Dust – often thought of as an impediment to lunar exploration – could be put to good use to build giant telescopes on the Moon – perhaps some large enough to fill entire craters, says a team of US researchers.

The team, led by Peter Chen of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, US, has devised a simple method to create a concrete-like substance using a mixture of carbon nanotubes, epoxy and a crushed rock material that NASA uses as a stand-in for Moon dust.

Using the mixture, they built a 30-centimetre disc. Then they added more liquid epoxy to its surface and spun it, coating it with aluminium in a vacuum. They believe the process could be scaled up to produce 20- to 50-metre-wide telescopes on the Moon.

That would be useful, says Chen, since the limited fuel and cargo capacities of rockets make it unfeasible to launch large mirrors or telescopes to the Moon. “By this technique, we are no longer restricted by how much a rocket can carry,” Chen said.

Concrete igloos

To make a 2.4-metre mirror like Hubble’s, Chen estimates the recipe would call for about 600 kilograms (1300 pounds) of Moon dust, 60 kg (130 pounds) of epoxy, 6 kg (13 pounds) of carbon nanotubes and less than a gram of aluminium.

The technique could also be used to build other structures on the Moon, including reflectors for an array of solar panels, as well as habitats, or igloos, for astronauts, they say.

But of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, says it’s one thing to make a substrate and a small mirror and quite another to build a telescope with a 20- to 50-metre mirror smooth enough to make precise astronomical observations.

“The surface of the entire mirror has to be formed and held to an accuracy of a fraction of a wavelength of light, which is mighty accurate,” he told 91av.

‘Not cheap’

And there are a host of problems that would have to be worked out. The team has yet to determine the type of devices that might be used to spin the mirrors, and the telescopes would need detectors, actuators, steel support structures and other components not made from Moon dust. Fabricant also notes, “In no conceivable way could this be cheap. It’s not cheap to do this on the Earth.”

Still, astronomers dream of the viewing opportunities such large telescopes on the Moon would offer. “It is an environment where you can do the best astronomy,” said of Cornell University in Ithaca, New York, US. “The Moon does not have an atmosphere and therefore we can see the sky very clearly at optical wavelengths.”

“In addition to that, the back side of the Moon that never sees the Earth is an ideal place to do radio astronomy . . . because there, you don’t have manmade radio interference,” he told 91av.

Chen presented the team’s work on Wednesday at a meeting of the American Astronomical Society in St Louis, Missouri, US.

The comet that suddenly became about a million times brighter nearly two weeks ago continues to “shine” with abnormal luminosity, leaving observers puzzled over what caused the outburst and whether the comet will perform an encore in the coming months.

Comet 17P/Holmes is normally an invisible runt of a comet, about 3.3 kilometres across and about 25,000 times too faint to be seen with the naked eye.

But following its sudden brightening on 23 October, the comet’s coma, a surrounding shell of gas and dust, has been expanding at a rate of about 0.5 kilometres per second, making the comet appear as a fuzzy “star” that can be seen with the naked eye in the constellation Perseus (see image at right and watch a ).

The comet was actually discovered during a similar, but less spectacular, brightening event in November 1892. It faded after a few weeks, only to dramatically brighten again in January 1893.

The comet orbits the Sun every seven years on a path that takes it from the distance of Jupiter’s orbit to about twice that of Earth’s. Interestingly, in both the 1892 event and the recent one, the comet initially brightened about five months after reaching perihelion – its closest approach to the Sun.

“It’s curious that the outburst came in the same period of orbit,” says Brian Marsden, former director of the Minor Planet Center at the Harvard-Smithsonian Center for Astrophysics in Massachusetts, US. “It will be interesting to see if it behaves in the same general way [this time as before].”

The common timing of the two mega-outbursts following perihelion suggests that the intensity of the Sun’s radiation is a key factor in the brightening. But that alone is not enough, as the comet reaches perihelion every seven years and hasn’t produced such an outburst in 115 years. There are also plenty of comets that make closer approaches to the Sun than Comet 17P/Holmes without brightening nearly as much.

