Skimming Phobos

Inputs from today’s blog post were provided by Thomas Duxbury, an interdisciplinary scientist on MEX for the Mars moons and Mars geodesy/cartography (and also a co-investigator on the HRSC scene team), Dmitri Titov, ESA’s Mars Express project scientist, and Simon Wood, from the MEX mission operations team at ESOC, ESA’s European Space Operations Centre, Darmstadt, Germany.

On Thursday, 14 January, ESA’s Mars Express spacecraft will make an unusually close flyby of the largest Martian moon, Phobos, passing the surface at just 53 km at closet approach at 16:00:21 UTC (17:00 CET) on orbit 15260.

The event will mark the spacecraft’s closest flyby of the moon in 2016, and, as a point of comparison, most of the other almost-60 Phobos flybys this year will occur between several hundred up to almost 2000 km. So it’s a real skimmer!

Phobos flyby 14012016

Predicted view from MEX for the 14 Jan 2016 Phobos flyby. The centre image is the predicted perspective view of Phobos at closest approach. This shows the view along Phobos’ shorter axes and it appears smaller than the other two images, which show the view along Phobos’ longest axis. Credit: T. Duxbury

The flyby will enable Mars Express instruments, especially the HRSC – the High Resolution Stereo Camera – to see points of the moon’s surface that have not previously been observed from such a close range.

“This flyby will provide very good viewing, within 1,000 km, of an area previously not seen well,” Dmitri Titov, ESA’s Mars Express project scientist. “HRSC will be taking images; the MARSIS radar and the ASPERA-3 particle instrument will operate as well to sound the subsurface and plasma environment of the moon.”

+ marks the spot

The “+” in the predicted images (see above) indicates a possible landing site for the future Russian Phobos Grunt sample return mission.

“This flyby is important as it will allow us to finally view this area on Phobos that has yet to be seen at high resolution and excellent lighting,” says Thomas Duxbury, professor in planetary science at George Mason University, USA.

In the past, Mars Express has made closer flybys, but not by much. On 29 December 2013, Mars Express flew by at just 45 km, close enough that the moon’s gravity pulled the spacecraft slightly off its course, enabling new estimates of the Phobos mass and density.

Phobos 2010

Phobos as seen by the HRSC nadir channel during Mars Express Orbit 7926 in 2010. Credit: ESA/DLR/FU Berlin (G. Neukum)

The flyby is an operational challenge as well as a scientific opportunity, as the positions of the moon and Mars Express must be known very, very precisely in order to safely make the ‘skim-by’.

Commands on board

Commands to trigger the instruments’ observations were uploaded  Thursday, 7 January, following last-minute optimisation of the expected position of Phobos relative to the spacecraft provided by the flight dynamics team at ESOC , Darmstadt.

“This is needed due to the high level of precision required to target Phobos with the instruments at such a close distance,” says Mars Express Spacecraft Operations Engineer Simon Wood.

“The activity will then take place fully automated and without intervention by the operations team at ESOC, who will be closely monitoring the flyby.”

Deciphering Phobos

Flybys such as this help generate evidence to understand how the moon was formed.

The mass of Phobos is estimated as 1.0603 x 10^16 kg (uncertainty less than 0.5 %) and the density is 1862 kg/m3 (uncertainty less than 2%). For comparison, the density of Mars is about 3930 kg/m3, and Earth has a density of around 5520 kg/m3.

The low density of Phobos is consistent with the moon having a high porosity with an uneven mass distribution; in other words, it is essentially a rubble pile with large empty spaces between the rocky blocks that make up the moon’s interior.

This favours the formation scenario in which Phobos was born in orbit around Mars from a disc of debris and is not a captured asteroid – one of the other leading theories for how Phobos and its sibling Deimos came into existence.

Mars Express in orbit around Mars. Credit: ESA/AOES Medialab

Mars Express in orbit around Mars. Credit: ESA/AOES Medialab

The debris disc could have resulted from a large impact on the surface of Mars, or perhaps Phobos (and maybe Deimos) formed from left-over debris from the formation of Mars itself.

Data from such flybys will also prove valuable in planning future robotic or even human missions to land on the moon, and ideal location from which to observe Mars.

It is expected that the initial results from this flyby will be available in the coming weeks.

Editor’s comment: It is interesting to note that, because the polar orbit of Mars Express intersects the equatorial orbit of Phobos, at some point in the future – long after Mars Express has depleted its fuel and has been shut down – the spacecraft is likely to impact the moon.

