the lander module performed the first successful landing on a comet, though its battery power ran out two days later. Communications with Philae were briefly restored in June and July 2015, but due to diminishing solar power, Rosetta's communications module with the lander was turned off on 27 July 2016.As of 2016, the mission continues to return data from the spacecraft in orbit. The probe is named after the Rosetta Stone, a stele of Egyptian origin featuring a decree in three scripts. The lander is named after the Philae obelisk, which bears a bilingual Greek and Egyptian hieroglyphic inscription. A comparison of its hieroglyphs with those on the Rosetta Stone catalysed the deciphering of the Egyptian writing system. READ MORE...
ALSO: WHAT IS A COMET?
WIKIPEDIA: A comet is an icy small Solar System body that, when passing close to the Sun, heats up and begins to outgas, displaying a visible atmosphere or coma, and sometimes also a tail. These phenomena are due to the effects of solar radiation and the solar wind upon the nucleus of the comet. Comet nuclei range from a few hundred metres to tens of kilometres across and are composed of loose collections of ice, dust, and small rocky particles. The coma and tail are much larger and, if sufficiently bright, may be seen from the Earth without the aid of a telescope. Comets have been observed and recorded since ancient times by many cultures. Comets usually have highly eccentric elliptical orbits, and they have a wide range of orbital periods, ranging from several years to potentially several millions of years. Short-period comets originate in the Kuiper belt or its associated scattered disc, which lie beyond the orbit of Neptune. Long-period comets are thought to originate in the Oort cloud, a spherical cloud of icy bodies extending from outside the Kuiper belt to halfway to the next nearest star. Long-period comets are directed towards the Sun from the Oort cloud by gravitational perturbations caused by passing stars and the galactic tide. Hyperbolic comets may pass once through the inner Solar System before being flung out to interstellar space. COMETS VS ASTEROIDS Comets are distinguished from asteroids by the presence of an extended, gravitationally unbound atmosphere surrounding their central nucleus. This atmosphere has parts termed the coma (the central part immediately surrounding the nucleus) and the tail (a typically linear section consisting of dust or gas blown out from the coma by the Sun's light pressure or outstreaming solar wind plasma). READ MORE...
READ FULL MEDIA REPORTS HERE:
Space probe finds lost Philae, the little space robot lander on comet
SEPTEMBER 6 -Philae is wedged in a dark crack on the surface of the frozen comet. The photos show Philae wedged in a shadowy crack, with its boxy, one-meter-wide body and two of its three legs clearly visible.
NASA, SEPTEMBER 5, 2016 (CNN WORLD) By Elizabeth Roberts for CNN September 6, 2016 - Philae, the little space robot that has captured hearts around the world, was thought to be forever lost.
But delighted scientists announced Sunday that the European Space Agency's comet lander has come back from the cosmic dead.
New images downloaded from the Rosetta probe in orbit around the awkwardly named Comet67P/Churyumov-Gerasimenko show the long-lost Philae wedged in a crack between some rocks.
Rosetta captured the images in the nick of time, as its mission is ending in less than a month.
"THE SEARCH IS OVER! I've found @Philae2014!!" announced the Rosetta Mission team on Twitter.
ROSETTA: THE COMET CHASER
ROSETTA in space
→New communication with Philae – commands executed successfully This report is provided by the German Aerospace Center, DLR.
The Philae lander communicated with the Rosetta orbiter again between 19:45 and 20:07 CEST on 9 July 2015 and transmitted measurement data from the COmet Nucleus Sounding Experiment by Radiowave Transmission (CONSERT) instrument. Although the connection failed repeatedly after that, it remained completely stable for those 12 minutes. “This sign of life from Philae proves to us that at least one of the lander’s communication units remains operational and receives our commands,” said Koen Geurts, a member of the lander control team at DLR Cologne.
The mood had been mixed over the last few days; Philae had not communicated with the team in the DLR Lander Control Center (LCC) since 24 June 2015.
After an initial test command to turn on the power to CONSERT on 5 July 2015, the lander did not respond. Philae’s team began to wonder if the lander had survived on Comet 67P/Churyumov-Gerasimenko.
“We never gave up on Philae and remained optimistic,” said Geurts. There was great excitement when Philae ‘reported in’ on 13 June 2015 after seven months of hibernation and sent data about its health. The lander was ready to perform its tasks, 300 million kilometres away from Earth.
However, Philae has to communicate with the ground stations through Rosetta, which acts as a radio relay.
Restrictions on the orbiter’s approach to and orbit around the comet have not permitted regular communication with the lander. The data sent on 24 June did not suggest that the lander had experienced technical difficulties. Now, Philae’s internal temperature of zero degrees Celsius gives the team hope that the lander can charge its batteries; this would make scientific work possible regardless of the ‘time of day’ on the comet.
The received data is currently being evaluated. “We can already see that the CONSERT instrument was successfully activated by the command we sent on 9 July,” explained Geurts. Even now, Philae is causing the team some puzzlement: “We do not yet have an explanation for why the lander has communicated now, but not over the past few days.”
The trajectory of the orbiter, for example, has not changed over the last three weeks.
However, one thing is certain; Philae has survived the harsh conditions on the comet and is responding to commands from the LCC team. “This is extremely good news for us,” said Geurts.
Watch the latest video update from the LCC, here.