Fresh ice

“The fact that it’s brightened by a factor of a million is just incredible,” says David Jewitt at the University of Hawaii in Honolulu, US. “What’s special about this one? We don’t know.”

Michael Mumma, director of the Goddard Center for Astrobiology in Greenbelt, Maryland, US, and colleagues are observing the comet with the 10-metre Keck II telescope’s infrared vision.

They find that the comet is still actively releasing gas. Most of it is composed of water vapour, but ethane, acetylene and hydrogen cyanide have also been seen in trace amounts. “We hope to learn a great deal about the material that was once in the interior of the comet nucleus,” Mumma told 91av.

The standard explanation of comet outbursts says that a sudden event exposes fresh ices from within the nucleus to solar radiation. This causes them to vaporise, dragging dust along with them. Sunlight reflecting off that dust then magnifies the comet’s brightness.

Built-up pressure

In an electronic telegram distributed by the International Astronomical Union, Zdenek Sekanina of NASA’s Jet Propulsion Laboratory in Pasadena, California, US, suggests that a flattened layer that was once part of the nucleus could have broken off and completely disintegrated to produce the outburst. The idea, which has not been peer-reviewed, is partially based on an estimate of the mass of particles in the coma.

Another hypothesis suggests that pockets of volatile gas somehow get trapped in the nucleus. Eventually, built-up pressure causes the surface to rupture, producing large outbursts.

Tom Van Flandern of Meta Research in Sequim, Washington, US, suggests a more outlandish possibility. He argues that comets are orbited by satellites and that occasionally a satellite crashes into its host comet, producing an outburst.

A similar idea was put forward more than 20 years ago by astronomer Fred Whipple, who suggested that the 1892 outburst of Comet Holmes was caused by a satellite grazing the surface of the nucleus and that the second brightening in 1893 was the result of the satellite crashing into the comet’s surface.

Race against the clock

Jewitt says the recent recurrence of the comet’s brightening strongly argues against the satellite model. “The fact that this has happened after 115 years – with nothing in the intervening years – makes the model more ad hoc,” he told 91av.

Marsden agrees, and believes that whatever caused the comet to suddenly brighten has to be intrinsic to the comet. “We need to continue observations and see how it goes,” he adds.

Mumma says the comet could fade at any time. In the case of the event 115 years ago, the comet rapidly faded after the second outburst. “That’s part of the challenge – to try to learn as much as we can before the darn thing vanishes,” he says.

Fortunately, the fading itself can also shed light on what caused the outburst. The comet is currently in the main asteroid belt, at a distance from the Sun of about two and a half times that of Earth (or 2.5 astronomical units). Water ice can no longer sublimate beyond about 3 astronomical units, so if the comet is still active past that point, something more volatile than water ice must be driving its activity.

Comets and Asteroids – Learn more in our .

NASA may alter the design of its upcoming Mars Science Laboratory rover so it will not only crush and analyse soil and rocks on the Red Planet, but will also store samples for a future mission to deliver to Earth. The possible change could shorten the wait time for a Mars sample return mission, which a new report ranks as the highest scientific priority for future Mars missions.

Scientists have been asking for a sample return mission since the 1960s, but cost, mission complexity and lack of appropriate technology have prevented any such missions from going forward. Still, they say studying Martian samples in labs on Earth could teach them much more about the climate, geochemistry and possibility of past or present life on Mars than remote studies with robots – even those as capable as the Mars Exploration Rovers.

That view is highlighted in a new NASA-commissioned report by the US National Research Council (NRC), which outlines the first comprehensive strategy devised for the detection of life on Mars since 1995. It states: “The highest-priority science objective for Mars exploration must be the analysis of a diverse suite of appropriate samples returned from carefully selected regions on Mars.”