How to hide behind a planet

Remember those raging snowball battles you had as a child in the school yard, during recess? The best strategy, if you were in the thick of it, is to hide behind something (or someone) massive – such as a tree – or another guy on your team. The fatter the guy, the more protection he offered.

Snowball fight on the National Mall, Washington, DC. Credit: CC by-nc-sa/2.0/Joe Newman http://www.cosmicsmudge.com

Snowball fight on the National Mall, Washington, DC. Credit: CC by-nc-sa/2.0/Joe Newman http://www.cosmicsmudge.com

Ah, those were the days… alas, long past. Or are they?

In fact, the situation we are preparing for now, or at least the situation our spacecraft is going to face on 19 October, isn’t all that different from the snowball fights of yore, at least in principle. Well, it is true that any cometary dust particles that might have MEX’s name on them are likely to be microscopic. On the other hand, they will be travelling at 56 km/s and at that speeds, even microscopic dust can pack a hefty punch.

There are no trees in the immediate vicinity of Mars Express, but there is a rather ‘fat guy’ to hide behind – one with a diameter of almost 6800 kilometres. That’s planet Mars, of course. The orbit of Mars Express around the planet is polar, it is eccentric (during every orbital revolution, which lasts 7 hours, the spacecraft passes as close as 350 km at the lowest point of the orbit (periares, in astronomers’ parlance) and as far as 10 500 km at the highest point (apoares) above the Martian surface.

The fundamental laws of celestial mechanics cannot be infringed – not even a tiny little bit and not even in an emergency or when no one is looking – dictate that MEX moves much faster at periares than it does at apoares. Other more complicated laws of physics dictate that a spacecraft orbit is not immutable: its orientation in space and also its shape are subject to variations due to external forces, known as perturbations. Some of these variations in the orbital parameters are periodic, some are not, and you can’t do much about them.

You might fight them – for a time – by using the rocket engines aboard the spacecraft and the propellant in its tanks, but that will be effective only if you have lots of propellant – precisely the one thing that Mars Express does not have. Our arrival at Mars and positioning onto the spacecraft’s science orbit used up 90% of the fuel that was on board. After all, the spacecraft was designed for a two-year mission and it has been at Mars for more than 10. This gain in mission duration is thanks largely to the skill and ingenuity of the Flight Dynamics team in making the spacecraft perform the observations that the science team required with the least possible fuel expenditure.

By 19 October, the evolution of the orbit will be such that when viewed from the direction from which the comet will be arriving (we know this direction quite accurately and can therefore simulate the encounter with a computer), it will look like in the image below. This shows where the spacecraft would be on its orbit if we did nothing.

Mars Express zipping behind Mars

Mars Express zipping behind Mars

The closest encounter takes place at 18:30. This is when we expect the concentration of mostly microscopic particles of comet dust to be greatest. About an hour and a half later, Mars will pass through the orbital plane of the comet. That’s where we foresee the highest risk of being hit by larger particles. Once again, there is a trade-off as the larger particles carry more energy but there are fewer of them. Let’s assume that the danger is greatest at closest approach (although as mentioned in a previous post, more observations are still needed to improve our models of what to expect in the encounter).

The approaching comet would see MEX pass behind Mars just after 16:30, and it would reappear about half an hour later. It so happens that the time when the spacecraft is behind Mars (and therefore protected from the dust that accompanies the comet) coincides with periares. If you recall, that is when it is also moving fastest on its orbit. The fact that the spacecraft is behind Mars for only half an hour, while the passage through the dust tail is likely to last a few hours, means that this alone is not a (full) answer to our problem of how to endure passing through a comet coma. It does, however, provide a short respite from exposure to potentially damaging particles.

There is one thing we can change, however. Note that if we do nothing, then at 18:30, at the closest encounter, MEX would be in the open, far from any protection. We can at least change the time of periares such that it occurs at 18:30, not 16:41. Though we can change neither the shape nor the orientation of the orbit, we can change this one parameter. And it will cost only very little propellant.

In essence, what we need to do is to delay the spacecraft passage behind Mars by 109 minutes. That sounds like a major task, and it is, if you try to do it just shortly before the comet is there. But we won’t wait until then.

We plan to do a manoeuvre using the MEX engine already several months in advance. We can cut this delay to, for example, 109 slices of 60 seconds each. If we increase the orbital period by this 60 seconds, then 109 orbits later we have accumulated exactly 109 minutes of delay and MEX will be behind Mars when the snowballs hail down – or rather, when the onslaught of cometary dust is expected to peak.

We could also split the delay into 218 slices of 30 seconds, or 327 slices of 20 seconds, and so on. The important part is that the earlier the change is made, the more revolutions are available and so the smaller the change needed and hence the smaller the fuel consumption.

