Keeping MEX warm

Today’s post was contributed by Luke Lucas, a Mars Express spacecraft operations engineer at ESA’s ESOC mission control centre. Read to the bottom for more info and registration for the upcoming Open Data Day, 28 October 2016, at ESOC! Link to live webcast also at foot of this post.

The vacuum of space is a challenging thermal environment. The illuminated side of an object may reach more than 250°C while the non-illuminated side may be less than -150°C.

Without careful consideration such temperature differences could cause parts of the spacecraft to break, twist or fail to function.

For example: the temperature difference between the front and back of the arrays may be hundreds of degrees and when the arrays go from the shadow of an eclipse, sudden and dramatic changes in temperature occur, which can lead to expansion, contraction, torsion and twisting. Selection of correct materials and good structural design are essential.

Mars Express eclipse Credit: ESA

Mars Express eclipse Credit: ESA

Maintaining an optimal thermal environment is the task of the craft’s thermal subsystem. The thermal subsystem includes electrical heaters where needed to keep the spacecraft warm (e.g. to prevent fuel lines from freezing) and passive radiators to keep other units cool. In this way Mars Express can perform scientific observations.

Cooling of some units to prevent overheating can be achieved by the clever siting of radiators. By placing radiators where they will always be facing deep space, passive cooling occurs.

The use of multi-layer insulating blankets around the spacecraft promotes a stable thermal environment and minimises the loss of heat to space.

Electrical heaters are used to heat Mars Express and maintain units, structures and instruments within their safe, warm operating range, with more than 200 thermistors continually measuring the temperature at various points around the spacecraft. By monitoring the measured temperatures, the heaters are turned on or off as needed.

As well as keeping MEX warm, it is important to know how much power is needed to achieve this. Power is supplied by either the solar arrays (when exposed to sunlight) by the batteries during an eclipse.

Power produced by either the solar arrays or batteries is used for platform operations, thermal operations and whatever remains is available for science. The energy provided by the batteries during an eclipse is finite, so knowing how much power is needed for the thermal subsystems means mission controllers can know how much power is available for science. Knowing accurately how much power is needed by the thermal subsystem means we can really maximise science observations.

So how much power does the thermal subsystem need to keep MEX warm?

Well it depends how cold the spacecraft is. And that, in turn, depends on many factors. The largest factors include:

  • Mars orbit Credit: ESA

    Mars orbit Credit: ESA

    How far away is MEX from the Sun? The Sun emits solar flux, measured in W/m2, (power per square meter received) and the further away from the Sun, the less flux received – and so the cooler is Mars Express

  • What is the solar aspect angle? This is the angle of the spacecraft with respect to the Sun? Where is the Sun is shining on MEX? On the top (+Z face), the bottom (-Z face) or the +X face, and at what angle to that face? This will affect the craft’s temperature.
  • How far is MEX from the surface of Mars? The albedo is the amount of energy reflected off the planet. ‘Mars shine’ refers to the amount of energy being reflected by the planet and can affect MEX’s thermal condition.
  • Mars Express orbit Credit: ESA

    Mars Express orbit Credit: ESA

    How far away is Mars from the Sun? This affects the Mars albedo, which in turn affects MEX.

  • Is there an eclipse happening? As seen above, during eclipse, MEX is in Mars’ shadow, receiving no illumination from the Sun, and this can cause a dramatic cool down.
  • What operations are on going? Certain operations, such as using the transmitter to communicate with Earth, warm up certain sections of the spacecraft.

Here are two thermally representative images of the -Y face, one seen during a communication pass and one when no communications were happening.

In a communication pass with temperatures up to 16°C

Mars Express -Y face, during a communication pass with a ground station Credit: ESA

Mars Express -Y face, during a communication pass with a ground station Credit: ESA

When not communicating with Earth and temperatures as low as -12°C

Mars Express -Y face, not during a communication pass with a ground station Credit: ESA

Mars Express -Y face, not during a communication pass with a ground station Credit: ESA

That is to say, that the thermal power required changes continuously as the craft orbits Mars and as Mars orbits the Sun; no two days are the same. Predicting the thermal power required is a puzzle of many parts. But it is a very important matter, because we want to perform as much science as possible.

We have a model we use to predict the power required, but wondered if anyone could derive something better.

Our engineering approach is to look at the factors involved and create an equation. But it is always good to look at any puzzle from more than one view. So we asked you, the public to look at this, as the Mars Express Power Challenge.

