Flying Over Mars on the Way to a Comet

Image of Mars taken with the camera onboard the Philae lander.

The European Space Agency’s Rosetta spacecraft did a flyby of Mars this weekend and sent back some way cool images. Rosetta is really headed toward a rendezvous with a comet 67/P Churyumov-Gerasimenko early in the year 2014. But, along the way, it has to pass by Mars and Earth to get a couple of much-needed velocity boosts so that it will be able to make the journey to the comet, some 800 million kilometers from the Sun. Since the spacecraft was going to be in the neighborhood of Mars, the mission teams decided to test out some of the instruments. The Philae lander, which will be settling onto the surface of the comet, was switched on and used in what they call “autonomous” mode (in which it relied on power from its batteries to run itself). The Rosetta Philae lander imaging system snapped the image of Mars, below.

Along with visual imagery, the spacecraft’s other instruments measured the planet’s atmosphere, studying it in ultraviolet light (which lets them see structures in the atmosphere, such as high-altitude clouds).

Rosetta’s next flyby is past Earth, which should be very interesting to see! I’m fascinated with this mission since its final destination is a comet. Back when I was in grad school, I spent a lot of time studying comets. They may be frigid balls of ice and dust and rock, but they do put on a good show when they get close to the Sun. For example, as the comet nears the Sun, radiation pressure strips dust particles from the comet as the surface ices melt. Those dust particles stream away from the comet’s nucleus and form the dust tail, that long, curving tail that makes a comet look the way it does.

Structures in the Mars atmosphere, as seen by the OSIRIS wide-angle camera on Rosetta. Credit: ESA © 2007 MPS for OSIRIS Team MPS/UPD/LAM/IAA/RSSD/INTA/UPM/DASP/IDA You can read more about this flyby here.

But, there’s another structure in that tail, the plasma tail. If the comet has enough volatiles (chemical elements or compounds) in its ices, it sends a stream of gas molecules out behind it. They encounter the solar wind, which is a stream of positive ions and electrons streaming out from the Sun and entrained in a magnetic field—making the plasma tail. The encounter causes heating, and the plasma tail will glow, especially in the ultraviolet. Like the dust tail of a comet, the plasma tail also points away from the Sun as the comet moves around it in its orbit.

Our interest was in the plasma tails because they actually act as “solar wind socks.” Study a plasma tail and what you find out will tell you about conditions in the solar wind. How does this work? As a comet moves through the solar wind, the plasma tail can grow, break off, and then regrow. We wanted to measure the growth and figure out what was happening in the solar wind to cause the plasma tail appear to break away. To do that, we needed a lot of images of comets in different parts of the solar wind.

We also wanted to measure the solar wind, and for that, we used data from the Ulysses spacecraft, which is looping around the Sun, over and over again, giving astronomers a long-term look at the Sun and the solar wind.

The Ulysses spacecraft and orbit information

It turns out that the solar wind is not the same all over. In other words, the solar wind that emanates from regions near the mid-section of the Sun has different particle “loads” and speeds than the solar wind from mid-latitudes and polar regions. Those “loads” and speeds are what affect a comet’s plasma tail. As a comet rounds the Sun, and particularly if it’s changing latitude, it will encounter different loads and velocities. It will also pass through areas where the direction of the magnetic field in the solar wind changes quite abruptly. When it does, the plasma tail breaks off and starts to regrow. You can actually see this happen over time, if you take a series of images of a plasma tail during a comet’s closest approach to the Sun. (You can read more about our work here.)

Now, the Rosetta mission is interested in the surface of the comet, which will tell us much more about the ices on its crust, as well as the dust component. It will get at them in a form we don’t often get to see: in their “natural” unheated state.

Cometary ices (and dust) are usually considered to be the oldest, and often the most primordial, bits of solar system material. Study those and you learn something about the conditions in the solar system at its birth. But, it’s good to do that study on materials that haven’t been heated by sunlight or otherwise partially destroyed or decomposed.