How do we delay the arrival of a spacecraft at a given position?

You won’t believe this – it’s done by increasing the orbital velocity. Nobody ever said that celestial mechanics was intuitive, right? You know that MEX’s orbit is an ellipse. Draw a line between the closest and the farthest point from Mars and you have the major axis; divide that by two and you have the semi-major axis. Every orbit represents a large amount of energy (that’s why it takes a big rocket to launch something into orbit).

The orbital period (the time it takes to complete one revolution) depends on the orbital energy around a given planet (here Mars), and on nothing else. The orbital energy in turn depends on the size of the semi-major axis, and on nothing else. The larger the semi-major axis, the higher the orbital energy, the longer the orbital period.

Now, if you speed up a spacecraft, you add orbital energy, and therefore, inevitably, you increase its orbital period. This is a simple physical fact, and we are going to make use of it to maximize the chances MEX has of weathering the encounter with Comet Siding Spring.

We can’t break the laws of physics but we can, and will, use physics to our advantage.

How to determine the orbit of a comet?

In movies about the impending end of the world due to a comet impact, one thing is certain: Detecting the comet and computing its orbit are dead easy. The scene starts with a computer screen showing a telescope image of a star field, where we can make out a faint, fuzzy little object that doesn’t quite look as if it belonged there.

Control room at ESA's Optical Ground Station Credit: ESA

Control room at ESA’s Optical Ground Station Credit: ESA

A scientist will throw a brief, disinterested glance at the screen and turn away. Then he stops. His eyes go wide and he snaps around to stare at the screen. He’ll call in his colleagues. Computer programs are started, and people frantically hack away at keyboards. In no time at all, they will have identified the fuzzy blob as a comet that is hurtling in from the frozen recesses of space.

Bruce Willis at the 2010 Comic Con in San Diego. Credit: Gage Skidmore

Better call this guy!

What’s more, in no time at all, they will have determined the comet’s trajectory and they can categorically state that it will hit Earth. A few more frantic calculations and they also know the date and time of impact – Quick, call Bruce Willis!

Neat. But does it really work like that?

In actual fact, one single picture of a comet is just that: a single picture of a comet. All this picture tells you is that there is a comet which, at a certain date and time – a certain epoch (astronomers like to use common words with meanings that are just different enough from what you think the word means to create confusion, so they say ‘epoch’) – appeared to be near stars A, B, and C in constellation X. No more.

From one picture, you can’t tell where it’s heading; you don’t know how close it will get to the Sun, nor if or when a close encounter with any other planet is due. To find out these things, you need more observations – many more of images that were taken at different dates, ideally spanning a long time frame. The longer the better.

Comet C/2013 A1 Siding Spring

There it is!

And even then, computing the orbit is not really a straightforward matter. In fact, it’s a drudgery and you have to go about it in a roundabout way. The pictures don’t tell you the comet’s path. All they do is to tell you where the comet appeared to be seen at the stated epochs from wherever the observatory was located. But that is not what you want to know.

So you have to make an educated guess at the parameters that describe the comet’s trajectory, also known – unsurprisingly – as its ‘orbital parameters’. This initial guess (as even the mathematicians rather candidly refer to it) in all likelihood will be quite far off. That’s OK, it’s just a guess. But at least, you have something to work with.
Euklid-von-Alexandria 1

A famous mathematician

Once you have the initial guess, you have a set of orbital parameters that apply to a given reference epoch that you are free to choose. The known laws of celestial mechanics allow you, if you have a full set of orbital parameters at one epoch, to compute where the object will be at other epochs in the past or in the future. So you can compute where it would have been at the epochs when the comet images were taken. Because the Earth’s orbit is well known, you can then compute in which constellation and next to which stars the comet would have appeared at these epochs, assuming that your initial guess holds.

For the initial guess, you will probably find out that the locations in the sky that you compute for the image epochs are quite different from what the images show. Hardly surprising, given that that was just the initial guess. But then you can make small changes to each of the orbital parameters for the initial guess to see which change makes the computed locations at the image epochs move closer to the actually observed locations. At all times, you can use the very accurately known positions of the background stars to get your bearings.

This procedure is known as ‘orbit determination’.  It is very time-consuming and involves a lot of complicated and repetitive mathematical calculations, which is why nowadays we let a computer handle most of it. The entire process is known as ‘parametric optimisation’ and each step is referred to as an ‘iteration’. As the optimisation process goes on and many iterations have been performed, you will see that for the epochs at which the images were taken, the computed locations, based on the current estimate of the orbital parameters, will move quite close to what you can see in the actual images.

