ASTEROIDS, COMETS and small moons

 

Asteroids vary in size by an order of magnitude more orders of magnitude, than do the terrestrial planets. 


This makes them difficult to classify; Vesta and Itokawa are in most respects (size, gravity, structure) less similar to one another than Jupiter is to the Earth. 


Yet Vesta is the parent body of thousands of Itokawa-sized asteroids, so there is connection across vast scales.


Vesta, Mathilde, Eros and Itokawa shown to scale.  Figure composite by Katherine Armstrong.

Keep scrolling for more images.  Those interested in small bodies may be interested in my forthcoming textbook “Asteroids: Their Origin, Geoohysics and Exploration”, and in this review article I wrote in 2009.

A striking image of the northern limb of Phoebe, one of the icy irregular moons of Saturn, thought to be a captured comet.


Knock yourself out on other high resolution NASA Cassini imagery of Phoebe and other novel satellites here.

In a dozen or so labs around the world, bits and chips of Wild 2’s outgassed dust grains are being studied with excruciating precision for clues to the origin of solar systems.  NASA’s Stardust mission, led by Donald Brownlee of the University of Washington, gave us the astonishing result that the ice-rich bodies of the outer solar system contain highly refractory materials that must have originated in the fires of the inner solar system.  The coma in this image is greatly contrast-enhanced to reveal the outgassing; the actual comet is black as coal

Here’s a Hubble Space Telescope view of asteroid Vesta, a ~500 km diameter differentiated body.  It is a tiny terrestrial planet with a rocky mantle over an iron core, and the second largest asteroid after Ceres.  An impact caused it to lose a big portion of its southern hemisphere -- not too different from what happened to Mars. NASA’s Dawn mission, led by Christopher Russell of UCLA, will arrive in a few short years to tell us all about it, and I think it is going to be one of the most fascinating space missions ever.


And now for something completely different.  Here is Ida (seen by NASA’s Galileo spacecraft en route to Jupiter) and its moon Dactyl (inset) -- the first confirmed asteroid satellite system.  Just look at that tiny thing. We now know that binaries are common.

Here is asteroid Phobos in a Viking Orbiter image.  Not an asteroid, you say? True enough -- it is a small satellite of Mars, the size of Mt. Shasta (about 20 km diameter), with a density of around 1.9 g/cm3 consistent with a rubble pile structure. 


Mars is close to the asteroid Main Belt, and binaries are common -- this makes it likely that Mars captured Phobos within the past ~0.1-1 Ga in a dynamical exchange with a binary system, in the Manner that Triton was captured around Neptune according to the 2005 model by Craig Agnor and Doug Hamilton.

Kaboom!  Actually, in space nobody can hear you go kaboom. On July 4, 2005 NASA perfected the art of crashing spacecraft into small bodies -- no small trick hitting something this small, this far away, this fast!  It’s like hitting a bullet, with another bullet, while riding a speeding train.


Shown is comet Tempel 1 moments after a ~350 kg copper projectile struck it at 10 km/s, imaged by the flyby spacecraft. A daring mission proposed and led by Michael A’Hearn of the University of Maryland, Deep Impact was the first active experiment on a minor body.

Speaking of comets going kaboom, here is an artist’s rendering (Don Davis) showing the fragments of comet Shoemaker-Levy 9 impacting Jupiter in July 1994.  The comet split apart 2 years earlier, in a tidal disruption event that inspired the hit-and-run idea for giant impacts.


I watched the impacts in front of Jeff Moore’s house in suburban Mountain View, through an 8” Newtonian telescope. Amidst street lamps and all that we gave it a try.  It was awe-inspiring to see the new blemishes on the surface of Jupiter rotate into view!

NASA’s only mission to a near-Earth object launched almost 13 years ago.  It’s time for another!  The Near Earth Asteroid Rendezvous mission first flew by the large (53 km) and seriously underdense (1.3 g/cm3) asteorid Mathilde -- a charcoal colored cosmic puff-ball, and the only C-type asteroid observed up close to date.

Above is Eros, the largest of the Amor asteorids, whose craters are all named for famous lovers!  It was the main event for the NEAR mission; check it out!