Topic: Late August Astronomy Bulletin

[Clive]

MARS ORBITERS TO ENCOUNTER COMET
The Planetary Society

Comet Siding Spring will pass within 134,000 kilometres of Mars on October 19. More importantly for us on Earth, the comet will pass within a few hundred thousand kilometres of a veritable fleet of scientific spacecraft. But they are not comet missions, they are Mars orbiters. Siding Spring is a long-period comet on its first visit to the inner Solar System. Will spacecraft designed to study a planet up close be able to get good observations of a tiny object that is much further away? Spacecraft safety is still the most important concern. Comets may be small, but they're surrounded by a vast cloud, or coma, of dust and ice particles shed from their active nuclei. For the rovers on the surface of Mars there is no problem: Mars' atmosphere is thick enough for such tiny particles to burn up without reaching the ground. The worry has been about the orbiters. Astronomers think that they are probably safe, but out of an abundance of caution, all three orbiters currently at Mars are shifting the timing of their orbits just slightly so that they will wind up on the far side of Mars from the comet when Mars' path through the tail reaches the region of highest dust density, about 100 minutes after the closest approach of the comet. The two orbiters on their way to Mars will also perform adjustments to keep on the safe side at the right time.

Every spacecraft plans to observe, some more than others, according to what their respective instruments can accomplish. Most of the work will focus on the comet's coma -- its extent, its composition, how it varies with time, how big are the particles in it, how it is supplied by jets from the nucleus. Another major area of study will be the comet's effect on the Martian atmosphere -- a comet's coma impinging on the atmosphere of a planet has never been studied before. And a single spacecraft may possibly be able to image the tiny nucleus of the comet as it passes by. The comet will be moving extremely fast, at a relative speed of 57 km/s. Its nucleus is likely to be only 1 or 2 kilometres across, but its neutral-gas coma extends for more than 100,000 kilometres in all directions from the nucleus. Ions may be farther away than that, and the tail stretches for millions of kilometres. So while most of the instruments will not even detect the comet's nucleus, most will be able to see the coma or the coma's effects on the atmosphere.

Here is a brief summary of the spacecraft at Mars, and the instruments they carry.

1. Mars Reconnaissance Orbiter, arrived 2006. Has 3 cameras plus an imaging spectrometer and a radar sounder.

2. Mars Express, arrived 2004. Will use HRSC camera and ultraviolet/infrared atmospheric spectrometer.

3. Mars Odyssey, arrived 2001. Will use THEMIS thermal emission imaging system.

4. MAVEN, arriving 2014. Has a suite of instruments devoted to Mars' upper atmosphere (no camera).

5. Mars Orbiter Mission, arriving 2014. Has a varied instrument suite but unknown if it will be performing Siding Spring observations.

The rovers are also planning to observe, but have constraints such that their plans will be fixed only the day before they are executed.

GRAINS CAPTURED BY STARDUST PROBABLY FROM INTERSTELLAR SPACE
University of California - Berkeley

Since 2006, when the Stardust spacecraft delivered its aerogel and aluminium-foil dust collectors to Earth, a team of scientists has combed through the collectors in search of rare, microscopic particles of interstellar dust. The team now reports that it has found seven dust motes that probably came from outside our Solar System, perhaps created in a supernova explosion millions of years ago and altered by aeons of exposure to the extremes of space. If that can be confirmed, they would be the first samples of contemporary interstellar dust. Additional tests must still be done before the team can say definitely that they are debris from interstellar space. But if they are, the particles could help to explain the origin and evolution of interstellar dust that until now could only be guessed from astronomical observations. Specifically, the particles are much more diverse in terms of chemical composition and structure than interstellar dust has been thought to be. The small ones differ considerably from the big ones, and may have had different histories, and many of the big ones have a fluffy structure, like snowflakes. The fact that the two largest fluffy particles have crystalline material -- a magnesium-iron-silicate mineral called olivine -- may imply that they came from discs around other stars and were modified in the interstellar medium. We seem to be getting our first glimpse of the surprising diversity of interstellar dust particles, which is impossible to explore through astronomical observations alone.

Two particles, each only about two microns in diameter, were isolated from the light, fluffy aerogel detectors after their tracks were discovered by volunteers calling themselves 'Dusters' who scanned more than a million images through Stardust@home, a citizen-science project that proved critical to finding those needles in a haystack. An additional 100 tracks found by Dusters have yet to be analyzed, and only 77 of the 132 aerogel panels have been scanned to date.  Scientists expect to find no more than a dozen particles of interstellar dust in all -- a millionth of the amount of cometary material picked up by other collectors that were on board Stardust.  The two aerogel-embedded particles are destined for further tests to determine their oxygen-isotope abundances, which could provide even stronger evidence for an extra-solar origin. Supernovae, red giants, and other evolved stars produce interstellar dust and generate heavy elements like carbon, nitrogen and oxygen.