And – Wow! – we were thrilled by the responses we received, the predictive models that were built and the amount of information shared among this wonderful community of data scientists, researchers, and space fans!

The challenge is now over and the winners will present their solution on 28 October 2016, at ESA’s ESOC mission control centre, Darmstadt, Germany. On this day, the Centre will host an ‘open data day’ for the candidates – and anyone interested is welcome to attend!

This will be an exciting, inspiring day, full of great ideas and exchanges!

A few tickets for the open data day are still available here via EventBrite.

Watch live 28 October, 10:00-12:00 and 14:00-16:30 CEST


Student of Mars

Today’s post – part of a series of reports marking the MEX 10th anniversary – was submitted by planetary geologist Damien Loizeau, who is on the hunt for water on Mars – Ed.

Damien Loizeau

I got involved in Mars Express when I started my PhD. Mars Express had been in orbit for a bit more than a year, the first results had just been published, and lots of new and exciting data were transmitted every week. Now I am part of two instrument teams for the mission: OMEGA, the imaging spectrometer, and HRSC, the high resolution stereo camera.

I work on the geology of the surface of Mars and these two instruments are perfect to study it. OMEGA helps us to determine the mineralogy of the surface, that is, the composition of the rocks, and we try to understand the age and the formation of the geological units with HRSC.

It was the first time that we had such a large dataset to understand the geology of Mars, and I was starting my scientific career inside this flow of new discoveries.

I could meet many of the leading European and American Mars scientists during the Mars Express instrument team meetings, where the most recent discoveries were presented and discussed. I also had the chance to work directly with the principal investigators of OMEGA and HRSC, in Orsay (France) and Berlin (Germany), respectively.

My first focus was on identifying minerals formed with liquid water. Liquid water is crucial for life on Earth, and it’s of utmost importance to evaluate if Mars was habitable, and if life had a chance to develop there. We mapped clays in different regions of Mars with OMEGA. On Earth, clay minerals mainly form over long periods by the interaction of rocks with liquid water. With the help of the orbiting high resolution cameras like HRSC, we observed that almost all the clay detections corresponded to rocks formed in the very early Martian history. This is a major sign of the drastic climate change that the Red Planet suffered more than 3 billion years ago.

I had the opportunity to make the map below for one of the Mars Express press conferences to illustrate our work, and I have been very happy to see it circulating on the web and in conferences for many years since.

Perspective view of clay-rich rocks (blue) on the old plateaus around the valley of Mawrth Vallis (left) and the crater Oyama (centre), made from a compilation of OMEGA, HRSC and MOLA (NASA Mars Global Surveyor) data. Credits : ESA/OMEGA/HRSC

Lately I had the opportunity to work for two years in one of ESA’s centres – ESTEC – in the Netherlands. I could follow more closely the missions with the scientists in charge of them, and the future projects like ExoMars. It was quite different from the academic world, with lots of new acronyms to remember!

Today, with the help of the instruments of the NASA Mars Reconnaissance Orbiter, we are discovering the diversity of environments were liquid water has been present in the past on Mars, not only at the surface, but also at kilometre depths. But there is still a lot to discover both within the datasets from the spacecraft still in orbit around Mars, and from future missions. Exciting times lie ahead!

First Contact! Mars Express’ first ‘conversation’ with Curiosity

As we reported yesterday, Mars Express had a busy Sunday evening, pointing first at NASA’s Curiosity rover on the surface of Mars and then swinging around to do another relay pass with Opportunity. We received the data from both of these passes this morning over ESA’s New Norcia ground station and, on first look, it seems that both relays were very successful.

First Laser-Zapped Rock on Mars

First Laser-Zapped Rock on Mars. This composite image, with magnified insets, depicts the first laser test by the Chemistry and Camera, or ChemCam, instrument aboard NASA’s Curiosity Mars rover. The composite incorporates a Navigation Camera image taken prior to the test, with insets taken by the camera in ChemCam. The circular insert highlights the rock before the laser test. The square inset is further magnified and processed to show the difference between images taken before and after the laser interrogation of the rock. The test took place on Aug. 19, 2012. Credit: NASA/JPL-Caltech/LANL/CNES/IRAP

In ESA’s MEX team, we’re particularly excited to have had our first contact with Curiosity – proof that the amazing new rover from the United States can talk with our veteran European Mars orbiter!

At the start of the contact, Mars Express was over 3600 km from Curiosity’s landing site in Gale Crater and closed in to only 1300 km by the end of the contact – streaking across the sky as seen from Curiosity.

