Category Archives: astronomy research

astronomy news, astronomy research, Ulysses mission

Goodbye, Ulysses

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You Served Well

https://i0.wp.com/science.nasa.gov/headlines/y2008/images/northpole/Ulysses_spacecraft.jpg?resize=389%2C292Back when I was in graduate school (in the last century, and I can say that without feeling TOO old), I worked on a team that used data from the Ulysses spacecraft to study the effects of the solar wind on comet plasma tails. Ulysses has been following an orbit that took it over the poles of the Sun, allowing the spacecraft’s instruments to sample every “regime” of the solar wind.

Now, unless you study the Sun, you probably don’t think about this, but it doesn’t blow its wind evenly in all directions. The speed and density (think of it as “loadedness” because it carries particles along) are not the same at each latitude.  In the Sun’s “midsection” (its mid-latitudes) the wind is heavily loaded (it’s more dense) and blows at an average speed of 450 km/sec. (970,000 miles per hour).  At higher latitudes (the closer you get to the poles) the wind is much less dense and blows out at around 750 km/sec. (1.6 million miles per hour). And of course, there are locally gusty conditions where winds can temporarily go faster.

What we were trying to do was correlate the appearance of a comet’s plasma tail to what latitude of the solar wind it was experiencing. A plasma tail is very sensitive to the solar wind and responds almost instantly to any changes. So, what we found (with the help of the Ulysses spacecraft) was that at equatorial latitudes, the plasma tail of a comet experiencing this part of the solar wind has a distinctly disturbed appearance. It varies over time and actually experiences what we called “disconnection events” (where the plasma tail breaks off and floats away) as it experiences reversals of the magnetic field at those latitudes.

When the same comet travels through the polar latitudes, it encounters that steady, less-dense and faster solar wind. It has smaller variations in speed and density, no magnetic field reversals, and no disconnection events. A plasma tail looks pretty boring at the higher latitudes (by contrast to its often disrupted, kinky appearance at mid-latitudes).  What we found was that the comet, by acting as a “solar wind sock,” can be used to map the conditions in different latitudes of the solar wind. And Ulysses gave us exquisite readings of wind conditions from its vantage point in the solar wind — which then allowed us almost to “predict” what the comet would look like when it encountered those conditions. Or, to take it another way, we could look at a picture of a comet and just about tell you where it was in the solar wind and we used Ulysses data to back it up.

It was fascinating work, and Ulysses performed well for us — even though our project was just one small one out of many that took advantage of this spacecraft’s work. It’s been a good run for Ulysses, and the news today that it will be switched off on July 1 is sad, but inevitable. The spacecraft has held up four times longer than originally expected, but its onboard generators are starting to fail (along with some other issues). It will likely be taking data right up to the end, and I think that’s a fitting tribute for a well-run, high-productivity mission.

astronomy news, astronomy research, exoplanets, exploration

Earth As An Exoplanet

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How We Might Look to the Neighbors*

*If They Had a Good Set of Instruments and a Clear View

RGB comp of deconvoluted frames of Earth and Moon

Astronomers are enjoying an unprecedented time of planetary discovery. As of May, 2008, here have been 293 planets found around other stars. That number will change, and in fact, it already has, and it’s only the 2nd of June.

These planetary discoveries have the potential to teach us a lot about how and where planets are scattered around the galaxy, and perhaps other galaxies as well. But, here’s what’s also cool–at least one mission out there, the former Deep Impact Mission to a comet–is now spending its time looking for other planets. And, once in a while it turns its eye back to Earth to look at our home planet as if it were an exoplanet. It’s not the first time a spacecraft has looked at Earth, but it is the first time one has done it with an array of instruments tuned to study the light from our planet (and others) in high detail.

The image here was taken by the spacecraft on May 29, 2008, and shows the Moon in its orbit around Earth. It shows our planet from the spacecraft-eye view; if a nearby civilization had a similar spacecraft and looked at us with it, they’d likely see a bright spot (if they could manage to separate us out from the glare of the Sun).  If they had a REALLY good instrument, they’d probably notice that our brightness would dim occasionally and periodically, almost like something is passing in front of us.

The idea behind looking at our home world as if it was an alien planet is so that we can use what we find to compare it to other planets we might find. If we can understand how we look to the neighbors (i.e. our color, atmospheric properties, distribution of continents and oceans) then it will help us recognize similar properties when we see them on other planets.

At the summer meeting of the American Astronomical Society, being held this week in Saint Louis, astronomers working on the EPOXI mission (using the Deep Impact craft) talked about their work. They said that later this summer they’ll release a time-lapse movie of the Moon transiting Earth. As it passes between Earth and the point of view of the spacecraft, it will cover up places like the deserts of North African, which reflect a lot of light.

To a distant observer, this kind of transit would show up as a dimming of the planet (which would actually appear pretty bright since its surface areas and clouds reflect light). This well-known transit effect is also how astronomers can “see” or actually infer the existence of larger planets around stars. The light blockage is what what helps them detect the existence of a planet. So, the story of planetary discovery keeps on ticking, like that well-known battery-operated bunny rabbit. There are, in fact, more results from the meeting that I’ll write about in another post. But for now, enjoy the pretty sight of Earth from “alien eyes” and think about what it will be like when we find another Earth out there.