Stardust was launched in 1999 to fly through the coma of Comet Wild-2 and capture cometary dust with aerogel tiles and aluminium foilsmounted on the front of a two-sided collector. Collectors mounted on the rear were designed to catch particles from the 'snowstorm' of interstellar dust streaming through the Galaxy. Such dust is relatively new, since the lifetime of interstellar dust is 'only' 50 to 100 million years, so we are sampling our contemporary Galaxy. The separate comet and interstellar dust collectors were dropped by parachute as Stardust flew by the Earth in 2006.

MILKY WAY LESS MASSIVE THAN PREVIOUSLY THOUGHT
RAS

The Milky Way is less massive than astronomers previously thought, according to new research. Scientists have recently been able to make a more accurate estimate than before of the mass of our Galaxy.  A team led by researchers at the University of Edinburgh has found that the Milky Way is approximately half the mass of the Andromeda galaxy, which has a similar structure to our own. The Milky Way and Andromeda are the two largest members of a cluster of galaxies which astronomers call the Local Group. Both galaxies have a spiral shape and appear to be of similar dimensions, but until now scientists had been unable to prove which is more massive, as previous studies were able to measure only the mass enclosed within both galaxies' inner regions. Astronomers used recently published data on the known distances between galaxies, as well as their velocities, to calculate the total masses of Andromeda and the Milky Way. They also found that so-called 'dark' matter makes up 90% of the mass in both systems.

Dark matter is a non-understood invisible substance which makes up most of the outer regions of galaxies and (it has been said) around 27% of the content of the Universe. The researchers estimate that Andromeda contains twice as much dark matter as the Milky Way, causing it to be about twice as massive in total. Their work may help astronomers learn more about how the outer regions of galaxies are structured. Astronomers had always suspected that Andromeda is more massive than the Milky Way, but determining their masses has been extremely challenging.

MAP OF DARK MATTER 4.5 BILLION LIGHT-YEARS AWAY
RAS

Using the Hubble telescope, a team of astronomers has mapped the mass within a cluster of galaxies more precisely than before. Created from Hubble's 'Frontier Fields' observing programme, the map shows the amount and distribution of mass within the cluster MCS J0416.1?2403.  Estimates of the amount and distribution of mass within distant objects is difficult, but attempts have been made by studying the gravitational effects the objects have on light from even-more-distant objects behind them. That is one of the main goals of 'Frontier Fields'.

Around three-quarters of all the matter in the Universe is thought to be so-called 'dark matter' [27% according to the previous item, so you can tell how much credence to put in these items! -- ED] which cannot be seen directly as it does not emit or reflect any light, and can pass through other matter without friction (it is collisionless).  It interacts only by gravity, and its presence must be deduced from its gravitational effects. One of the effects was predicted by Einstein and makes large clumps of mass warp and distort the space-time around them. Acting like lenses, they appear to magnify and bend light that travels through them from more-distant objects.  Despite their large masses, galaxy clusters' effects on their surroundings are usually quite minimal. For the most part they cause what is known as weak lensing, making even-more-distant sources appear as only slightly more elliptical or smeared across the sky.  However, when the cluster is large and dense enough and the alignment of cluster and distant object is just right, the effects can be more dramatic. The images of normal galaxies can be transformed into rings and sweeping arcs of light, even appearing several times within the same image. That effect is known as strong lensing, and it is that phenomenon, seen around the six galaxy clusters targeted by the Frontier Fields programme, that has been used to map the mass distribution of MCS J0416.1?2403. Even though strong lensing magnifies the background galaxies, they are still very far away and very faint.

The astronomers identified 51 multiply-imaged galaxies around the cluster. Because the galaxies are seen several times, there are almost 200 individual strongly lensed images which can be seen across the frame. That has allowed the team to calculate the distribution of visible and dark matter in the cluster and produce a highly constrained map of its mass. The total mass within MCS J0416.1-2403, modelled to be over 200 kiloparsecs across, was found to be 1.6 x 10 to the 14 times the mass of the Sun, with an unrealistically small claimed uncertainty of 0.5%. By pinpointing where the mass resides within clusters like that one, the astronomers are also measuring the warping of space-time with some precision. The team will continue to study the cluster using imaging and lensing information to try to map the outer regions of the cluster as well as its inner core, and may thus be able to detect sub-structures in the cluster's surroundings.