During this overflight by Mars Express, it ‘hailed’ Curiosity in Gale Crater and the rover responded. The two spacecraft then autonomously established a link with each other and Curiosity flowed data back to Mars Express for nearly 15 minutes. This international chat between two spacecraft in deep space is proof of all our preparation, standardisation and cooperation work in action – so it’s something both agencies can be proud of.

ESA's first 35-metre deep-space ground station is situated at New Norcia, 140 kilometres north of Perth in Australia. The 630 tonne antenna will be used to track Rosetta and Mars Express, the latter to be launched in 2003, as well as other missions in deep space. The ground station was officially opened on 5 March 2003 by the Premier of Western Australia, Hon Dr Geoff Gallop. Credits: ESA

ESA’s first 35-metre deep-space ground station is situated at New Norcia, 140 kilometres north of Perth in Australia. The 630 tonne antenna will be used to track Rosetta and Mars Express, the latter to be launched in 2003, as well as other missions in deep space. The ground station was officially opened on 5 March 2003 by the Premier of Western Australia, Hon Dr Geoff Gallop.
Credits: ESA

The actual data that flowed back was made available to NASA earlier today, who will now retrieve and process the data.

Hopefully we’ll have some info from them in the next couple of days about what exactly was contained within. We’ll also receive (within Tuesday) the ‘housekeeping’ telemetry of Melacom – information on how our radio performed. This will allow us to double-check the performance of this first important contact with Curiosity.

The data was sent at a rate of only 8 kbps – 125 times slower than the 1-Mbit/second Internet connection you might have at home!

We wanted to take things easy to start with, though, and test the performance of the link. Nonetheless, we received 955 data packets from Curiosity, totalling 867 kilobytes of data.

This will be the first of several contacts with Curiosity in the future, as we better learn how to use and optimise this relay link between the two craft and the two space agencies. Watch this space for more details as we get them on this pass and the future contacts between Mars Express and Curiosity.

 

Time delay between Mars and Earth

Spacecraft event time vs. Earth receive time

Mars Express Light Time Delay Display

A photo of the Mars Express delay display on the control system, showing us the critical numbers of one-way light time, two-way light time and the distance from Earth.

One of the most difficult things about operating a spacecraft around Mars (not to mention the different time zones), compared with the Earth, is that it’s so far away!

Mars is so far away in fact that it takes radio signals quite a long time to get from the spacecraft back to Earth. During Curiosity EDL, this delay will be 13 minutes, 48 seconds, about mid-way between the minimum delay of around 4 minutes and the maximum of around 24 minutes.

This makes it a challenge to operate Mars Express because it’s hard to have a conversation with the spacecraft, or react if anything happens on board. If there is a problem and the spacecraft tells us, we won’t know for 13 minutes, and then even if we react straight away it’ll be another 13 minutes before our instructions get back to Mars – there’s a lot that can happen in half an hour at Mars (for example a whole Curiosity landing)!

To keep Mars Express flying safely, we load all the commands for the mission in advance and built in lots of autonomy to let the spacecraft take care of itself – you could say that for the Curiosity landing we’re running completely on autopilot!

The delay is nothing to do with the spacecraft or the hardware on the ground – it can’t be improved by a faster computer or a more powerful radio. In fact it is obeying the fundamental speed limit of the universe – the speed of light.

At 1,079,000,000 km/hour, light is pretty quick; you could get from here to the Moon in a little over a second! But that just underlines how far away Mars is.

All light (or electromagnetic radiation, which includes radio signals) travels up to this speed, and radio waves from Earth to Mars Express and back are no exception. Take a look at the Wikipedia article on the speed of light and you’ll see how, in 1905, Einstein came upon the concept of this cosmic speed limit.

Above all, for tomorrow’s coverage of the Curiosity landing it makes it challenging for us to work out when to tell you what’s happening (as you’ve seen in our three column timeline)!

At ESOC, we talk about two different times – Spacecraft Event Time (SCET) and Earth Received Time (ERT). The former is what’s actually happening at Mars right now, although we won’t hear about it until over 13 minutes later, a time we call ERT.

The delay between the two is usually called the One-Way Light Time (OWLT) and the time for a message to go to Mars and come back is the Two-Way Light Time (TWLT), or round-trip time.

During all our coverage we’ll follow NASA’s lead and generally communicate events here and on Twitter to you in ERT because that’s when we’ll actually know what’s happened. If we do communicate something in SCET we’ll let you know so you (and us too) don’t get confused – it’s all part of the fun of exploring the Solar System!