Rosetta is an ESA mission with contributions from its Member States and NASA. Rosetta's Philae lander is contributed by a consortium led by DLR, MPS, CNES and ASI.
Comet Probe Rosetta -A NASA instrument aboard the European Space Agency's Rosetta orbiter has successfully made its first delivery of science data from comet 67P/Churyumov-Gerasimenko. — jpl.nasa.gov
In November 2014, Philae completed a spectacular landing on the frozen surface of the comet, despite a harpoon system that did not function after its 10-year journey through space.
The lander hopped across the comet's surface, and as people across the globe watched its progress, engineers and scientists announced that Philae was communicating with its ground stations, some 317 million miles (510 million kilometers) away on Earth.
Google dedicated its search page to the lander, and instead of the second letter O in its logo, depicted Philae extending its three legs.
Image matchMark as Adult Google | Default Picture 2016 1440 x 810 · jpeg·dlr.de
Scientists carried out over 60 hours of research with Philae's instruments, acquiring images, sensing molecules and attempting to hammer the unexpectedly hard surface of the comet.
Scientists describe the pictures as definitive: There's no doubt the object is Philae.
Among the invaluable scientific data gathered was the dramatic discovery of 16 "carbon and nitrogen-rich" organic compounds, supporting the theory that the building blocks of life could have been brought to Earth by comets.
But then the intrepid little probe ran out of juice. Unable to get the energy it needed from the sun to power its solar panels, its battery went flat and it fell into hibernation mode.
As the comet came closer to the Sun in June and July 2015, the lander briefly revived and communicated once again.
When once again it fell silent, it was assumed Philae had stopped functioning due to the extremely cold environment. The three-legged probe even sent a heartbreaking farewell tweet in July.
"Unfortunately, the probability of Philae re-establishing contact with our team at the DLR Lander Control Center is almost zero, and we will no longer be sending any commands," Stephan Ulamec, Philae project manager, announced in February this year. "It would be very surprising if we received a signal now."
Philae seemed to be doomed to life lost and alone traveling through the solar system.
But when analysis was completed of the latest pictures from Rosetta's Osiris camera, which were downlinked to Earth on Sunday night, there came a delightful surprise.
There has been an outpouring of delight at the remarkable news.
"With only a month left of the Rosetta mission, we are so happy to have finally imaged Philae, and to see it in such amazing detail," said Cecilia Tubiana of the Osiris camera team
Although there is no hope of reviving the lander, knowing exactly where it is resting will help scientists make better sense of the data it returned during its three days of operation back in 2014.
Matt Taylor, ESA's Rosetta project scientist added: "This wonderful news means that we now have the missing 'ground-truth' information needed to put Philae's three days of science into proper context, now that we know where that ground actually is."
The discovery comes just a few weeks before the European Space Agency plans to crash Rosetta on the comet Sept 30 to bring its 12-year mission to an end.
"Now that the lander search is finished we feel ready for Rosetta's landing, and look forward to capturing even closer images of Rosetta's touchdown site," said Holger Sierks, principal investigator of the Osiris camera.
VIDEO: PHILAE ON COMET
Pieces of the Puzzle – Philae on Comet 67P .
Published on Aug 12, 2015
Philae’s landing on comet 67P/Churyumov-Gerasimenko (#CometLanding) on 12 November 2014 was a historic moment – the first time in the history of space exploration that a spacecraft landed on a comet. Millions of people across the world followed the Rosetta mission via the Internet. The DLR Video ‘Pieces of the Puzzle – Philae on Comet 67P’ provides an insight into the ‘roller coaster ride’ on the day of the #CometLanding: “We had to make decisions, develop concepts, alter schedules, sleep briefly and return – and then do the whole thing again and again. There was not a moment to breathe.” In the video, Koen Geurts, Philae’s Technical Manager, looks at the days immediately after the landing and the following seven months of waiting for a renewed sign of life from Philae. The ‘crazy year’ was to continue, as on 14 June 2015, the comet lander once again reported back. However, the connections thus far have been irregular and unstable. And so, all those involved in the Rosetta mission must examine the pieces of the puzzle together to decipher what is happening 266 million kilometres from Earth. More: http://www.dlr.de/blogs/en/desktopdef...
A short film by DLR German Aerospace Center Directed by Peter Folie, Fabian Walker
Category Science & Technology
THE GUARDIAN UK - Science Across the universe
No alien life on Philae comet Stuart Clark @DrStuClark Monday 6 July 2015 18.29 BST Last modified on Monday 6 July 2015 19.32 BST
A sensational claim that ESA’s Philae spacecraft has landed on a comet teeming with life doesn’t hold water....
No aliens here. Comet scientists sceptical of sensational claim. Photograph: ESA/ESA via Getty Images
The Guardian’s story “Philae comet could be home to alien life, say scientists” has been met with scepticism and outright dismissal by leading comet experts.
The people behind the headline are Chandra Wickramasinghe, University of Buckingham, and Max Wallis, University of Cardiff. Today, the Daily Mirror reported Wickramasinghe as saying, “Data from the comet seems to unequivocally point to micro-organisms being involved.”
However, the evidence for any life on Philae’s comet is flimsy at best. Even Wickramasinghe’s own colleague fell short of agreeing with him outright.
At the academic lecture that triggered the story, Paul Sutherland of SEN reported Wallis as saying, “If there is any active biology in the comet, we’d hope to detect it”.
Certainly, the vast majority of comet scientists would agree that comet 67P’s surface features are much more easily explained by non-biological mechanisms.