The report suggests using a series of spacecraft for sample return rather than a single mission, an approach that would reduce the complexity and weight of each individual probe. In this scenario, one or more missions could collect and store, or “cache”, samples; one could retrieve a scientifically promising cache and launch it into orbit around Mars; and a third might then bring the sample back to Earth for detailed analysis. The report also calls for increased funding from NASA to develop the technology needed for such missions.

Alan Stern, NASA’s associate administrator for science, is hoping to speed up the development of such a programme by seeing if rover missions already on the drawing board could be altered to perform the first task of caching samples. He has asked NASA’s Mars Science Laboratory (MSL) team to study the feasibility of adding such a capability to the mega-rover, which is due to launch in 2009 to study whether the planet could ever have sustained microbial life.

Late addition

He has also started preliminary talks with the European Space Agency about providing a caching system for ExoMars, a rover mission scheduled for launch in 2013, though he told 91av that so far ESA has not expressed an opinion on the matter.

At an NRC colloquium on astrobiology and Mars exploration in Pasadena, California, US, on Sunday, many researchers applauded putting a sample return programme in the spotlight. But the idea of caching samples on MSL raised a few eyebrows. For one thing, the rover has already passed its critical design review – a milestone after which significant mission design changes are unusual.

“It comes very late in MSL’s development,” says Bruce Jakosky, a researcher at the University of Colorado in Boulder, US, and chair of the NRC committee that put together the astrobiology strategy report. “We should question whether the astrobiology science objectives can be addressed by the type of sample MSL can obtain. … I’d like to see it discussed within the community.”

Go deeper

Kenneth Nealson of the University of Southern California agrees. He says some scientists are concerned that the powdery samples MSL will collect are the most likely to change with atmospheric exposure on the surface. “It would be more of a technology demonstration,” he told 91av. “[Still], one sample is always better than zero.”

Another session on the exploration of the Martian subsurface came to the conclusion that the return of an MSL cache probably would not meet the requirements to help scientists directly address astrobiological questions.

They said such a mission would be valuable for other areas of scientific interest, but that any discoveries of possible Martian life would most likely require a sample collected from the subsurface – probably between 1 and 10 metres down, where samples would be relatively untouched by oxidising agents at the surface (see Life may lie deep below Martian surface).

Mars Rovers – Mars is full of surprises. Learn more in our continually updated .

The question is an important one. While giant telescopes and sensitive detectors hundreds of metres across would provide an incredible window on the Universe, they would be prohibitively expensive and cumbersome to launch and maintain in space.

So several next-generation space missions propose to use more than one, and in some cases dozens, of smaller spacecraft flying in formation to cover the area between them.

For example, NASA’s MAXIM (Micro-Arcsecond X-ray Imaging Mission) envisions as many as 33 satellites flying in concert to create a huge X-ray interferometer with a resolution high enough to image black holes. The SPECS (Submillimeter Probe of the Evolution of Cosmic Structure) mission calls for three satellites spaced 1 kilometre apart to study the first galaxies to form in the early Universe.

The problem is how to maintain the configurations needed to make the separate spacecraft behave as though they were a single unit. Physicist Young Bae is proposing a system that relies on lasers and tethers. “With these two [factors] working in combination, we can achieve nanometre accuracy over a kilometre formation,” claims Bae, at the Bae Institute in California, US.

Laser thrust

The idea is that lasers are beamed between pairs of spacecraft within a formation. Mirrors on each satellite reflect the laser beams back and forth, providing an energy-efficient thrust.

This outward push is balanced by the inward pull of tension in Kevlar tethers linking the satellites. The system also uses the laser beams to measure and maintain the desired distance between satellites very accurately. “The alignment accuracy cannot be achieved by any other means,” Bae says.

The NASA Institute for Advanced Concepts funded the first stage of the laser-tether project and is considering the concept for a second, more in-depth study.

But Robert Cassanova, director of the institute based in Atlanta, Georgia, says Bae’s is not the only proposal for how to control formation flying. “We’re testing ideas in parallel,” he says. “We don’t want to put all our bets into one concept. And there may be applications for two different types of systems where one would be better than the other.”

