Mars Express, Phobos and the occult … ation

Today’s post contributed by Michael Khan, a mission analyst at ESOC and an avid amateur astronomer – Ed.

Imagine you’re playing Scrabble. The tiles on your rack don’t look as if there is much you could do with them. No vowels. Three ‘Y’s, a ‘Z’, and a ‘G’. Oh no! You might as well pass and exchange all tiles for new ones, Right?

You think you’re good at online Scrabble? Well, I just put “syzygy” on the board – see if you can beat that! Credit: E. Yourdon CC BY-NC-SA

Wrong! (Don’t quit yet!)

Just look for a letter ‘S’ somewhere on the game board and you can append five of your tiles to spell the word SYZYGY (preferably such that it covers a triple word score). Thus you get rid of the ‘Z’ and the ‘Y’s and you earn heaps of points. There will be complaints around the table, but you can relax and let them complain: You really cleaned their clocks with this one. You win!

In astronomy, a syzygy defines a situation where at least three bodies are aligned. Eclipses, such as the 15 April lunar eclipse, are a perfect example of a syzygy in action.

Via Flickr E. Yourdon

In spacecraft operations, syzygies abound, and our Mars Express is no exception. Often, they are regarded as an unavoidable nuisance. When the spacecraft passes through the shadow of Mars and has to run on batteries, that is a syzygy of Mars Express, Mars and the Sun. When MEX is occulted by Mars and communications are interrupted, that is a syzygy of MEX, Mars and the Earth.

Other syzygies, however, offer real science opportunities. Stellar occultations by bodies in the Solar System have led to very important scientific discoveries.

To name but a few:

• The fact that the Saturnian moon Titan has a dense atmosphere (unlike all other planetary moons) was established thanks to stellar occultation measurements – a distant star was observed as Titan appeared to pass in front of it. The starlight did not just wink off and then back on as one could have expected, but rather faded gradually and then equally gradually returned to its original brightness. This was as long ago as 1903. A later stellar occultation, in 1944, permitted spectroscopic measurements that not only confirmed the presence of an atmosphere but also showed that it was composed mainly of nitrogen with a significant methane content. This in turn means that most of the chemical building blocks of life are present there, which arguably is an eminently interesting finding. All thanks to syzygies.
• More recently, in 1977, a stellar occultation of planet Uranus led to the discovery of its very faint rings. This was unexpected (‘serendipitous’ in science parlance): the main purpose of this observation was to study the properties of the Uranus atmosphere. It was also somewhat embarrassing to other astronomers who had published a paper some years earlier, explaining in detail why Uranus does not have rings.
• In 2013, the minor body 10199/Chariklo, a so-called Centaur (a transneptunian object that must have undergone a close encounter with planet Neptune long ago and ended up closer to the Sun, between Saturn and Uranus) was observed during a stellar occultation and was found to have rings – the first proven case of a ringed minor body.
• ESA’s ExoMars 2016 spacecraft will be orbiting Mars and actually performing its major science task at the time of eclipse entry and exit, i.e., at every syzygy of the spacecraft, Mars and the Sun. It will analyse the sunlight that is filtered by the Mars atmosphere before reaching the detectors on board the spacecraft (to analyse even minuscule whiffs of trace gases). It will do that more than 13 thousand times at as many different locations over the period of one Mars year, which will lead to an immense advance in our understanding of how that planet’s atmosphere works, perhaps also leading to crucial clues in the quest for the greatest scientific prize of all: the search for life on celestial bodies other than the Earth.

Back to MEX…

On 28 April, MEX will be occulted not by Mars (that happens all the time), but by tiny Phobos, a roughly potato-shaped body measuring 27 km across its longest axis and 18 across its shortest (a recent low-res webcam pic of Phobos – Ed).

Close-up of Phobos, acquired on 28 July 2008. Credit: ESA/DLR/FU Berlin (G. Neukum)

Phobos has already been imaged and studied many times by different spacecraft. There have even been (unfortunately, unsuccessful) missions dedicated to landing on this Moon and even returning samples from its surface to Earth. Currently the Russian space agency is planning another sample return mission and ESA is studying the possibility of a similar mission of its own, or a collaboration with Russia. Phobos, and its sister moon, Deimos, pose a formidable scientific enigma. There is no convincing theory of their origin that offers a satisfactory explanation for these moons’ current orbits and appearances.

As Phobos has already been extensively observed, its occultation of MEX on 28 April is not expected to yield dramatic discoveries. What we will see is that the radio signal from MEX is cut off and then reappears, around nine seconds later. But science lies in the smallest things.

Though the orbits of Phobos and Deimos are well-known, there is one orbital parameter – the semi-major axis, i.e., its mean distance from Mars – that is notoriously difficult to pin down. The semi-major axis determines the orbital period. Any uncertainty in this parameter leads to an uncertainty in the orbital period, and the problem with that is that the resulting errors add up with time, as orbital revolution follows orbital revolution. A fraction of a second becomes an error of several seconds, then of minutes, and pretty soon you’re faced with a significant difference between where you expect Phobos to be and where it actually is.

This is closely akin to clocks, which also tend to accumulate minuscule errors that end up in giving a completely wrong time, if you don’t reset them regularly. And therein lies the rub. The Phobos orbit data (ephemeris) requires regular re-setting, and re-setting requires accurate calibration data (just as resetting a clock does… in that case, another clock which you know to be accurate). For Phobos, one way of obtaining such data is by measuring exactly when it occults the signal from MEX.

We know MEX’s orbit very precisely because it is computed regularly based on data we get from the radio signals it transmits, so that is our reference. With the accurately measured occultation time, of course taking into account the time the signal needs to travel from Mars to the Earth, we can reset the Phobos ephemeris.