“No scientist active in any of the Rosetta instrument science teams assumes the presence of living micro-organisms beneath the cometary surface crust,” Uwe Meierhenrich of Université Nice Sophia Antipolis, France, told me in an email exchange on Monday afternoon.
Meierhenrich serves as a co-investigator on Philae’s COSAC instrument, which was designed to chemically analyse the comet. He told me that a comet’s black surface crust was a prediction made in 1986 by J. Mayo Greenberg (Nature 321, 385), who calculated what would happen to naturally occurring organic molecules on the comet when they were struck by cosmic rays and light.
“These explanations seem to be valid, also with regard to new data of the cometary Rosetta mission,” wrote Meierhenrich.
Several of the news reports have quoted Wickramasinghe as saying that he was not allowed to place life detection equipment on the Philae lander. They go on to say that the spacecraft cannot detect life, even if it is there. This last assertion is simply not true.
Life is quite picky about which chemicals it utilises; therefore, if life were present on the comet, this would recognisably boost a number of key molecules. COSAC and the PTOLEMY instrument on Philae could measure this enhancement. “We can thereby well distinguish between the biological and astrochemical formation of organics,” wrote Meierhenrich.
A first list of molecules identified on the cometary nucleus by COSAC, authored by Meierhenrich and colleagues, will be published at the end of July in the journal Science. According to Meierhenrich, this list hints at non-biological formation mechanisms.
Wickramasinghe has a history of claiming to have detected extraterrestrial microbes. In 2001, he claimed to find extraterrestrial microbes in stratospheric dust collected at 41km in altitude. In 2003, he suggested that the SARS virus came from outer space. None of this work has been accepted by the mainstream.
However, an earlier hypothesis of Wickramasinghe’s that the molecular building blocks of life could have been brought to Earth by comets is now widely thought plausible. Evidence to back up this claim has been amassed by independent investigations, of which Rosetta and Philae are the latest round.
However, Wickramasinghe appears to see this acceptance as a weapon to be used against his bolder ideas. In a paper published in 2014, he wrote that the acceptance and promotion of this idea by scientific journals, “serves as a deliberately chosen device to keep the full force of evidence for ingress of extraterrestrial life from coming to the public’s notice.”
Today’s story began in a press release issued by the Royal Astronomical Society at 23:01 BST on Sunday 5 July. “Do micro-organisms explain features on comets?” ran the bold headline. It was to publicise a talk at the National Astronomy Meeting. However, the abstract of that talk mentions life only in passing.
Planetary scientist Professor Dave Rothery of the Open University posted in a comment on Facebook, “The Guardian and the RAS disgraced themselves today with the ‘top scientists’ argue case for life on comet’ piece today. I’ve just sat through the talk behind the press release and I think it fair to say that the audience was polite but entirely unconvinced. Diatoms [a type of micro-organism] in comets, my arse!”
Stuart Clark is the author of Is There Life on Mars? (Quercus). Follow him on Twitter.
ABOUT Rosetta (spacecraft)
Rosetta is a space probe built by the European Space Agency launched on 2 March 2004. Along with Philae, its lander module, Rosetta is performing a detailed study of comet 67P/Churyumov–Gerasimenko During its journey to the comet, the spacecraft flew by Mars and the asteroids 21 Lutetia and 2867 Šteins.
On 6 August 2014, the spacecraft reached the comet and performed a series of manoeuvres to be captured in its orbit. On 12 November, the lander module performed the first successful landing on a comet, though its battery power ran out two days later.
Communications with Philae were briefly restored in June and July 2015, but due to diminishing solar power, Rosetta's communications module with the lander was turned off on 27 July 2016.
As of 2016, the mission continues to return data from the spacecraft in orbit.
The probe is named after the Rosetta Stone, a stele of Egyptian origin featuring a decree in three scripts. The lander is named after the Philae obelisk, which bears a bilingual Greek and Egyptian hieroglyphic inscription. A comparison of its hieroglyphs with those on the Rosetta Stone catalysed the deciphering of the Egyptian writing system.
Similarly, it is hoped that these spacecraft will result in better understanding of comets and the early Solar System.In a more direct analogy to its namesake, the Rosetta spacecraft also carries a micro-etched nickel alloy Rosetta disc donated by the Long Now Foundation inscribed with 13,000 pages of text in 1,200 languages.
Comet Churyumov–Gerasimenko in September 2014 as imaged by Rosetta
Rosetta was launched on 2 March 2004 from the Guiana Space Centre in French Guiana on an Ariane 5 rocket and reached Comet Churyumov–Gerasimenko on 6 August 2014,becoming the first spacecraft to orbit a comet. (Previous missions had conducted successful flybys of seven other comets.)
It is one of ESA's Horizon 2000 cornerstone missions. The spacecraft consists of the Rosetta orbiter, which features 12 instruments, and the Philae lander, with nine additional instruments. The Rosetta mission will orbit Comet Churyumov–Gerasimenko for 17 months and is designed to complete the most detailed study of a comet ever attempted. The spacecraft is controlled from the European Space Operations Centre (ESOC), in Darmstadt, Germany.
The planning for the operation of the scientific payload, together with the data retrieval, calibration, archiving and distribution, is performed from the European Space Astronomy Centre (ESAC), in Villanueva de la Cañada, near Madrid, Spain. It has been estimated that in the decade preceding 2014, some 2,000 people assisted in the mission in some capacity.
