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Category Archives: 08. Pluto, comets, and space debris

In an article in the New York Times, January 31, 2015, Peter Brannen reports on the ongoing debate about what caused the extinction of the non-bird dinosaurs at the end of the Cretaceous period (see A Closer Look 8.5, p. 220).

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Credit: Emiliano Ponzi/NY Times

While to many, the NEO impact theory and the discovery of the Chicxulub impact crater is compelling evidence, some geologists point to enormous floods of lava in India, called the Deccan Traps, as an alternate explanation.

Read more about the debate here.

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From a JAXA press release, December 3, 2014:

Mitsubishi Heavy Industries, Ltd. and the Japan Aerospace Exploration Agency (JAXA) successfully launched the H-IIA Launch Vehicle No. 26 with the Asteroid Explorer “Hayabusa2” on board at 1:22 p.m. on December 3, 2014 (Japan Standard Time) from the Tanegashima Space Center. The launch vehicle flew as planned, and at approximately one hour, 47 minutes and 21 seconds after liftoff, the separation of the Hayabusa2 to Earth-escape trajectory was confirmed.

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Credit: JAXA

The asteroid explorer “Hayabusa2” is a successor to the “Hayabusa”, which verified various new exploration technologies and returned to Earth in June 2010. “Hayabusa2” is setting out on a journey to clarify the origin and evolution of the Solar System as well as search for organic matter.

Links: JAXA press release, including detailed flight sequence.

Abridged from a New Scientist article by Rebecca Boyle, September 30, 2014:

A newly discovered asteroid called 2014 OL339 is the latest quasi-satellite of Earth – a space rock that orbits the Sun but is close enough to Earth to look like a companion. The asteroid has been hanging out near Earth for about 775 years, but its orbit is unstable – it will probably move on about 165 years from now.

Credit: NASA

Credit: NASA

Quasi-satellites orbit in resonance with Earth, allowing our planet’s gravity to shift the rock’s position. The asteroid orbits the Sun every 365 days, as Earth does, but Earth’s gravity guides it into an eccentric wobble, which causes the rock to appear to circle backward around the planet.

The asteroid, which is between 90 and 200 metres in diameter, is among several different categories of space rock in Earth’s retinue besides our one satellite, the Moon. Rocks that hang out at a gravitational middle ground known as a Lagrange point, where they follow or lead Earth in its orbit, are called Trojans.

Links: The full New Scientist article; NASA’s Near-Earth Object program.

Astronomy Picture of the Day (APOD) for November 1, 2014, shows Mars the day after Comet Siding Spring’s close encounter, with the comet visible at the edge of its overexposed disk.

Credit & copyright: Rolando Ligustri (CARA Project, CAST)

The caption describes: “this comet [came] within 86,700 miles or so of Mars, about one-third the Earth-Moon distance. Earth’s spacecraft and rovers in Mars orbit and on the surface reported no ill effects though, and had a ringside seat as a visitor from the outer Solar System passed by.”

Adapted from a European Space Agency press release, September 26, 2014:

ESA’s Rosetta mission will deploy its lander, Philae, to the surface of Comet 67P/Churyumov–Gerasimenko on November 12, 2014. Philae’s landing site, currently known as Site J, is located on the smaller of the comet’s two ‘lobes’, with a backup site on the larger lobe. The sites were selected just six weeks after Rosetta arrived at the comet on August 6, following its 10-year journey through the Solar System.

Philae’s primary landing site

Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

The main focus to date has been to survey 67P/Churyumov–Gerasimenko in order to prepare for the first ever attempt to soft-land on a comet. Site J was chosen unanimously over four other candidate sites as the primary landing site because the majority of terrain within a square kilometre area has slopes of less than 30° relative to the local vertical and because there are relatively few large boulders. The area also receives sufficient daily illumination to recharge Philae and continue surface science operations beyond the initial 64-hour battery-powered phase.

Final confirmation of the primary landing site and its landing scenario will be made on October 14 after a formal review, which will include the results of additional high-resolution analysis of the landing site and its back-up conducted in the meantime. Should the backup site be chosen at this stage, the landing attempt can still take place on November 12.

Links: the ESA press release, including links to further resources on Rosetta and Philae and this short movie showing Philae’s planned descent.