Getting the data back – Store and Forward

This video shows the view of Mars Express from the Earth before, during and after the Curiosity landing. It demonstrates perfectly why we need to use a method called ‘store and forward’ to get the recording of the descent back to Earth.

At the start and end of the video, you can see Mars Express’ big 1.6-m High Gain Antenna (the grey circle on the front of the spacecraft) pointed right at us. We need that to be pointed at us to be able to talk to Mars Express from Earth.

Unfortunately, to support the landing of Curiosity, we need to point our Melacom antennas at the incoming lander, and they’re fixed perpendicular to the High Gain antenna. That’s why during the middle of the video you see the spacecraft turn the High Gain Antenna away from us – it’s so it can get the best possible view of the incoming lander.

In order to relay the recording of the descent, we store the data in our on-board memory – a bit like saving a picture to the memory card on your digital camera.

We have 12 Gigabits of on-board memory, which might sound small compared to your home computer, but it’s plenty of space for what we need. Once we turn back to Earth, we can tell the spacecraft to forward the recorded data back to Earth, just like plugging in your camera and downloading the results from the memory card. In fact, due to the criticality of the Curiosity recording, we’ll transmit it to Earth three times to make sure it reaches us safely.

So when you’re watching the landing tomorrow, note that’s why it’ll take us a bit of time to swing the spacecraft around and dump the recorded data to ground. The JPL orbiter Mars Reconnaissance Orbiter will do the same thing and so will experience a similar delay.

In contrast, the live relay from Mars to Earth will be provided by JPL’s venerable Mars Odyssey orbiter, the oldest spacecraft currently operating around Mars. It uses a different mode, called ‘bent pipe’, where it takes the incoming data and ‘bends’ it around and blasts it back towards Earth more or less simultaneously.

If all goes according to plan, this direct relay will be NASA’s first confirmation of a successful landing, and the detailed recordings made in ‘store and forward’ by the other two orbiters will follow shortly after to provide us a full picture of this historic landing.

Melacom – Europe’s voice & ears at Mars

Melacom

A photo of the Melacom UHF communications package carried on Mars Express. Credit: QinetiQ

When Curiosity lands on Mars, the radio receiver on Mars Express which will be listening in is Melacom. This radio was developed for Mars Express by the UK company, QinetiQ in order to support the Beagle-2 lander which was carried on Mars Express.

Sadly the Beagle-2 lander failed to land successfully, but the Melacom lander communications package was not wasted and has been used to contact every single Mars lander to successfully land since the Mars Express launch in 2003.

Mars Express has a large X-Band and S-Band radio system that lets it talk to Earth, but Melacom was specially designed as a separate UHF radio system to let it talk to landers on the surface of Mars. The radio supports a number of different modes, including the ability to hold a two-way data communication with a lander and the open loop mode we described earlier. It implements a standard known as Proximity-1, developed by CCSDS – an international committee that works on standards such as this to ensure that any spacecraft can talk to any other, such as the European Mars Express and the American Curiosity [more details on the excellent work done at CCSDS by ESA, NASA and other agencies here – Ed.].

Melacom Communications System Installed On- board Mars Express

Another shot of Melacom after installation on Mars Express, taken while the spacecraft was being built.

The radio has been used successfully many times, including open loop recording of JPL’s Phoenix lander as it landed on Mars in 2008.

In preparation for the arrival of Curiosity, our in-flight testing intensified and we’ve conducted a number of demonstration passes with NASA’s Opportunity Mars Exploration Rover, operated by JPL. During these passes we demonstrated the ability of spacecraft from two agencies to coordinate and work together at Mars, exchanging telemetry data and commands and conducting recordings.

In anticipation of the arrival, a team from QinetiQ also took a test model of the Melacom radio to JPL to perform ground compatibility testing with a similar model of the Curiosity radio. Through all of these activities, we’re confident that we’ll all be speaking the same language at Mars when Curiosity arrives tomorrow.

To learn a lot more in depth information about the Melacom radio and our support of the Curiosity mission using it, take a look at this conference paper by our Melacom engineer, Olivier Reboud.

What is Open Loop Recording?

How Mars Express will listen to Curiosity

3-D waterfall diagram showing the open loop recording made by Mars Express of MER-B (Opportunity) during the rehearsal overflight for Curiosity EDL.

You’ll see a lot on our coverage of the Curiosity landing about Open Loop Recording,’ something which was hinted at in a previous post about the difference between ‘signal’ and ‘data’.