This diagram is indicative of how the orbit determination accuracy for comet C/2013 A1 (Siding Spring) evolved over time.

This diagram is indicative of how the orbit determination accuracy for comet C/2013 A1 (Siding Spring) evolved over time. It shows a derived quantity: the expected encounter distance from Mars. Source of data: NASA/JPL

At some point, there will be no further improvement. The process will have ‘converged’ in mathematicians’ parlance. But wait: this does not mean that you’re done. It just means that you can do no better with the available information, in this case, the available set of photographic images. There is always some uncertainty in measuring just where an object is with respect to the background stars. Comets appear fuzzy and blurred. Who’s to say what their exact position in the celestial vault is on that image you’re gazing at? Might it not be a few hundredths of a degree further up? But, on a cosmic scale, such minute differences matter – a lot!

Furthermore, if the time span between the earliest and the latest available image is short (just a few weeks or even a few days) the information they contain may be ambiguous. Comets on a wide variety of orbits might still appear to be in that location over a short period of time. But you will see that in the results of the orbit determination, because the mathematical process involved will not only give you a best estimate of the orbital parameters, it will also tell you how accurate (or inaccurate) that best estimate is.

The problem is that if you know the orbit only approximately at that reference epoch you selected at the start of the process, and you want to propagate ( i.e. compute the time change in the orbit, subject to the laws of celestial mechanics and any effects that may change the orbit) this orbit to another epoch, you will find that any uncertainty in the initial parameters will balloon, the further you propagate.

In the diagram above, it took almost 200 days to find out that comet Siding Spring would not hit Mars. At that time, the uncertainty in the predicted encounter distance still ran into hundreds of thousands of kilometres. Though the most probable encounter distance was established fairly early, the uncertainty was still significant after more than a year of observation. It took 44 days of observation to achieve even a semblance of an orbit determination – one that was still all over the place, with a predicted mean Mars distance at flyby 900,000 km, with a high guess of 3.6 million!

Sincere thanks to Michael Khan for today’s post – Ed.

 

Phobos flyby

Early in the morning (GMT time) on Sunday, 29 December, ESA’s Mars Express will make the closest-ever flyby of Phobos, one of Mars’ two moons.

The breathtakingly close passage will see Mars Express skim past the moon just 45 km from its surface and promises to provide valuable scientific insight into the unresolved origins of the two Martian moons, Phobos and Deimos (see Mars Express heading toward daring flyby of Phobos).

As the spacecraft passes close to Phobos, it will be pulled slightly off course by the moon’s gravity, changing the spacecraft’s velocity by no more than a few centimetres per second. These small deviations will be reflected in the spacecraft’s radio signals as they are beamed back to Earth, and scientists can then translate them into measurements of the mass and density structure inside the moon.

Earlier flybys, including the previous closest approach of 67 km in March 2010, have already suggested that the moon could be between a quarter and a third empty space – essentially a rubble pile with large spaces between the rocky blocks that make up the moon’s interior.

Knowing the structure of the roughly 27 x 22 x 18 km Phobos will help to solve a big mystery concerning its origin and that of its more distant sibling, Deimos, which orbits Mars at approximately three times greater distance.

Artist’s impression of Mars Express set against a 35 km-wide crater in the Vastitas Borealis region of Mars at approximately 70.5°N / 103°E.

Artist’s impression of Mars Express set against a 35 km-wide crater in the Vastitas Borealis region of Mars at approximately 70.5°N / 103°E. Credit: ESA/DLR/FU-Berlin-G.Neukum

The flyby is not only a scientific challenge, but also an operations one as well, which led our director general earlier this year to mention, ‘I hope that my colleagues at ESOC will prove that they are the best pilots‘.

In fact, Mars Express Spacecraft Operations Manager Michel Denis and the extended ‘team of teams’ responsible for MEX flight operations at ESOC – including the flight operations team, the flight dynamics experts and the ground tracking station specialists – are treating the flyby as a mini ‘mission within a mission’.

“For 35 hours around the time of closest approach [08:09 CET 29-12-13], MEX will conduct a science mission completely different from its routine and highly automated operations for observing Mars,” says Michel.

“Months of preparation will come to fruition, but the scientific prize will be worth the work.”

Here’s the skinny on how the flyby will work.

Continue reading

We are at Maaaaaaaars!

Today’s post – part of a series of reports marking the MEX 10th anniversary – was submitted by Mars Express Operations Manager Michel Denis, who was in the Main Control Room at ESOC during the night of 24/25 December 2003 when Europe arrived at Mars – Ed.