Like the Rosetta Stone, which was the key that helped people understand the language and cultures of ancient Egypt, the Rosetta spacecraft will provide the key that will unlock the mysteries behind the building blocks of the solar system: the chemical elements and compounds hidden away in comets for the past 4.6 billion years.

I always thought it was kind of ironic that each time we studied a planet’s plasma tail, we were watching bits of solar system history flash before our eyes. Now, it’s exciting to know that a spacecraft will land on one and sample materials that haven’t changed much since they first formed the comet.

I had an onslaught of relatives over the long holiday weekend (here in the U.S. we celebrate Presidents’ birthdays on the third Tuesday in February). My nephew brought his telescope along, and we did a little stargazing outside for as long as we could stand the cold weather. He’s starting to learn the constellations and a few of the brighter stars. Not that he needed the telescope to show us those things, but I think he wanted to show off his new acquisition.
Their visit was a nice break from show production. We’re working on a series of shows about stargazing and how easy it is, and we needed a bit of a break.

Orion in the wintry sky

Stargazing is one of those things that I used to think everybody knew how to do. That was before I did lectures in the planetarium, back in grad school. Then I found out that people really don’t know much about the sky, other than “the sun sets in the west” kind of information. It’s kind of a shame that this knowledge isn’t really prized as much as it used to be, because the sky is up there, free for the gazing. Even through all the glow of light pollution, if you live in the city. I noticed when I was living and working in New York City, I was still able to make out a fair number of bright stars and planets each night (when it was clear).

I like winter stargazing because (for me, anyway) it brings Orion, the Hunter to my attention. My favorite constellation. It’s got it all: bright stars, a starbirth nebula, and an easily recognizable pattern.

A false-color view of the Orion Nebula from the Spitzer Space Telescope.

So, go out tonight and check it out. If you have a pair of binoculars or a small telescope, find the line of three stars (the “Belt” of Orion) and then look just below the belt. You’ll spot a fuzzy patch of light; that’s the Orion Nebula. It lies about 1,500 light-years away and is the nearest star-forming region to us. Here’s a false-color view of the nebula, as seen by the Spitzer Space Telescope in several wavelengths of infrared light.

Warming Your Hands at The Fire of a Dying Star

Since it’s snowing like mad and the wind is blowing like a banshee here (and the temperature is a toasty 21 degrees Fahrenheit) right now, I want to talk about hot stars. Specifically, let’s talk about hot, dying stars that were once like the Sun. Here’s NGC 2440 to help me take my mind off the cold weather! (For you folks in sunnier climes, count your blessings!)

HST Looks at NGC 2440 again

So, NGC 2440 first came to my attention back when I was working on my first book with Jack Brandt, called Hubble Vision. We wanted to show a nice array of stars at different stages in their lives. Star lives, by the way, are way longer than ours, but like us, they proceed in stages. There’s the infancy part—that takes place in a cocoon of gas and dust. Then, there’s the “living” part, where the star consumes nuclear fuel in its core for some amount of time. Then, there’s the old age part, where the star starts to lose mass in huge quantities and finally gives up the ghost. If the star is massive (like more than 8 or 10 times the mass of the Sun), then it sheds lots of its atmosphere before blowing itself to smithereens in a supernova explosion.

If the star is like the Sun, then it litters its environment with material that it blows away from itself. It does it maybe once, or maybe several times, creating shells of gas (nitrogen, hydrogen, oxygen, and helium, for example). That “exhaled” matter forms a shell around the star. Then, the interior star contracts (shrinks), and in the process, heats up.

So, NGC 2440 (what’s left of it) is very hot—like 400,000 degrees Fahrenheit (try more than 400,000 times hotter than your oven gets). All that heat has to go somewhere and do something, so it’s lighting up the huge cloud of mass that the star lost earlier in its life.

There was more than one outburst from NGC 2440 during its old age, which is why we see two “lobes” of material surrounding the central star.

If you want to see a huge version of the image above, go here (the HST news center). The highest-resolution image almost looks three-dimensional.

Now, I feel a bit warmer. How about you?