In 2007, Rosetta made a Mars gravity assist (flyby) on its way to Comet Churyumov–Gerasimenko. The spacecraft also performed two asteroid flybys. The craft completed its flyby of asteroid 2867 Šteins in September 2008 and of 21 Lutetia in July 2010. Later, on 20 January 2014, Rosetta was taken out of a 31-month hibernation mode as it approached Comet Churyumov–Gerasimenko.
Rosetta's Philae lander successfully made the first soft landing on a comet nucleus when it touched down on Comet Churyumov–Gerasimenko on 12 November 2014.
On September 5th, 2016 ESA announced that the lander was discovered by the narrow-angle camera aboard Rosetta as it was slowly making its descent to the comet. The lander sits on its side wedged into a dark crevice of the comet, explaining the lack of electrical power to etablish proper communication with the probe.
During the 1986 approach of Halley's Comet, international space probes were sent to explore the comet, most prominent among them being ESA's Giotto. After the probes returned valuable scientific information, it became obvious that follow-ons were needed that would shed more light on cometary composition and answer new questions.
Both ESA and NASA started cooperatively developing new probes. The NASA project was the Comet Rendezvous Asteroid Flyby (CRAF) mission. The ESA project was the follow-on Comet Nucleus Sample Return (CNSR) mission. Both missions were to share the Mariner Mark II spacecraft design, thus minimising costs.
In 1992, after NASA cancelled CRAF due to budgetary limitations, ESA decided to develop a CRAF-style project on its own.
By 1993 it was evident that the ambitious sample return mission was infeasible with the existing ESA budget, so the mission was redesigned and subsequently approved by the ESA, with the final flight plan resembling the cancelled CRAF mission: an asteroid flyby followed by a comet rendezvous with in-situ examination, including a lander. After the spacecraft launch, Gerhard Schwehm was named mission manager; he retired in March 2014.
The Rosetta mission planned to achieve many historic firsts.
On its way to comet 67P, Rosetta passed through the main asteroid belt, and made the first European close encounter with several of these primitive objects. Rosetta was the first spacecraft to fly close to Jupiter's orbit using solar cells as its main power source.
Rosetta is the first spacecraft to orbit a comet nucleus, and is the first spacecraft to fly alongside a comet as it heads towards the inner Solar System. It is planned to be the first spacecraft to examine at close proximity how a frozen comet is transformed by the warmth of the Sun. Shortly after its arrival at 67P, the Rosetta orbiter dispatched the Philae lander for the first controlled touchdown on a comet nucleus. The robotic lander's instruments obtained the first images from a comet's surface and made the first in-situ analysis of its composition.
Trajectory of the Rosetta space probe
Rosetta was set to be launched on 12 January 2003 to rendezvous with the comet 46P/Wirtanen in 2011.
This plan was abandoned after the failure of an Ariane 5 carrier rocket during Hot Bird 7's launch on 11 December 2002, grounding it until the cause of the failure could be determined. A new plan was formed to target the comet Churyumov–Gerasimenko, with a revised launch date of 26 February 2004 and comet rendezvous in 2014. The larger mass and the resulting increased impact velocity made modification of the landing gear necessary.
After two scrubbed launch attempts, Rosetta was launched on 2 March 2004 at 7:17 GMT from the Guiana Space Centre in French Guiana. Aside from the changes made to launch time and target, the mission profile remained almost identical.
The first Earth flyby was on 4 March 2005.
On 25 February 2007, the craft was scheduled for a low-altitude flyby of Mars, to correct the trajectory. This was not without risk, as the estimated altitude of the flyby was a mere 250 kilometres (160 mi). During that encounter, the solar panels could not be used since the craft was in the planet's shadow, where it would not receive any solar light for 15 minutes, causing a dangerous shortage of power.
The craft was therefore put into standby mode, with no possibility to communicate, flying on batteries that were originally not designed for this task. This Mars manoeuvre was therefore nicknamed "The Billion Euro Gamble". The flyby was successful, with Rosetta even returning detailed images of the surface and atmosphere of the planet, and the mission continued as planned.
The second Earth flyby was on 13 November 2007 at a distance of 5,700 km (3,500 mi). In observations made on 7 and 8 November, Rosetta was briefly mistaken for a near-Earth asteroid about 20 m (66 ft) in diameter by an astronomer of the Catalina Sky Survey and was given the provisional designation 2007 VN84.
Calculations showed that it would pass very close to Earth, which led to speculation that it could impact Earth. However, astronomer Denis Denisenko recognised that the trajectory matched that of Rosetta, which the Minor Planet Center confirmed in an editorial release on 9 November.
The spacecraft performed a close flyby of asteroid 2867 Šteins on 5 September 2008.
Its onboard cameras were used to fine-tune the trajectory, achieving a minimum separation of less than 800 km (500 mi). Onboard instruments measured the asteroid from 4 August to 10 September.
Maximum relative speed between the two objects during the flyby was 8.6 km/s (19,000 mph; 31,000 km/h).
Rosetta's signal received at ESOC in Darmstadt, Germany, on 20 January 2014
Rosetta's third and final flyby of Earth happened on 12 November 2009
On 10 July 2010, Rosetta flew by 21 Lutetia, a large main-belt asteroid, at a minimum distance of 3,168±7.5 km (1,969±4.7 mi) at a velocity of 15 kilometres per second (9.3 mi/s).