Another NIAC-funded project, led by MIT’s Raymond Sedwick and David Miller, suggests using electromagnets rather than traditional thrusters to control the pointing and relative positions of the individual satellites.

Cassanova says this project has completed NIAC’s two phases of feasibility studies and has received additional funding from NASA to continue development.

About 170 extrasolar planets have been found so far, but direct detection remains extremely difficult. Isolating light reflected towards the Earth from the exoplanet itself would give astronomers valuable information about what that world was like.

Now, a European team led by Alessandro Berton at the Max Planck Institute of Astronomy in Heidelberg, Germany has simulated highly complex Earth-based telescope configurations to see how useful they might be in the hunt.

Their findings suggest the current class of 8-metre telescopes – such as the Very Large Telescope in Chile – can only hope to directly detect young, bright planets that are at least five times the size of Jupiter, and nothing smaller.

The team’s simulations incorporated the latest adaptive optics – which compensate for distortions produced by turbulence in Earth’s atmosphere – and two specialised techniques for reducing signal noise.

Berton’s system also considered special hardware and techniques which allow astronomers to observe the spectra of the light bounced off the planet from its central star, while subtracting much of a central star’s glare.

Cold and small

The simulation showed that the combined system would be able to detect objects at all distances measured so far, up to 130 light years, as long as they had with masses 30 times that of Jupiter. Similarly, the system should handle very young objects – which are hot and bright – such as only about 10 million years old.

The team concludes: “This technique appears to be extremely useful, if not for detecting very cold and small objects, for a better definition and characterisation of planets younger than 1 billion years old or more massive than a few Jupiter masses.”

Berton told 91av he has also completed the simulation for a similar system on the next generation of telescopes – the Extremely Large Telescopes. “The result was that, even with a 100-metre telescope, we could detect Jupiter-like planets for sure, but not Earth-like planets”.

Work in progress

That comes as no surprise to Wesley Traub, chief scientist of NASA’s Navigator programme, the agency’s missions that share the goal of finding Earth-like planets around other stars. “You’ll never be able to detect Earth-like planets from the ground,” he says.

That is not to say that Jupiter-like planets and hot, young Jupiters are not of interest to astronomers. “It would be extremely valuable to just know more about what’s out there,” Traub says. “Self-luminous, young Jupiters begin to tell us that our theories of how these things form and how they cool off and what they’re made of are correct. That would be extremely useful.”

Berton notes that space-based missions like Kepler and Darwin – scheduled for launch in 2008 and 2015, respectively – are bound to gather better results than ground-based telescopes without atmospheric interferences. “But as a preparation for these missions, these [ground-based] instruments can be extremely useful and much cheaper.”

The $110 million DART mission – Demonstration of Autonomous Rendezvous Technology – went awry in April 2005. At the time of the crash, reads the report: “DART was flying toward [the satellite] at 1.5 metres per second while its navigational system thought it was 130 metres away and retreating at 0.3 metres per second.”

The summary report – which will not be released in full due to the sensitivity of its contents – cites a lack of engineering experience and lack consultation with advisors, pressure to meet deadlines and a failure to adequately test all technical decisions as key factors in the failure.

The spacecraft was originally envisioned as a low-cost, high-risk project to test the ability of two spacecraft to manoeuvre in close proximity without human guidance. But by the time DART launched, President George W Bush had set out the Vision for Space Exploration, aiming eventually to send astronauts to Mars.

DART then became a high-profile proving ground for the type of technology this vision would require. As it turned out, DART managed to fulfil only 11 of its 27 mission objectives and none of its “critical technology objectives” before prematurely placing itself in a retirement orbit. This happened 11 hours into its planned 24-hour mission.

Consistent error

But not everything went badly for the DART team. Launch from Vandenberg Air Force Base went smoothly, as did the early orbit and the initial lining up with its satellite rendezvous target, called MUBLCOM. But when DART prepared operations close to MUBLCOM, the report summary says ground operators noted it was using more of its nitrogen fuel than expected.

