Helene at Saturn

The Cassini spacecraft captured this closeup image of Helene on June 18, 2011. Helene is a small satellite of Saturn about 20 miles across. Helene is a coorbital satellite, which means that it shares an orbit with the much larger satellite Dione that is ~30 times larger and ~30,000 times more massive. Helene resides at Dione's L4 Lagrange point, which is a stable niche in Dione's orbit that leads that satellite by 60 degrees. The other stable niche is of course the L5 Lagrange point that trails Dione by 60 degrees. The orbit of a coorbital satellite is analogous to the Trojan asteroids that lead or trail Jupiter by 60 degrees in its orbit about the Sun.

No one knows how a coorbital satellite like Helene came to reside in such a special orbit. But it is conceivable that a coorbital satellite is debris that was excavated when the larger satellite Dione was stuck by a comet long ago. If this scenario is correct, then a lucky fraction of that debris managed to find and settle into one or both Lagrange points where it could have reassembled into a small coorbital satellite like Helene.

To see more images of Helene, as well as the rest of the Saturnian system, visit Cassini's CICLOPS website.

The Dawn spacecraft approaches asteroid Vesta

The is the Dawn spacecraft's view of the asteroid Vesta. Dawn is the NASA mission that will visit two asteroids, beginning with Vesta in a few weeks, and then Ceres in 2015. Dawn will go into orbit about Vesta on July 16 to study that asteroid's surface for about a year. Vesta is about 300 miles across and is the fourth largest asteroid. Vesta's surface is composed of basaltic rock, or lava, which makes this asteroid quite unique and very interesting. Evidently Vesta was volcanically active in the past, likely when it first formed 4.5 billion years ago.

In July 2012, Dawn will then fire up its ion engine and depart Vesta for its 3 year trip to asteroid Ceres, which is the largest asteroid, one that appears to be quite rich in water. Meanwhile, keep an eye on the Dawn mission website for better pictures of Vesta that will soon get much more interesting when the spacecraft goes into orbit.

Free floating planets might outnumber the stars in the Galaxy

A recent gravitational microlensing survey indicates that there may be twice as many free floating planets in our Galaxy than stars. Gravitational microlensing is the brightening that occurs when a dim but massive object passes along the line of sight to a more distant brighter object. According to Einstein's theory of relativity, mass bends spacetime, so the path followed by a light ray is deflected (ie lensed) if passing near enough to a star or a planet. So an astronomer observing a lensed star will see it brighten for a month or two if a very dim star (such as a white dwarf or neutron star) passes near the line of sight (LOS). This also occurs if a planet passes near the LOS, but the lower mass planet has a smaller gravitational influence, so the lensing event is briefer, only a few days.

This is illustrated in the above figure, which shows an otherwise steady star brightening by 40% during three days. These planetary microlensing events are quite rare, so astronomers must continuously monitor millions of stars just to detect 10 such microlensing events in one year. From the observed frequency of these microlensing events, it can be shown that most of the lensing objects are free-floating Jupiter-mass planets that are not bound to any star. But this unusual finding is consistent with some models of planet formation, which predict that when multiple planets form around a star, the planets' gravitational interactions can eject one or more planets from the system. Those ejected bodies are free-floating planets, and their fate is to roam the Galaxy unseen, except in these microlensing surveys. These results were obtained by astrophysicists T. Sumi and K. Kamiya (Osaka Japan) and others, with further details reported in their preprint.

Propellers and wakes in Saturn's rings

This figure shows results from an Nbody simulation of a small patch in Saturn's rings; click figure to zoom in. Small dots represent meter sized ring particles, while the circle at the center is a 150m moonlet that is embedded in the ring. All bodies are travelling to the right as they orbit Saturn, but keep in mind that those nearer Saturn (which is far downwards in this figure) orbit faster, so ring particles in the lower x<0 half of this figure are drifting towards the right side of the moonlight, while those in the upper x>0 half are drifting left of the moonlet. This Nbody simulation was performed by Shugo Michikoshi and Eiichiro Kokubo, and their results are detailed in this preprint.

The upper figure shows what happens in a low mass ring having a surface density of 60 grams/cm^2. As particles drift past the moonlet they receive a kick due to the moonlet's gravity, which in turn opens a propeller-shaped gap in the ring. The Cassini spacecraft has in fact observed many such propellers orbiting in Saturn's A ring, like the one seen below; see the CICLOPS website for more details about this image. Curiously, the model predicts that the propeller should appear as a dark gap in the ring, while the Cassini image below shows that a propeller is bright. The meaning of this is unclear, but it may indicate that the propeller gap is also filled with sunlight-reflecting dust grains that are produced as ring particles collide near the moonlet.

The lower Nbody simulation (in the lower half of the top graphic) shows results for a high mass ring of surface density 400 grams/cm^2. In this case, the higher ring gravity cause the ring particles to condense into ropy or taffy-like structures that are known as wakes. These wakes dominate the ring's appearance and completely wash-out the propeller that the moonlet is trying to form. The fact that propellers are seen in Saturn's A ring, while none have been observed in Saturn's B ring, suggest that the A ring is a relatively low mass ring that allows moonlets to form propellers, while the B ring is massive and full of gravitating wakes that inhibit any such propellers.

Messenger's first image from Mercury orbit

The planet Mercury acquired its first artificial satellite on March 17, when the Messenger spacecraft went into orbit about the planet. Two weeks later, the spacecraft acquired its first image from Mercury orbit, with 75,000 more images to be acquired during the next year. See this press release for more details and a much larger image, or visit the Messenger website.

Rare elongated crater on Mars

ESA's Mars Express imaged this elongated crater on Mars. Note that most craters are circular, even when the impactor strikes the planet at a shallow angle. However a train of interplanetary debris can leave an elongated scar, which might account for the crater seen here. But accounting for the origin of that hypothetical debris train can be problematic---perhaps this is debris from a comet or asteroid that was tidally disrupted by Mars? Or perhaps this debris is from a tidally disrupted satellite that had spiraled inwards and onto the planet due to the martian tide. Although this might seem farfetched, this in fact will be the ultimate fact of the Martian satellite Phobos, which will eventually impact Mars in tens of million years, due to its slow orbital decay that is driven by the martian tidal forces. See the Mars Express website for more details.

Monster solar flare imaged by Solar Dynamics Observatory

This massive solar flare erupted from the Sun's surface on February 24, and was imaged by NASA's Solar Dynamics Observatory. Check the SDO website for more info, plus a very dramatic movie of this explosion in space.