The flyby provided images of up to 60 metres (200 ft) per pixel resolution and covered about 50% of the surface, mostly in the northern hemisphere. The 462 images were obtained in 21 narrow- and broad-band filters extending from 0.24 to 1 μm. Lutetia was also observed by the visible–near-infrared imaging spectrometer VIRTIS, and measurements of the magnetic field and plasma environment were taken as well.
In May 2014, Rosetta began a series of eight burns. These reduced the relative velocity between the spacecraft and 67P from 775 m/s (2,540 ft/s) to 7.9 m/s (26 ft/s).
Orbit around 67P
In August 2014, Rosetta rendezvoused with the comet 67P/Churyumov–Gerasimenko (67P) and commenced a series of manoeuvres that took it on two successive triangular paths, averaging 100 and 50 kilometres (62 and 31 mi) from the nucleus, whose segments are hyperbolic escape trajectories alternating with thruster burns. After closing to within about 30 km (19 mi) from the comet on 10 September, the spacecraft entered actual orbit about it.
The surface layout of 67P was unknown before Rosetta's arrival. The orbiter mapped the comet in anticipation of detaching its lander. By 25 August 2014, five potential landing sites had been determined. On 15 September 2014, ESA announced Site J, named Agilkia in honour of Agilkia Island by an ESA public contest and located on the "head" of the comet, as the lander's destination.
Rosetta and Philae
Philae detached from Rosetta on 12 November 2014 at 08:35 UTC, and approached 67P at a relative speed of about 1 m/s (3.6 km/h; 2.2 mph). It initially landed on 67P at 15:33 UTC, but bounced twice, coming to rest at 17:33 UTC. Confirmation of contact with 67P reached Earth at 16:03 UTC.
On contact with the surface, two harpoons were to be fired into the comet to prevent the lander from bouncing off as the comet's escape velocity is only around 1 m/s (3.6 km/h; 2.2 mph).
Analysis of telemetry indicated that the surface at the initial touchdown site is relatively soft, covered with a layer of granular material about 0.82 feet (0.25 meters) deep, and that the harpoons had not fired upon landing.
After landing on the comet, Philae had been scheduled to commence its science mission, which included:
•Determination of the chemical compounds present, including amino acid enantiomers
•Study of comet activities and developments over time
•Philae landed oddly, in the shadow of a nearby cliff and canted at an angle of around 30 degrees.
This made it unable to adequately collect solar power, and it lost contact with Rosetta when its batteries ran out after two days, well before much of the planned science objectives could be attempted. Contact was briefly and intermittently reestablished several months later at various times between 13 June and 9 July, before contact was lost once again.
There was no communication afterwards, and the transmitter to communicate with Philae was switched off in July 2016 to reduce power consumption of the probe. The precise location of the lander was discovered in September 2016 when Rosetta came closer to the comet and took high-resolution pictures of its surface.
Knowing its exact location provides information needed to put Philae's two days of science into proper context.
One of the first discoveries was that the magnetic field of 67P oscillated at 40–50 millihertz.
Scientists modified the signal by speeding it up 10,000 times so that people could hear a rendition of it. Although it is a natural phenomenon, it has been described as a "song" and has been compared to Continuum for harpsichord by György Ligeti.
However, results from Philae's landing show that the comet's nucleus has no magnetic field, and that the field originally detected by Rosetta is likely caused by the solar wind.
The isotopic signature of water vapour from comet 67P, as determined by the Rosetta spacecraft, is substantially different from that found on Earth. That is, the ratio of deuterium to hydrogen in the water from the comet was determined to be three times that found for terrestrial water.
This makes it very unlikely that water found on Earth came from comets such as comet 67P, according to the scientists.
On 22 January 2015, NASA reported that, between June and August 2014, the rate at which water vapor was released by the comet increased up to tenfold.
On 2 June 2015, NASA reported that the ALICE spectrograph on Rosetta determined that electrons within 1 km (0.62 mi) above the comet nucleus — produced from photoionization of water molecules by solar radiation, and not photons from the Sun as thought earlier — are responsible for the degradation of water and carbon dioxide molecules released from the comet nucleus into its coma.
WHAT IS A COMET?
Comet 17P/Holmes and its blue ionized tail
Halley's Comet in 1910
A comet is an icy small Solar System body that, when passing close to the Sun, heats up and begins to outgas, displaying a visible atmosphere or coma, and sometimes also a tail.
These phenomena are due to the effects of solar radiation and the solar wind upon the nucleus of the comet. Comet nuclei range from a few hundred metres to tens of kilometres across and are composed of loose collections of ice, dust, and small rocky particles. The coma and tail are much larger and, if sufficiently bright, may be seen from the Earth without the aid of a telescope. Comets have been observed and recorded since ancient times by many cultures.
Comets usually have highly eccentric elliptical orbits, and they have a wide range of orbital periods, ranging from several years to potentially several millions of years. Short-period comets originate in the Kuiper belt or its associated scattered disc, which lie beyond the orbit of Neptune. Long-period comets are thought to originate in the Oort cloud, a spherical cloud of icy bodies extending from outside the Kuiper belt to halfway to the next nearest star. Long-period comets are directed towards the Sun from the Oort cloud by gravitational perturbations caused by passing stars and the galactic tide. Hyperbolic comets may pass once through the inner Solar System before being flung out to interstellar space.