This was due to faulty data being fed to the craft from its primary global positioning system receiver, which caused it to repeatedly fire its thrusters in an attempt to correct its path. The report says the receiver “consistently produced a measured velocity that was offset or ‘biased’ about 0.6 metres per second from what it should have been.”

That error was worsened by a known computational bug in the navigational software that the DART team had never fixed. The software was keeping its own estimates of the spacecraft’s speed and position. But when the GPS measurements and the predicted velocity differed too greatly, this navigational software was designed to reset and begin its estimates anew using the GPS measurements.

The report summary says the reset occurred about once every 3 minutes, each time feeding the faulty navigational data into the system,

leading to excessive firings and fuel loss.

Inappropriate gain

The erroneous GPS data would still have fallen within the acceptable range if the navigational software had met the design requirements. “The design requirements stated that the measured velocity data only had to be accurate within 2 metres per second,” the summary says. The GPS data was off by less than 1 metre per second.

Furthermore, the investigation board found that the chain of resets and correcting firings would not have continued as it did if a feature called the “gain” had been properly set. The gain controlled how much importance DART placed on the measured and estimated velocities.

But the gain was changed to place an inappropriately high level of weight on the GPS measurements late in the spacecraft’s development and was not properly tested.

The crash itself after the navigational problems meant DART had failed to enter a 12.6-metre-wide spherical space behind MUBLCOM. It had to be this near to close in on its target, but missed by 2 metres. Ironically, this ultimately meant the disoriented craft hit the satellite. There was a collision-avoidance system, but that relied on the same erroneous positioning data.

When astronauts return to the Moon, they should keep their eyes peeled for extraterrestrial artefacts – pieces of technology from alien civilisations that have wound up on the lunar surface either by chance or design.

So says Ian Crawford, a researcher from University of London’s Birkbeck College in the UK. He told a SETI specialist meeting of the Royal Astronomical Society (RAS) in London last week that although he considers such a find a long-shot, it is definitely worth bearing in mind.

“This is not a primary reason to go back to the Moon – there are very strong scientific reasons for going back. But if we go back to the Moon in the next 20 or 30 years, then amongst those things we might like to keep our eyes open for are alien artefacts,” Crawford told 91av.

The focus of the RAS discussion was the history and status of the search for extraterrestrial intelligence – an endeavour that has largely relied so far on large radio telescopes listening for electromagnetic signals from other technological civilisations.

Little additional cost

Crawford thinks scientists will be keen for the next lunar astronauts to sift through the lunar soil in greater quantities and in more detail was possible during the Apollo era. So there would be little additional cost to remain open to the idea that alien material may exist within those upper metres of the moon’s regolith.

Seth Shostak, senior astronomer at the SETI Institute in Mountain View, California, says the possibility of such an interesting payoff for little additional cost makes the idea of looking for artefacts worth considering.

“On the Moon, I think it’s certainly worthwhile taking a couple hundred square feet or so of material and looking it over,” he says. But SETI researchers “probably wouldn’t bet their mortgages on finding anything”.

Message in a bottle

Looking for small artefacts, or even probes or time capsules, in our solar system is not a new idea in the SETI community, Shostak notes. Indeed, our own civilisation has already taken this approach several times.

NASA’s Pioneer 10 and 11 spacecraft carry metal plaques showing the spacecraft’s time and place of origin. And the Voyager 1 and 2 probes carry gold-plated records (pictured) bearing messages, images and sounds depicting life on Earth for any extraterrestrials who might encounter them thousands of years from now.

In 2004, an engineer and a physicist published a paper in Nature suggesting that if extraterrestrials were not in a hurry, their best shot at making contact with other civilisations would not be with radio waves, but with an interstellar message in a bottle – a physical artefact to tell other intelligent life forms something about their existence.

Crawford says the moon would be a good target for such “inscribed matter”, given its lack of geologic activity and airless environment. But if an intentional targeting of a relatively small body sounds a little too like 2001: A Space Odyssey, he says there is also a possibility first brought to his attention by a 1998 paper by Ukrainian astronomer Alexy Arkhipov.