FARTHEST LENSING GALAXY YIELDS CLUES TO EARLY UNIVERSE
Space Telescope Science Institute (STScI)

Astronomers have unexpectedly discovered the most distant galaxy that acts as a cosmic lens. Seen in a new image as it looked 9.6 billion years ago, the monster elliptical galaxy breaks the previous record-holder by 200 million years. The object behind the cosmic lens is a tiny spiral galaxy undergoing a rapid burst of star formation. Its light has taken 10.7 billion years to arrive here. The team used the gravitational lensing from the chance alignment to estimate the giant galaxy's total mass, including the amount of dark matter, from the intensity of its lensing effects on the background galaxy's light.  The giant foreground galaxy has a mass of 18 to the 11 solar masses and is a massive galaxy for its time. It is also one of the brightest members of a distant cluster of galaxies, called IRC 0218. Hundreds of lens galaxies are known, but almost all of them are relatively 'nearby' in cosmic terms.

The team suspects that the lensing galaxy must have continued to grow over the past 9 billion years, gaining stars and dark matter by cannibalizing neighbouring galaxies. Recent studies suggest that massive galaxies gain more dark matter than stars as they continue to grow. Astronomers had assumed that dark matter and normal matter build up equally in a galaxy over time, but they now believe that the ratio of dark matter to normal matter changes with time. The newly discovered distant lensing galaxy will eventually become much more massive than the Milky Way and will have more dark matter, too.

The team was studying star formation in two distant galaxy clusters, one of which was IRC 0218, when they stumbled upon the gravitational lens. While analyzing spectrographic data from the Keck Observatory in Hawaii, the astronomers detected hot hydrogen gas that appeared to arise from a giant elliptical galaxy. The detection was surprising because hot hydrogen gas is a clear signature of star birth. Previous observations had shown that the giant elliptical galaxy was an old, sedate galaxy that had stopped making stars a long time ago. Another puzzling discovery was that the young stars were at a much farther distance than the elliptical galaxy. Hubble images taken in blue wavelengths revealed the glow of young stars. The images showed a blue, eyebrow-shaped object next to a smeared blue dot around the massive elliptical which the astronomers recognized as the distorted, magnified images of a more distant galaxy behind the elliptical galaxy, the signature of a gravitational lens. To confirm the gravitational-lens hypothesis, the team analyzed Hubble archival data from previous observing programmes, which turned up another finger-print of hot gas connected to the more distant galaxy. The distant galaxy is too small and far away for Hubble to determine its structure.

MARS ROVER SETS OFF-WORLD DRIVING RECORD
NASA

The 'Opportunity' rover on Mars, which landed there in 2004, now holds the off-Earth roving distance record after accruing 40 kilometres of driving, and is not far from completing the first extra-terrestrial marathon. The previous record was held by the Soviet Lunokhod 2 rover. It is remarkable, since Opportunity was intended to drive only about one kilometre and was never designed for distance. The rover had driven more than 32 kilometres before arriving at Endeavour Crater in 2011, where it has examined outcrops on the crater's rim containing clay and sulphate-bearing minerals. The sites are yielding evidence of ancient environments with less-acidic water than those examined at Opportunity's landing site. If the rover can continue to operate the distance of a marathon -- about 42.2 kilometres -- it will approach the next major investigation site, dubbed by mission scientists "Marathon Valley'. Observations from spacecraft orbiting Mars suggest that several clay minerals are exposed close together there, surrounded by steep slopes where the relationships between different layers may be evident. The Russian Lunokhod 2 rover, a successor to the first Lunokhod mission in 1970, landed on the Moon on 1973 Jan. 15 and drove about 39 kilometres in less than five months, according to measurements recently made on images from Lunar Reconnaissance Orbiter cameras that show Lunokhod 2's tracks.

MARS 2020 ROVER TO EXPLORE PLANET AS NEVER BEFORE
NASA

The next rover that NASA hopes to send to Mars in 2020 will carry seven instruments; its design will be based on that of the highly successful rover, Curiosity, which landed almost two years ago. The new rover will carry improved hardware and new instruments to conduct assessments of the rover's landing site, determine the potential habitability of the environment, and search directly for signs of ancient Martian life. Scientists will use the Mars 2020 rover to identify and select a collection of rock and soil samples that will be stored for potential return to Earth by a future mission. The rover might also help to assess whether future human explorers could use natural resources available on Mars

Designers of future human expeditions could use the mission to understand the hazards posed by Martian dust and demonstrate technology to process carbon dioxide from the atmosphere to produce oxygen. The experiments will help engineers to learn how to use Martian resources to produce oxygen for human respiration and potentially for use as an oxidizer for rocket fuel. The rover will help to answer questions about the Martian environment that astronauts may face, and test technologies they would need. Mars has resources which could reduce the amount of supplies that human missions will need to carry, and better understanding of Martian dust and weather would be of value for planning human Mars missions.