COMETS VS ASTEROIDS
Comets are distinguished from asteroids by the presence of an extended, gravitationally unbound atmosphere surrounding their central nucleus. This atmosphere has parts termed the coma (the central part immediately surrounding the nucleus) and the tail (a typically linear section consisting of dust or gas blown out from the coma by the Sun's light pressure or outstreaming solar wind plasma).
However, extinct comets that have passed close to the Sun many times have lost nearly all of their volatile ices and dust and may come to resemble small asteroids. Asteroids are thought to have a different origin from comets, having formed inside the orbit of Jupiter rather than in the outer Solar System.
The discovery of main-belt comets and active centaurs has blurred the distinction between asteroids and comets.
As of November 2014 there are 5,253 known comets, a number that is steadily increasing. However, this represents only a tiny fraction of the total potential comet population, as the reservoir of comet-like bodies in the outer Solar System (in the Oort cloud) is estimated to be one trillion.
Roughly one comet per year is visible to the naked eye, though many of these are faint and unspectacular. Particularly bright examples are called "Great Comets".
Comets have been visited by unmanned probes such as the European Space Agency's Rosetta, which became the first ever to land a robotic spacecraft on a comet, and NASA's Deep Impact, which blasted a crater on Comet Tempel 1 to study its interior.
Nucleus of 103P/Hartley as imaged during a spacecraft flyby. The nucleus is about 2 km in length.
The solid, core structure of a comet is known as the nucleus. Cometary nuclei are composed of an amalgamation of rock, dust, water ice, and frozen gases such as carbon dioxide, carbon monoxide, methane, and ammonia. As such, they are popularly described as "dirty snowballs" after Fred Whipple's model.
However, some comets may have a higher dust content, leading them to be called "icy dirtballs". Research conducted in 2014 suggests that comets are like "deep fried ice cream", in that their surfaces are formed of dense crystalline ice mixed with organic compounds, while the interior ice is colder and less dense.
The surface of the nucleus is generally dry, dusty or rocky, suggesting that the ices are hidden beneath a surface crust several metres thick. In addition to the gases already mentioned, the nuclei contain a variety of organic compounds, which may include methanol, hydrogen cyanide, formaldehyde, ethanol, and ethane and perhaps more complex molecules such as long-chain hydrocarbons and amino acids.
In 2009, it was confirmed that the amino acid glycine had been found in the comet dust recovered by NASA's Stardust mission.
In August 2011, a report, based on NASA studies of meteorites found on Earth, was published suggesting DNA and RNA components (adenine, guanine, and related organic molecules) may have been formed on asteroids and comets.
The outer surfaces of cometary nuclei have a very low albedo, making them among the least reflective objects found in the Solar System. The Giotto space probe found that the nucleus of Halley's Comet reflects about four percent of the light that falls on it, and Deep Space 1 discovered that Comet Borrelly's surface reflects less than 3.0% of the light that falls on it; by comparison, asphalt reflects seven percent of the light that falls on it.
The dark surface material of the nucleus may consist of complex organic compounds. Solar heating drives off lighter volatile compounds, leaving behind larger organic compounds that tend to be very dark, like tar or crude oil. The low reflectivity of cometary surfaces enables them to absorb the heat necessary to drive their outgassing processes.
Comet nuclei with radii of up to 30 kilometres (19 mi) have been observed, but ascertaining their exact size is difficult.
Roughly six percent of the near-Earth asteroids are thought to be extinct nuclei of comets that no longer experience outgassing, including 14827 Hypnos and 3552 Don Quixote.
Results from the Rosetta and Philae spacecraft show that the nucleus of 67P/Churyumov–Gerasimenko has no magnetic field, which suggests that magnetism may not have played a role in the early formation of planetesimals.
Further, the ALICE spectrograph on Rosetta determined that electrons (within 1 km (0.62 mi) above the comet nucleus) produced from photoionization of water molecules by solar radiation, and not photons from the Sun as thought earlier, are responsible for the degradation of water and carbon dioxide molecules released from the comet nucleus into its coma.
Instruments on the Philae lander found at least sixteen organic compounds at the comet's surface, four of which (acetamide, acetone, methyl isocyanate and propionaldehyde) have been detected for the first time on a comet.
Hubble image of Comet ISON shortly before perihelion.
The streams of dust and gas thus released form a huge and extremely thin atmosphere around the comet called the "coma", and the force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous "tail" to form pointing away from the Sun.
The coma is generally made of H2O and dust, with water making up to 90% of the volatiles that outflow from the nucleus when the comet is within 3 to 4 astronomical units (450,000,000 to 600,000,000 km; 280,000,000 to 370,000,000 mi) of the Sun.
The H2O parent molecule is destroyed primarily through photodissociation and to a much smaller extent photoionization, with the solar wind playing a minor role in the destruction of water compared to photochemistry. Larger dust particles are left along the comet's orbital path whereas smaller particles are pushed away from the Sun into the comet's tail by light pressure.
Both the coma and tail are illuminated by the Sun and may become visible when a comet passes through the inner Solar System, the dust reflecting Sunlight directly and the gases glowing from ionisation.
Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible to the naked eye. Occasionally a comet may experience a huge and sudden outburst of gas and dust, during which the size of the coma greatly increases for a period of time. This happened in 2007 to Comet Holmes.