This suggests that if enough space faring societies have existed, even if they never travelled beyond their own planetary systems, they may have produced enough space debris to make it possible that micron-sized particles could have reached our own solar system. And Crawford says such tiny material could have fallen to the moon, remaining there within the top 10 or 15 metres of regolith.

Excruciating detail

But several lunar experts describe the prospect of finding extraterrestrial artefacts on the moon as “far-fetched”. Gary Lofgren, lunar curator in charge of the scientific preservation of the Apollo lunar samples at the Johnson Space Center in Houston says even if such extraterrestrial micron-sized particles are there, it would be incredibly difficult to find them, requiring special instrumentation that has never been sent to space before.

Apollo astronauts brought a total of 842 pounds (382 kilograms) of lunar material to Earth and researchers are still studying the samples today. Lofgren says: “People have gone through the samples in excruciating detail and haven’t found anything that would suggest extraterrestrial activity.”

In fact, he says, scientists have even been surprised by how little of the material has been assigned an origin off the moon – rocks from Earth or Mars thrown up by meteorite impacts, for example.

Still, Lofgren says, scientists will be eager to study new samples from the Moon, noting that the Apollo samples only cover six sites clustered around the equator. And he says, a discovery of material that cannot be explained by an origin and evolution within our solar system “would be an incredible find”.

Scott Hubbard, former director of NASA’s Ames Research Center and now a SETI Institute researcher says: “While nothing is impossible, I find the payoff from seeking a cometary in-fall in the Moon’s permanently shadowed craters much more compelling than extraterrestrial civilisation artefacts.”

Giant structures stretching more than a billion light years across have been revealed by two new maps of the distribution of galaxies in the universe. The updated atlases lend more support to the idea that the universe is dominated by dark matter and dark energy.

Both studies used data from the Sloan Digital Sky Survey to gather the colour and position in the sky of more than a million galaxies. But in order to understand galaxy distribution in three dimensions, researchers must also know the distance between each galaxy and the Earth.

That can be done by collecting a spectrum for each and every galaxy to see how the expansion of the universe stretches its light waves on the way to Earth, but is very time consuming and expensive.

So the teams that produced the new maps sidestepped that process by finding automatic ways to assign distances to hundreds of thousands of galaxies without having to collect all those spectra. They looked at a class of bright, very old galaxies – called luminous red galaxies – which have well known true colours. Distortions in their colour are therefore easily measured, meaning their distances from Earth can be obtained.

One of the teams, led by Chris Blake of the University of British Columbia, Canada, has created the largest map of the heavens ever produced. It includes more than a million galaxies and the farthest sits more than 5 billion light years from Earth.

The other group, led by Nikhil Padmanabhan at Princeton University in the US looked at 600,000 galaxies covering a similar region – about one-tenth of the sky. But both final maps rely on only 10,000 spectral measurements.

Trained algorithm

Padmanabhan’s team used the measurements of the luminous red galaxies to calibrate the correlation between galaxy colour and distance. To produce the “million-galaxy-map”, Adrian Collister and Ofer Lahav, of the University of Cambridge, UK, “trained” a computer algorithm to take the 10,000 galaxies – with their coordinates, colours and the measured distances from Earth – and come up with approximate distances for other galaxies based on the SDSS data.

“The point is we have this technique that uses only the colours and positions of the 10,000 galaxies and we get distances for more than a million objects,” says Lahav, now head of the astrophysics group at University College London, UK.

Both teams were able to identify mathematical patterns in the distribution of galaxies throughout the universe indicating the presence of large structures. “This is one of the first detections of structures that are so large,” Padmanabhan says. “The only other place that they have been detected is in the cosmic microwave background”, the afterglow of the big bang.

So he says, by comparing the structures in the background, from a time when the universe was only hundreds of thousands of years old, with those from the new surveys dating to a period when the universe was about 8 billion years old, researchers can assess test theories about how the universe evolved.