OLR refers to the type of recording that will be made by Mars Express as Curiosity descends towards Mars, and in parallel by ESA’s New Norcia station here on Earth.

In open loop recording, we don’t try to decode the bits and bytes being sent by the descending lander but instead try and listen to as much of the radio spectrum as we can, hopefully detecting the tone of the lander’s transmissions within this spectrum. Think of it like listening to a crowd of people – you can either focus on the words one person is saying, or listen to the whole crowd to get a full picture of what’s going on; that’s what we’ll do with open loop recording.

On Mars Express we’ll use our UHF Melacom radio to listen in on the UHF part of the spectrum – usually used on Earth for radio and television transmissions; it’s also used at Mars as the frequency that different orbiters and landers use to talk to each other.

From New Norcia we’ll be listening to the X-Band part of the spectrum – used on Earth mainly for radar systems but also as a way of communicating with spacecraft across the solar system (Mars Express uses X-Band for its main link back to Earth).

Each of these parts of the spectrum is actually a wide range of frequencies and in open loop we listen to as many as possible, creating a diagram like the one in the picture above.

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Mars Express – timeline for MSL support

Early on 6 August, Mars Express will receive crucial signals from NASA’s Mars Science Laboratory mission as it delivers the car-sized Curiosity rover onto the Red Planet. The ESA spacecraft will begin tracking the NASA mission 45 minutes before it enters the martian atmosphere; an ESA ground station will also record vital signals.

The highlight of ESA’s support for NASA’s Curiosity landing happens at 06:29 on Monday, 6 August, when the Mars Express Lander Communication (MELACOM) system is switched on. Recording of the radio signals transmitted by the Mars Science Laboratory (MSL) is planned to begin at 07:09 and end at 07:37 (all times shown as ground event time in CEST).

ESA’s ground tracking station in New Norcia, Australia, will also listen and record signals from the NASA mission at the same time.

Notes:
CEST = UTC + 2 hours
Earth time = Mars time + 13min:48sec
MEX: Mars Express
MSL: Mars Science Laboratory
NNO: ESA New Norcia station
AOS: Acquisition of signal
S/C: Spacecraft
All times subject to change

Event Earth CEST Earth UTC S/C UTC Notes
DSS-15 (G/S) AOS 4:03:00 2:03:00
NNO AOS MEX 4:03:00 2:03:00
NNO AOS MSL 4:05:00 2:05:00
DSS-15 LOS MEX 6:05:00 4:05:00
NNO LOS MEX 6:05:00 4:05:00
NNO start recording MSL signals 6:25:00 4:25:00
MEX starts slew to point at MSL 6:06:30 4:06:30 3:52:42
MEX MELACOM reciever ON 6:28:48 4:28:48 4:15:00
MEX ends slew to point at MSL 6:36:38 4:36:38 4:22:50 Now pointing at MSL
MEX starts recording MSL signals 7:08:48 5:08:48 4:55:00
MSL Cruise Stage separation 7:14:34 5:14:34 5:00:46 MSL starts transmitting
MRO UHF TM capture starts 7:21:34 5:21:34 5:07:46
MSL Atmosphere entry 7:24:34 5:24:34 5:10:46
MSL Start plasma attenuation 7:26:13 5:26:13 5:12:25
ODY UHF bent pipe relay start 7:26:34 5:26:34 5:12:46
MSL End plasma attenuation 7:27:13 5:27:13 5:13:25
MSL Parachute deploys 7:28:46 5:28:46 5:14:58
MSL Heat shield separation 7:29:07 5:29:07 5:15:19
MSL Backshell separation 7:30:40 5:30:40 5:16:52
MSL Curiosity separation 7:31:17 5:31:17 5:17:29
Curiosity touchdown 7:31:37 5:31:37 5:17:49 Planned
MEX MELACOM reciever OFF 7:36:48 5:36:48 5:23:00
ODY UHF bent pipe relay end 7:37:37 5:37:37 5:23:49
MEX starts slew to point at Earth 7:38:58 5:38:58 5:25:10
NNO stop recording MSL signals 7:40:00 5:40:00
MEX ends slew to point at Earth 8:09:46 6:09:46 5:55:58 Now pointing at Earth
MEX transmitter ON 8:09:48 6:09:48 5:56:00
MEX start sending TM 8:15:00 6:15:00 6:01:12
MEX start recording download 8:15:31 6:15:31 6:01:43
MEX stop recording download 8:40:31 6:40:31 6:26:43
ESOC team passes data to NASA 8:42:00 6:42:00