Adapted from an ESA press release, August 6, 2014:
After a decade-long journey chasing its target, ESA’s Rosetta has today become the first spacecraft to rendezvous with a comet, opening a new chapter in Solar System exploration. Comet 67P/Churyumov–Gerasimenko and Rosetta now lie 405 million kilometers from Earth, about half way between the orbits of Jupiter and Mars, rushing towards the inner Solar System at nearly 55,000 kilometers per hour.

Comet 67P/Churyumov-Gerasimenko on August 3, 2014

Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

The comet is in an elliptical 6.5-year orbit that takes it from beyond Jupiter at its furthest point, to between the orbits of Mars and Earth at its closest to the Sun. Rosetta will accompany it for over a year as they swing around the Sun and back out towards Jupiter again.
The journey to the comet was not straightforward, however. Since its launch in 2004, Rosetta had to make three gravity-assist flybys of Earth and one of Mars to help it on course to its rendezvous with the comet. It has traveled for ten years, five months and four days, clocking up 6.4 billion kilometers. Its complex course also allowed Rosetta to pass by asteroids Šteins and Lutetia, obtaining unprecedented views and scientific data on these two objects.

August 6 saw the last of a series of ten rendezvous manoeuvres that began in May to adjust Rosetta’s speed and trajectory gradually to match those of the comet. If any of these manoeuvres had failed, the mission would have been lost, and the spacecraft would simply have flown by the comet.

Links: the ESA press release, including further images; Rosetta fact-sheet.

From a press release of the Max Planck Institute for Solar System Research, July 17, 2014:
As ESA’s spacecraft Rosetta is slowly approaching its destination, Comet 67P/Churyumov-Gerasimenko (see p. 213) is again proving to be full of surprises. New images obtained by OSIRIS, the onboard scientific imaging system, confirm the body’s peculiar shape that earlier pictures had hinted at. 67P is obviously quite unlike any other comet visited so far. (See also previous post.)

“The distance still separating Rosetta from 67P is now far from astronomical,” says OSIRIS principal investigator Holger Sierks from the Max Planck Institute for Solar System Research (MPS) in Germany. “It’s a trip of less than 12,000 kilometers. That’s comparable to travelling from Germany to Hawaii.”

However, while taking a snapshot of Mauna Kea, Hawaii’s highest mountain, from Germany is an impossible feat, Rosetta’s camera OSIRIS is doing a great job at catching ever clearer glimpses of its similarly sized destination. Images obtained on July 14th clearly show a tantalizing shape. The comet’s nucleus consists of two distinctly separated parts.

Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

“This is unlike any other comet we have ever seen before,” says OSIRIS project manager Carsten Güttler from the MPS. “The images faintly remind me of a rubber ducky with a body and a head,” he adds with a laugh. How 67P received this duck-like shape is still unclear. “At this point we know too little about 67P to allow for more than an educated guess,” says Sierks. In the next months, the scientists hope to determine more of the comet’s physical and mineralogical properties. These could help decide, whether the comet’s body and head were formerly two individual bodies.

In order to get an idea of what seems to be a very unique body, the observed image data can be interpolated to create a smoother shape. “There is, of course, still uncertainty in these processed, filtered images and the surface will not be as smooth as it now appears,” Güttler points out. “But they help us the get a first idea.”

Links: the original MPS press release and media files; an interactive movie (requires a Flash player) depicting Rosetta’s journey through the Solar System to reach Comet 67/P C-G.

Adapted from an ESA press release, July 17, 2014:
New images of Comet 67P/Churyumov-Gerasimenko reveal an extraordinarily irregular shape (see p. 213). It has become clear that this is no ordinary comet. Like its name, it seems that comet 67P/C-G is in two parts.

Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

ESA scientists have made this movie, using a sequence of 36 interpolated images each separated by 20 minutes, providing a truly stunning 360-degree preview of the overall complex shape of the comet. It supports the presence of two definite components – one segment seems to be rather elongated, while the other appears more bulbous. Indeed, some people have already likened the shape to a duck, with a distinct body and head. Note that the comet’s surface features won’t be as smooth as these processed images imply.