Both studies confirm that the galaxy distribution and structures of the universe match best the models in which normal matter makes up only a few percent of the universe, with about one-quarter taken up by dark matter and the rest dark energy.

“With new measurements, our emerging picture of a universe dominated by dark matter and dark energy had a chance to fall on its face,” says Uros Seljak, another Princeton. “Instead, it passed a new test with flying colours.”

Cosmic sound waves

Both groups also detected in their maps the surviving imprint of cosmic sound waves, released shortly after the big bang. Evidence of the waves, which are thought to have been trapped by the early universe’s dense fog of gas and photons, was first reported last year by SDSS and Two Degree Field galaxy redshift survey teams.

“We can detect the surviving imprint of these sound waves in the distribution of galaxies. They are echoes of a quite remarkable epoch of the universe that occurred less than 300,000 years after the big bang, when the cosmos was so hot and dense that hydrogen atoms were ionised into protons and electrons,” Blake says.

Scientists have found that ripples from the waves slightly increase the probability that galaxies will be located 500 million light years apart than any other distance. Such a “ruler” could help astronomers map the expansion history of the universe and assess the accuracy of astronomical distance measurements.

The new studies are the first to detect the waves so far from Earth. “Measuring this ‘standard ruler’ at different epochs is one of the best tools we have for studying the dark energy, the component of modern cosmology we understand least,” says David Schlegel, another member of Padmanabhan’s team at Lawrence Berkeley National Laboratory in the US.

Astronomers have discovered two new satellite galaxies of the Milky Way and one could claim the title of the faintest yet found.

In the past two years, the Sloan Digital Sky Survey’s measurements of stars stretching across a quarter of the sky have enabled astronomers to find several new companion dwarf galaxies for the Milky Way and its near-twin galaxy, Andromeda.

“The advent of very wide-area surveys means we’re certainly moving to a new regime in terms of what we’re able to discover,” says Daniel Zucker at the University of Cambridge, UK.

Zucker’s team located one of the new dwarf galaxies in the constellation Canes Venatici by finding an “over-density” of old, metal-poor stars in the Sloan data. All of the stars have similar chemical abundances and temperature, but they are quite spread out, spanning more than 6000 light years.

The dim galaxy structure is therefore not apparent in photographic images. “You would need a very, very deep exposure to bring out some structure,” Zucker says.

Boo bizarre

The second galaxy, nicknamed “Boo” for its location in the constellation Bootes, was also identified in Sloan data by another Cambridge astronomer, Vasily Belokurov. His team believes the object is a dwarf galaxy with an extremely low luminosity – two to three times fainter than UMajor, the faintest galaxy previously identified.

Belokurov describes Boo as “a really bizarre object”, especially because of its observed shape. The galaxy is believed to have been gravitationally disrupted by the forces of the Milky Way. Normally, such disruption leads to very elongated galaxies with stars forming two narrow tails.

But rather than a single pair of tails, Boo appears to have two pairs that form a cross. Data from the Cerro Tololo Inter-American Observatory in Chile have confirmed the galaxy’s irregular shape. The team will now collect additional spectroscopic and photometric data to better understand the oddball galaxy.

Bridging the gap

The recent discoveries bring the number of Milky Way satellite galaxies identified to about a dozen. But theory predicts there should be between 300 and 500 such satellites.

Scientists have proposed various explanations for the discrepancy. Some say the galaxies that have been seen are embedded in much larger dark matter clumps that obscure many of the faintest (see “Mini-galaxies may reveal dark matter stream“). Others have suggested that as-yet undiscovered dwarf galaxies’ star formation process has been cut off, making them virtually impossible to see.

“The question is: are we going to just keep finding handfuls of these things and that’s it?” Zucker says. “That’s certainly not going to bridge the gap between what some of the models predict and what we see.”

His hunch is that astronomers will continue to find more faint galaxies, but not hundreds. “Theory and observation are going to have to meet somewhere in between,” he says.