It was 25 December 2003, in the very early morning hours. As Spacecraft Operations Manager, I was invited from the Main Control Room to the large Conference Centre (where the main event at ESOC was happening – Ed.) to report on the ongoing Mars orbit injection manoeuvre. We know it has started, but we didn’t yet know whether it had completed successfully.

Team in ESOC Main Control Room 24 Dec 2003 (pana, left) Credit: ESA/M. Denis

Team in ESOC Main Control Room 24/25 Dec 2003 (panorama, left) Credit: ESA/M. Denis

Whatever my innermost emotions and questions, I talked to the officials and the journalists in the tone you need for these circumstances. I told them that the 39 commands that perform the ‘now or never’ orbit-injection manoeuvre have been verified innumerable times down to the last bit by the best experts; I repeated that the manoeuvre had been rehearsed exhaustively, using extreme simulations of the software and harsh tests of the spacecraft’s main engine by the manufacturer.

As a rational engineer I know that 100% certainty is impossible to achieve.

I pointed out that, if required, the small thrusters can automatically step in to help reduce our speed by almost 3000 km/h to help us get ‘caught’ by the Red Planet’s gravity.

Team in ESOC Main Control Room 24 Dec 2003 (pana, right) Credit: ESA/M. Denis

Team in ESOC Main Control Room 24 Dec 2003 (panorama, right) Credit: ESA/M. Denis

As a rational engineer, I know that 100% certainty is impossible to achieve, and that much can happen in such a 40-minute-long manoeuvre…

Now I had to return back on console, and went back down to the Main Control Room, where my deputy, Alan Moorhouse, was in charge – mainly of waiting at that particular moment.

If you know ESOC, you certainly know the rotunda – a large spiral staircase leading to the Conference Centre (it’s in the H Building; the MCR is in the E Building – Ed.).

Michel Denis Credit: ESA/J. Mai

Michel Denis Credit: ESA/J. Mai

I start going down the steps, floating between two worlds equally tense; from the glossy world of the public event to the protected world of the Main Control Room – a busy cocoon where we had lived already ten days and nights, where the entry manoeuvre has been prepared based on the computations by Flight Dynamics, where all critical commands have finally been assembled and up loaded to our little spaceship 150 million kilometres away.

Flight Dynamics confirm capture, within 0.5% accuracy.

In the middle of the stairs, between the floor of talks and the floor of acts, the mobile phone wiggles in my pocket. A message from Alan: “Flight Dynamics confirm capture, within 0.5% accuracy.”

In everyone’s private or professional life there are turning points which, however planned and expected, represent ‘a giant leap’, to paraphrase a glorious quote. A point with a Before and an After; ‘after’,  our existence is changed, irreversibly.

In these instants, the present is more intense; more present than ever. Overwhelming.

Rotunda staircase at ESOC. Yelling is normally not permitted. Credit: ESA/J. Mai

Rotunda staircase at ESOC. Yelling is normally not permitted. Credit: ESA/J. Mai

So overwhelming, that when you remember this moment years or decades later, you revive it as it were the first time again.

I am overwhelmed, alone in the huge rotunda, perfectly empty, everyone at ESOC is either sitting in the Conference Centre or standing in the control rooms, waiting for the news. Alone, for a few seconds, in this resonant space that makes sounds impressive, where I often sang Christmas carols with the ESOC Choir. Today is Christmas day; whether child or adult, whether you believe or not, in our lives a special date, very emotional.

“We are at Maaaaaaaars!” I could not refrain from yelling, with my loudest voice, to expel from my chest all the emotions of the night and the years of preparation and the last-minute doubts and angst and the incredible joy that seizes me now, just like Mars has seized Mars Express, just like Europe has seized Mars. We are at Mars: now it is true, and nothing can make this not to have happened.

Merry Christmas Europe!  Welcome to Mars!

Curiosity’s Daily Update: MSL Right on Course – TCM-5 Cancelled

Via NASA

With less than three days to go before touchdown on the Red Planet, Curiosity remains in good health, with all systems operating as expected. Given the Mars Science Laboratory spacecraft’s consistent and stable course, today the project decided that the planned Trajectory Correction Maneuver 5 (TCM-5) and its corresponding update to parameters for the autonomous software controlling events during entry, descent and landing will not be necessary. As of 21:35 CEST, the Mars Science Laboratory spacecraft was approximately 753,200 km from Mars, or a little less than twice the distance from Earth to the Moon. It is travelling at about 3,576 m/second. It will gradually increase in speed to about 5,900 m/second) by the time it reaches the top of the Martian atmosphere.