In 1996, comets were found to emit X-rays. This greatly surprised astronomers because X-ray emission is usually associated with very high-temperature bodies. The X-rays are generated by the interaction between comets and the solar wind: when highly charged solar wind ions fly through a cometary atmosphere, they collide with cometary atoms and molecules, "stealing" one or more electrons from the atom in a process called "charge exchange".
This exchange or transfer of an electron to the solar wind ion is followed by its de-excitation into the ground state of the ion, leading to the emission of X-rays and far ultraviolet photons.
In the outer Solar System, comets remain frozen and inactive and are extremely difficult or impossible to detect from Earth due to their small size. As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them.
The streams of dust and gas each form their own distinct tail, pointing in slightly different directions. The tail of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail called the type II or dust tail. At the same time, the ion or type I tail, made of gases, always points directly away from the Sun because this gas is more strongly affected by the solar wind than is dust, following magnetic field lines rather than an orbital trajectory. On occasions - such as when the Earth passes through a comet's orbital plane, and we see the track of the comet edge-on, a tail pointing in the opposite direction to the ion and dust tails may be seen – the antitail.
The observation of antitails contributed significantly to the discovery of solar wind. The ion tail is formed as a result of the ionisation by solar ultra-violet radiation of particles in the coma. Once the particles have been ionized, they attain a net positive electrical charge, which in turn gives rise to an "induced magnetosphere" around the comet.
The comet and its induced magnetic field form an obstacle to outward flowing solar wind particles. Because the relative orbital speed of the comet and the solar wind is supersonic, a bow shock is formed upstream of the comet in the flow direction of the solar wind. In this bow shock, large concentrations of cometary ions (called "pick-up ions") congregate and act to "load" the solar magnetic field with plasma, such that the field lines "drape" around the comet forming the ion tail.
If the ion tail loading is sufficient, then the magnetic field lines are squeezed together to the point where, at some distance along the ion tail, magnetic reconnection occurs. This leads to a "tail disconnection event". This has been observed on a number of occasions, one notable event being recorded on 20 April 2007, when the ion tail of Encke's Comet was completely severed while the comet passed through a coronal mass ejection. This event was observed by the STEREO space probe.
In 2013 ESA scientists reported that the ionosphere of the planet Venus streams outwards in a manner similar to the ion tail seen streaming from a comet under similar conditions."
Uneven heating can cause newly generated gases to break out of a weak spot on the surface of comet's nucleus, like a geyser. These streams of gas and dust can cause the nucleus to spin, and even split apart.
In 2010 it was revealed dry ice (frozen carbon dioxide) can power jets of material flowing out of a comet nucleus. This is known because a spacecraft got so close that it could see where the jets were coming out, and then measure the infrared spectrum at that point which shows what some of the materials are.
Most comets are small Solar System bodies with elongated elliptical orbits that take them close to the Sun for a part of their orbit and then out into the further reaches of the Solar System for the remainder.
Comets are often classified according to the length of their orbital periods: The longer the period the more elongated the ellipse.
Periodic comets or short-period comets are generally defined as having orbital periods of less than 200 years. They usually orbit more-or-less in the ecliptic plane in the same direction as the planets.
Their orbits typically take them out to the region of the outer planets (Jupiter and beyond) at aphelion; for example, the aphelion of Halley's Comet is a little beyond the orbit of Neptune. Comets whose aphelia are near a major planet's orbit are called its "family". Such families are thought to arise from the planet capturing formerly long-period comets into shorter orbits.
At the shorter extreme, Encke's Comet has an orbit that does not reach the orbit of Jupiter, and is known as an Encke-type comet. Short-period comets with orbital periods shorter than 20 years and low inclinations (up to 30 degrees) are called Jupiter-family comets (JFCs). Those like Halley, with orbital periods of between 20 and 200 years and inclinations extending from zero to more than 90 degrees, are called Halley-type comets (HTCs).
As of 2015, only 75 HTCs have been observed, compared with 511 identified JFCs.
Recently discovered main-belt comets form a distinct class, orbiting in more circular orbits within the asteroid belt.
Because their elliptical orbits frequently take them close to the giant planets, comets are subject to further gravitational perturbations. Short-period comets have a tendency for their aphelia to coincide with a giant planet's semi-major axis, with the JFCs being the largest group. It is clear that comets coming in from the Oort cloud often have their orbits strongly influenced by the gravity of giant planets as a result of a close encounter. Jupiter is the source of the greatest perturbations, being more than twice as massive as all the other planets combined. These perturbations can deflect long-period comets into shorter orbital periods.
Long-period comets have highly eccentric orbits and periods ranging from 200 years to thousands of years. An eccentricity greater than 1 when near perihelion does not necessarily mean that a comet will leave the Solar System. For example, Comet McNaught had a heliocentric osculating eccentricity of 1.000019 near its perihelion passage epoch in January 2007 but is bound to the Sun with roughly a 92,600-year orbit because the eccentricity drops below 1 as it moves further from the Sun.
The future orbit of a long-period comet is properly obtained when the osculating orbit is computed at an epoch after leaving the planetary region and is calculated with respect to the center of mass of the Solar System.
By definition long-period comets remain gravitationally bound to the Sun; those comets that are ejected from the Solar System due to close passes by major planets are no longer properly considered as having "periods".