Dual objects like this – known as ‘contact binaries’ in comet and asteroid terminology – are not uncommon. Indeed, Comet 8P/Tuttle is thought to be such a contact binary; radio imaging by the ground-based Arecibo telescope in Puerto Rico in 2008 suggested that it comprises two sphere-like objects. Meanwhile, the bone-shaped Comet 103P/Hartley 2, imaged during NASA’s EPOXI flyby in 2011, revealed a comet with two distinct halves separated by a smooth region. In addition, observations of asteroid 25143 Itokawa by JAXA’s Hayabusa mission, combined with ground-based data, suggest an asteroid comprising two sections of highly contrasting densities.

Is Rosetta en-route to rendezvous with a similar breed of comet? The scientific rewards of studying such a comet would be high, as a number of possibilities exist as to how they form.

Links and further resources: the full ESA press release; link to unprocessed (still) image; link to movie.

Adapted from Carnegie Institution of Science press release, March 26, 2014:

A new distant dwarf planet, called 2012 VP113, has been discovered beyond the known edge of the Solar System. It is likely one of thousands of distant objects that are thought to form the so-called inner Oort cloud (see Section 8.2, p. 202). The findings were published March 27 in the journal Nature.

Credit: Scott Sheppard (Carnegie Institution of Science)

The known Solar System can be divided into three parts: the rocky planets like Earth, which are close to the Sun; the gas giant planets, which are further out; and the frozen objects of the Kuiper belt, which lie just beyond Neptune’s orbit. Beyond this, there appears to be an edge to the Solar System where only one object, Sedna, was previously known to exist for its entire orbit. But the newly found 2012 VP113 has an orbit that beyond Sedna’s, making it the furthest known in the solar system. The discovery of 2012 VP113 shows us that Sedna is not unique and is likely the second known member of the hypothesized inner Oort cloud, the likely origin of some comets.

2012 VP113’s closest orbit point to the Sun brings it to about 80 times the distance of the Earth from the Sun (80 AU). The Kuiper belt (composed of thousands of icy objects, including Pluto) ranges from 30 to 50 AU. The Solar System has a distinct edge at 50 AU – prior to this discovery, Sedna was the only object known to stay significantly beyond this outer boundary at 76 AU for its entire orbit.

Links: Carnegie Institution press release; NY Time coverage.

Our Solar System seems like a neat and orderly place, with small, rocky worlds near the Sun and big, gaseous worlds farther out, all eight planets following orbital paths unchanged since they formed. However, the true history of the Solar System is far more riotous. Giant planets migrated in and out, tossing interplanetary flotsam and jetsam far and wide.

New clues to this tumultuous past come from the asteroid belt. Millions of asteroids circle the Sun between the orbits of Mars and Jupiter, in a region known as the main asteroid belt. Traditionally, they were viewed as the pieces of a failed planet that was prevented from forming by the influence of Jupiter’s powerful gravity. Their compositions seemed to vary methodically from drier to wetter, due to the drop in temperature as you move away from the Sun.

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Credit: David A. Aguilar (CfA)

This traditional view has changed, as astronomers have recognized that the current residents of the main asteroid belt weren’t all there from the start. In the early history of the Solar System the giant planets ran amok, migrating inward and outward substantially. Jupiter may have moved as close to the Sun as Mars is now. In the process, it swept the asteroid belt nearly clean, leaving only a tenth of one percent of its original population. As the planets migrated, they stirred the contents of the Solar System. Objects from as close to the Sun as Mercury, and as far out as Neptune, all collected in the main asteroid belt.

Using data from the Sloan Digital Sky Survey, astronomers have examined the compositions of thousands of asteroids within the main belt. They found that the asteroid belt is more diverse than previously realized, especially when you look at the smaller asteroids. This finding has interesting implications for the history of Earth. Astronomers have theorized that long-ago asteroid impacts delivered much of the water now filling Earth’s oceans. If true, the stirring provided by migrating planets may have been essential to bringing those asteroids.

This raises the question of whether an Earth-like exoplanet would also require a rain of asteroids to bring water and make it habitable. If so, then Earth-like worlds might be rarer than we thought.  The paper describing these findings appears in the January 30, 2014 issue of Nature.

Links: Harvard-Smithsonian Center for Astrophysics press release; Nature article.