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The Planetary Society Blog

By Emily Lakdawalla

Mars to fly by Rosetta

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by Doug Ellison in Darmstadt

Has it really only been 4 months since I tapped away at the same keyboard from the other side of Europe reporting on events at the IAC in Valencia? That week was a first for me – and in the same way I wanted to retell both the goings on of the conference and my reflections on the experience of being there, hopefully over the next 12 hours I can tell you not only what's going on with Rosetta, but what it's like to be at this sort of event. As Valencia was my first ‘conference' visit, this is my first ‘live' space event, and I'm more than a little excited!

As I write this, Rosetta is just over a quarter of a million kilometres out from Mars and has already begun the intensive science campaign that has been scheduled for the flyby. In the next 8 hours Isaac Newton will take that 250,000 km and turn it to 250 km -– an average of more than 8.6 km/sec between now and closest approach. But as with every mention of orbital mechanics, it all depends on how you look at it. If you were right under the trajectory (longitude 298.2° E and latitude 43.5° N is the point of closest approach) Rosetta would be only 250 km above you, travelling more than 36,000kph. That's lower -– and faster -– than the International Space Station orbits the Earth which is currently at 334km and travels at 27,000 kph. It's even lower than the orbiting Mars Odyssey. Rosetta would indeed seem to fly past Mars if Mars is where you view things from.

But take it to the third person perspective at some point hundreds of millions of miles above all the planets, with their orbits sketched out below you like a set of perfectly stacked (if not perfectly round) dinner plates. Rosetta is, from that perspective, travelling slower than Mars and indeed Mars will run over Rosetta. Cast your mind back to the days of the Deep Impact mission and the same was true then, the comet ran the impactor over. If you think that's confusing –- consider this: When a new orbiter arrives at Mars you will hear the press releases say that the engines will slow the spacecraft down. In relation to Mars, yes...but in actual fact in terms of its orbit from the solar perspective -- Mars is about to run the orbiter over and the engine burn lets the spacecraft play catchup by speeding it up in its orbit around the sun. Orbital mechanics is a strange and often unintuitive and frankly deep, dark and mysterious science. David Seal is one of the people who not only understands it all but designs spacecraft trajectories ( especially Cassini ) – and Emily and I have both been making great use of his Solar System Simulator where you can see the perspective of these events from all sorts of angles. Without it, I'd find it hard to visualize what's going on in our back-garden -– it's like watching the radar at the air traffic control. But once you've seen a rendered view of Mars from Rosetta –- perhaps your mind wanders as mine did on the plane to Frankfurt today –- what would that flyby be like if you were actually THERE?

If you were sat onboard Rosetta now Mars would already look three times larger than a full moon does from Earth and a hairs-breadth to one side of that ochre disc, at about the 11 o'clock position is an appointment that the navigators at ESA and Isaac Newton dictate we're not going to miss. We've all seen spectacular views of the Earth scrolling under the feet of spacewalking astronauts -- tonight would be much more dramatic, and just at closest approach you would slip from the lit side of Mars to the night side -– with the Sun setting through the thin atmosphere, possibly picking out the high altitude clouds as it set over the dusty horizon. The Rosetta orbiter has to be turned off for this phase, but its lander, Philae, doesn't – and it's wide-angle cameras will hopefully provide some amazing images of that closest approach phase.

Emily and I worked through some of the fairly 'not suitable for public consumption' planning documents the Rosetta team were kind enough to send us in advance and Emily put together a detailed timeline that I've already found useful. For a bigger-picture overview I made my own timeline that isn't as detailed but gives a wider overview of events in a graphical form

Click to enlarge > Rosetta Mars flyby timeline Graphical outline of imaging sequences during the Rosetta Mars Flyby Credit: Doug Ellison & Emily Lakdawalla from data supplied by the Rosetta Science Operations Centre at ESA

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