Jupiter’s moon Callisto is not the largest moon in the solar system, or even the largest of Jupiter’s natural satellites. But it does rank highest in one category: it is the most heavily cratered body in our solar system.

Callisto’s surface is about as evenly blemished as you could expect it to be from the random impacts it has received over the eons. Made up of equal parts of rock and ice, it’s surface captures the history of the violent past of the solar system. The outermost of Jupiter’s four Galilean satellites, it is nearly the same size as the planet Mercury, though it has only about one-third the mass of the speedy inner planet.

Callisto orbits Jupiter at a distance of nearly 3 million kilometers (1.8 million miles), considerably farther away from the planet than the other Galilean moons. That relative isolation is one of the reasons for the golf ball-like appearance of the moon. At that distance, Jupiter’s strong gravity exerts less force on Callisto than on the other large moons. Io, Europa and Ganymede pay for their proximity to Jupiter with geological processes like volcanism, plate tectonics and other resurfacing events. Callisto’s surface hasn’t had any such recent rejuvenation treatments, and it carries scars from the early solar system, dating back some four billion years.

This is the only complete full-color view of Callisto obtained by the Galileo spacecraft, which studied Jupiter and its moons from 1995 to 2003.


Image credit: NASA/JPL/DLR


Star on a Hubble diet

The star cluster Pismis 24 lies in the core of the large emission nebula NGC 6357 that extends one degree on the sky in the direction of the Scorpius constellation. Part of the nebula is ionised by the youngest (bluest) heavy stars in Pismis 24. The intense ultraviolet radiation from the blazing stars heats the gas surrounding the cluster and creates a bubble in NGC 6357. The presence of these surrounding gas clouds makes probing into the region even harder.

One of the top candidates for the title of “Milky Way stellar heavyweight champion” was, until now, Pismis 24-1, a bright young star that lies in the core of the small open star cluster Pismis 24 (the bright stars in the Hubble image) about 8,000 light-years away from Earth. Pismis 24-1 was thought to have an incredibly large mass of 200 to 300 solar masses. New NASA/ESA Hubble measurements of the star, have, however, resolved Pismis 24-1 into two separate stars, and, in doing so, have “halved” its mass to around 100 solar masses.

Credit: NASA, ESA and Jesús Maíz Apellániz (Instituto de Astrofísica de Andalucía, Spain). Acknowledgement: Davide De Martin (ESA/Hubble)

Light from Cygnus A

Celebrating astronomy in this International Year of Light, the detailed image reveals spectacular active galaxy Cygnus A in light across the electromagnetic spectrum. Incorporating X-ray data (blue) from the orbiting Chandra Observatory, Cygnus A is seen to be a prodigious source of high energy x-rays. But it is actually more famous at the low energy end of the electromagnetic spectrum. One of the brightest celestial sources visible to radio telescopes, at 600 million light-years distant Cygnus A is the closest powerful radio galaxy. Radio emission (red) extends to either side along the same axis for nearly 300,000 light-years powered by jets of relativistic particles emanating from the galaxy’s central supermassive black hole. Hot spots likely mark the ends of the jets impacting surrounding cool, dense material. Confined to yellow hues, optical wavelength data of the galaxy from Hubble and the surrounding field in the Digital Sky Survey complete a  remarkable multiwavelength view.

Image Credit: X-ray: NASA/CXC/SAO; Optical: NASA/STScI; Radio: NSF/NRAO/AUI/VLA


Why is our relationship with the moon so one-sided? Let MinuteEarth explain. 

There’s no dark side of the moon, but there is a side we don’t see often. But because of the moons rock-and-rolling libration, that unseen portion is actually less than half the moon’s face:

Follow that with this history of our moon’s violent birth, and then check out this post to tour through a whole catalog of maps of the moon.


See that tiny spot at image center? That is NASA’s Curiosity rover on the surface of Mars, as seen by the Mars Reconnaissance Orbiter. This view from the High Resolution Imaging Science Experiment (HiRISE) was obtained on 13 December 2014, the 835th Mars day, or sol, of the rover’s mission.

At the time, Curiosity was at the Pahrump Hills area of the Gale Crater. The rover was near a feature known as Whale Rock in an outcrop at the base of Mount Sharp. This area contains sedimentary rocks that scientists believe formed in the presence of water.

While this may look like some vast expanse of the Martian surface, you have to keep in mind that Curiosity is only about 3 meters (10 ft) long. The view here covers only about 330 meters (360 yards) across. If you would to see a map of how Curiosity got here, you can look at this HiRise image that has been annotated with the last 150 sols worth of travel:


Image credit: NASA/JPL-Caltech/University of Arizona



The closest conjunction of the year

"If you’ve got the skies and the equipment, go and have a look; the Universe will never again appear exactly as it does at that moment again, so don’t miss your chance to experience it!"

When you look up at the sky, all the twinkling lights — the stars — appear to be fixed. But littered among them are the non-twinkling wanderers: the planets. Just 11 days ago, the Mars/Venus conjunction occurred, giving us a spectacular view of two naked-eye planets separated by a mere half-a-degree. But tomorrow night, particularly for skywatchers in Europe and northern Africa, an even closer conjunction, of just 0.1 degrees, occurs between Venus and Uranus, the closest one of the year.

Seeing New Sights Through the Trifid

In visible light, the Trifid Nebula (M20) in the constellation Sagittarius is quite a sight. Its three-part structure is created by dusty lanes, and it glows in pink emissions from ionized hydrogen and a blue haze from the light of hot young stars scattering off the dust around it.

But that beautiful nebulosity also poses some difficulties for astronomers. Since it is opaque to optical wavelengths, we can’t see what lies behind it. With the Trifid lying near the galactic plane in the direction of our galaxy’s center at a distance of 5,200 light light-years, it blocks our vision of the central bulge and beyond.

Now, using the European Southern Observatory’s Visible and Infrared Survey Telescope for Astronomy (VISTA), astronomers are conducting a survey known as VISTA Variables in the Via Lactea, or the VVV survey. By observing in infrared light, the new survey peers right through the nebula and the dust of the galactic disk beyond. The VVV survey visits at the same patch of sky over and over, looking for objects that vary in brightness over time. The new image shown here is just part of the VVV field, and it features the barely recognizable Trifid Nebula on the right.

Using this new dataset, astronomers are seeing things that have been hidden until now. One of their first findings are two previously unknown Cepheid variable stars that lie along the central plane of the Milky Way, close to the Trifid Nebula from our perspective. But these whereas the nebula is relatively nearby, these variables are quite distant. In fact, they lie on the other side of the galaxy, at an estimated distance of 37,000 light-years. That puts them about 10,000 light-years beyond the Milky Way’s center, making them the only Cepheid variables detected thus far that are close to the galactic plane and located on the far side of the galaxy.

For a great way to see the difference between the optical and infrared views of this patch of sky, you really need to take a look at this treat from the ESO.

We are pleased to be able to bring you this news release as part of our ongoing collaboration with @ESO Astronomy. Keep following us for more amazing images and breaking astronomy news from the European Southern Observatory.


Image credit: ESO/VVV consortium/D. Minniti