The orbits of long-period comets take them far beyond the outer planets at aphelia, and the plane of their orbits need not lie near the ecliptic. Long-period comets such as Comet West and C/1999 F1 can have aphelion distances of nearly 70,000 AU with orbital periods estimated around 6 million years.
Some authorities use the term "periodic comet" to refer to any comet with a periodic orbit (that is, all short-period comets plus all long-period comets), whereas others use it to mean exclusively short-period comets.
Similarly, although the literal meaning of "non-periodic comet" is the same as "single-apparition comet", some use it to mean all comets that are not "periodic" in the second sense (that is, to also include all comets with a period greater than 200 years).
Early observations have revealed a few genuinely hyperbolic (i.e. non-periodic) trajectories, but no more than could be accounted for by perturbations from Jupiter.
If comets pervaded interstellar space, they would be moving with velocities of the same order as the relative velocities of stars near the Sun (a few tens of km per second). If such objects entered the Solar System, they would have positive specific orbital energy and would be observed to have genuinely hyperbolic trajectories.
A rough calculation shows that there might be four hyperbolic comets per century within Jupiter's orbit, give or take one and perhaps two orders of magnitude.
Effects of comets
Diagram of Perseids meteors
As a result of outgassing, comets leave in their wake a trail of solid debris too large to be swept away by radiation pressure and the solar wind.
If the comet's path crosses the path the Earth follows in orbit around the Sun, then at that point there are likely to be meteor showers as Earth passes through the trail of debris. The Perseid meteor shower, for example, occurs every year between 9 and 13 August, when Earth passes through the orbit of Comet Swift–Tuttle.
Halley's Comet is the source of the Orionid shower in October.
Comets and impact on life
Many comets and asteroids collided into Earth in its early stages. Many scientists think that comets bombarding the young Earth about 4 billion years ago brought the vast quantities of water that now fill the Earth's oceans, or at least a significant portion of it. Other researchers have cast doubt on this idea.
The detection of organic molecules, including polycyclic aromatic hydrocarbons, in significant quantities in comets has led some to speculate that comets or meteorites may have brought the precursors of life—or even life itself—to Earth.
In 2013 it was suggested that impacts between rocky and icy surfaces, such as comets, had the potential to create the amino acids that make up proteins through shock synthesis.
In 2015, scientists found significant amounts of molecular oxygen in outgassings from comet 67P, an indicator that presence of that molecule may occur naturally more often than it had been thought, and thus that it may not be as strong an indicator of alien life as has been supposed.
It is suspected that comet impacts have, over long timescales, also delivered significant quantities of water to the Earth's Moon, some of which may have survived as lunar ice.
Comet and meteoroid impacts are also thought to be responsible for the existence of tektites and australites.
Fate of comets
Departure (ejection) from Solar System
If a comet is traveling fast enough, it may leave the Solar System; such is the case for hyperbolic comets.
To date, comets are only known to be ejected by interacting with another object in the Solar System, such as Jupiter. An example of this is thought to be Comet C/1980 E1, which was shifted from a predicted orbit of 7.1 million years around the Sun, to a hyperbolic trajectory, after a 1980 encounter with the planet Jupiter.
Jupiter-family comets and long-period comets appear to follow very different fading laws. The JFCs are active over a lifetime of about 10,000 years or ~1,000 orbits whereas long-period comets fade much faster. Only 10% of the long-period comets survive more than 50 passages to small perihelion and only 1% of them survive more than 2,000 passages.
Eventually most of the volatile material contained in a comet nucleus evaporates away, and the comet becomes a small, dark, inert lump of rock or rubble that can resemble an asteroid.
Some asteroids in elliptical orbits are now identified as extinct comets. Roughly six percent of the near-Earth asteroids are thought to be extinct nuclei of comets that no longer emit gas.
Breakup and collisions
The nucleus of some comets may be fragile, a conclusion supported by the observation of comets splitting apart.
A significant cometary disruption was that of Comet Shoemaker–Levy 9, which was discovered in 1993. A close encounter in July 1992 had broken it into pieces, and over a period of six days in July 1994, these pieces fell into Jupiter's atmosphere—the first time astronomers had observed a collision between two objects in the Solar System.
Other splitting comets include 3D/Biela in 1846 and 73P/Schwassmann–Wachmann from 1995 to 2006. Greek historian Ephorus reported that a comet split apart as far back as the winter of 372–373 BC. Comets are suspected of splitting due to thermal stress, internal gas pressure, or impact.
Comets 42P/Neujmin and 53P/Van Biesbroeck appear to be fragments of a parent comet. Numerical integrations have shown that both comets had a rather close approach to Jupiter in January 1850, and that, before 1850, the two orbits were nearly identical.
Some comets have been observed to break up during their perihelion passage, including great comets West and Ikeya–Seki. Biela's Comet was one significant example, when it broke into two pieces during its passage through the perihelion in 1846.
These two comets were seen separately in 1852, but never again afterward. Instead, spectacular meteor showers were seen in 1872 and 1885 when the comet should have been visible.
A minor meteor shower, the Andromedids, occurs annually in November, and it is caused when the Earth crosses the orbit of Biela's Comet.
Some comets meet a more spectacular end – either falling into the Sun or smashing into a planet or other body.
Collisions between comets and planets or moons were common in the early Solar System: some of the many craters on the Moon, for example, may have been caused by comets.
A recent collision of a comet with a planet occurred in July 1994 when Comet Shoemaker–Levy 9 broke up into pieces and collided with Jupiter.