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SYFY WIRE Bad Astronomy

Update ExoMars: Red Planet goes behind Sun, still infested with alien robots

By Phil Plait
Schiaparelli descends

The European Space Agency’s current ExoMars mission has had a bumpy ride from the third to the fourth planet from the Sun, but right now things are looking good.

Launched in 2016, the mission had two parts: The Trace Gas Orbiter (or TGO), which was designed to orbit Mars and investigate the planet’s atmosphere, and the Schiaparelli lander, which was mostly a technology testbed to better understand how to land robotic explorers on Mars. NASA has done the latter many times — not always successfully —  but ESA hasn’t done so yet.

TGO is doing fine. It was initially on a long, elliptical orbit around Mars (the easiest kind to establish upon arrival) but has been executing a series of short engine burns that drop the low point in its orbit (called periareion, for “near to Mars”) into the upper parts of the atmosphere. That causes drag with the thinly distributed molecules there, taking energy away from the spacecraft’s orbit, lowering and circularizing it. Called aerobraking, this maneuver will eventually put TGO into a circular orbit at a height of about 400 km above the surface. The spacecraft will then orbit the planet once every two hours or so.

That will happen in 2018. Right now, it’s still in the elliptical orbit that stretches from about 200 km above the surface to 33,000 km out. That’s still a useful path! For example, it passed about 7700 km from the Martian moon Phobos, and dusty, battered space potato of a satellite, and took this pretty nifty image in October of 2016:


It also has been observing the thin air of Mars, detecting carbon dioxide (the major component) as well as small amounts of water vapor. Eventually it will look for traces of methane, which has been positively detected in the Martian atmosphere but is poorly understood. On Earth, the major source of methane is biological activity, including livestock (by, um, outgassing) and human activity, including production and use of coal. It’s a greenhouse gas, but methane molecules are fragile and tend to react easily when oxygen is present. In Earth’s air the amount of methane is more or less stable, with the destruction of the molecules balancing their creation. That’s good, because methane is a very strong greenhouse gas, stronger than CO2.

Its presence on Mars is more difficult to explain. It’s due to some sort of geological process, but just what isn’t well known. TGO will map its concentration and location, hopefully providing needed clues to the gas’s origin.

Mellish crater

Things, however, are not so good for the Schiaparelli lander. In fact, that part of the mission, in some sense, ended before it really began: It crashed into the planet on October 19, 2016, instead of softly touching down.

The crash investigation recently ended, and they found that a confused measurement device on board Schiaparelli instigated the impact. The lander deployed from the orbiter cleanly on the way to Mars. As the lander entered the upper atmosphere, the parachute also deployed as designed. However, it caused the lander to vibrate, or oscillate, for a few moments. A device called an inertial measurement unit confused that motion for a rotation of the spacecraft and got a reading far higher than it was designed for. It saturated, basically pegging the needle.

This only lasted for about a second, but that was enough. The odd reading was interpreted by the lander as its being upside-down, and the software wasn’t designed to handle that. When it did the math, it incorrectly calculated that it had a negative altitude, and so it interpreted this as being on the surface. It ejected the parachute and fired its landing thruster, but it was far too early and for too short a time.

This happened while it was still 3.7 kilometers (over two miles) above the surface. It free-fell the rest of the way, impacting Mars at a speed of 370 km/hour — nearly four times faster than a car on the highway. It didn’t survive. The impact scar and debris have been spotted by other orbiters.

Schiaparelli crash site

The good news is that the engineers learned a lot from the event, and won’t make those same mistakes a second time. In fact, they’ve said that some of the errors leading to the crash wouldn’t have been detected if the lander had made it down safely, and that could have led to disaster on a future mission. Since Schiaparelli was designed to test the hard- and software, in one way, this was fortunate. Better Schiaparelli than a far more sophisticated and expensive science lander.

Not that this crash was a good thing, but when it comes to space travel, every mistake is a chance to learn. At least, in this case, the loss was minimized.

Mars and the Sun

Right now, Mars is nearly on the opposite side of the Sun as seen from Earth. Our orbit is closer to the Sun, and faster. Once every 26 months or so, we pass between the Sun and Mars, and then, roughly 13 months later, the Earth is on the opposite side of its orbit from Mars (remember, Mars moves, too, so it takes a while for us to pull ahead); this means that, from Earth, Mars and the Sun are very close together in the sky (called solar conjunction). That means it’s more difficult to communicate with spacecraft there — the Sun is the brightest radio source in the sky — so TGO has been commanded to sit tight in its current orbit for now. In a few weeks, it’ll start dipping its orbit again, hopefully on its way to a nice, stable circular science orbit.

It joins a veritable fleet of other robotic craft there, including some from the U.S., one from India and another by the ESA. It may be quite some time before humans go to the Red Planet, but our uncrewed proxies are still working apace. Mars is dry and cold and probably lifeless, but that doesn’t mean it’s not a dynamic and interesting world. I’m glad humans all over our planet are still interested in exploring it.

[Top image: ESA/ATG medialab]