After a 7-month journey from Earth and a 139-minute rocket burn, the ExoMars Trace Gas Orbiter has settled in around its new home.
It was tense day at the European Space Agency's main control center in Darmstadt, Germany. Some 87 million km away, two spacecraft dispatched from Earth together last March atop a Russian-built Proton rocket were attempting to get to their respective destinations.
The main ExoMars craft, called the Trace Gas Orbiter (TGO), needed to fire its braking rocket for 139 minutes and slow down by more than 1.5 km/s — enough to be captured by the Red Planet's gravity. For its hitchhiking lander, Schiaparelli, the challenge lasted only 6 minutes but was more involved. It had to streak through the thin Martian atmosphere, deploy a parachute, fire thrusters to slow its descent, and then plop unceremoniously onto the surface. The anxiety at mission control was heightened because TGO slipped behind Mars and out of radio contact during its orbital capture.
Not until 16:34 Universal Time (12:34 p.m. EDT) did ground stations receive a strong radio signal showing that TGO had executed its arrival as planned. However, for now the situation is not rosy for Schiaparelli, which had detached from the orbiter on October 16th. Some telemetry indicated that portions of the descent sequence took place, but there's been radio silence from Schiaparelli in the hours since its landing should have occurred.
For now, TGO's orbit will be a highly elongated loop that ranges in altitude from 300 to 96,000 km (190 to 60,000 miles). The instrument teams are looking forward to two single-orbit sets of initial observations during November. Then, beginning next March, the orbiter will repeatedly dip into the uppermost Martian atmosphere, creating a controlled drag that will gradually bleed away orbital energy and gradually shrink the ellipse. By March 2018, the near-polar orbit should be circular, with an altitude of 400 km. Then the real science can begin.
Trace Gases at Mars
ExoMars is actually a two-mission undertaking, with a sophisticated lander to follow in 2020. But TGO, which is roughly the size of a small car and weighs about 3½ tons, is no mere precursor.
It carries four instruments to understand the role of trace gases in the Martian atmosphere — compounds that, despite their tiny abundances, play roles in the bigger story of how Mars evolved and reached its physical state today. Russia's Space Research Institute (IKI) supplied the first two of these in the list below:
NOMAD (short for Nadir and Occultation for Mars Discovery) combines three spectrometers that cover ultraviolet and visible wavelengths (0.2 to 0.65 micron) and infrared (2.2 to 4.3 microns). Besides looking straight down, it will also measure how gases absorb sunlight as the Sun slips behind the planet's limb.
ACS (Atmospheric Chemistry Suite), a trio of infrared spectrometers, complement NOMAD by extending observations out to 17 microns.
FREND (Fine-Resolution Epithermal Neutron Detector) measures neutrons created when cosmic rays bombard the Martian surface. Epithermal ("slow") neutrons indicate the presence of hydrogen, almost exclusively in water, so these can be used to map near-surface deposits of water ice.
CaSSIS (Colour and Stereo Surface Imaging System) carries an telescopic lens with a 880-mm focal length to capture color and stereo imagery of the Red Planet's geologic features. It'll be on the lookout for the onset of dust storms, which are likely to arise in the months ahead.
On the molecular checklists for ACS and NOMAD are water vapor (H2O), methane (CH4), nitrogen oxides (NO and NO2), sulfur dioxide (SO2), and acetylene (C2H2), among others. In addition, these instruments will assess the isotopic ratios of hydrogen, carbon, and oxygen. For example, determining the relative abundance of deuterium ("heavy hydrogen") in atmospheric water vapor is key to establishing both how much water was delivered to Mars by impacting comets and how much of it has escaped to space over the eons.
The most provocative observational objective, without doubt, will be to map the abundance of methane in the Martian atmosphere and over which locations, if any, it appears to be concentrated. Methane is a gas that is inherently unstable in the Martian atmosphere, because it should combine with oxygen (derived from either CO2 or H2O). And yet it's been detected there by various ground-based searches and from the Curiosity lander — so it's being replenished somehow.
Could that "somehow" involve biological processes? We just don't know, but TGO's spectrometers (and, in particular, their ability to distinguish different isotopes of carbon) should help learn the answer. Click here and here for more background on the Martian methane mystery.
Red Planet Redemption
The successful arrival of TGO represents redemption of sorts for ESA's partner, Roskosmos (Russian Space Agency). For all of their decades of space-exploration achievement, the Russians have had a particularly bad string of luck when it comes to visiting Mars.
Meanwhile, ESA managers are anxious about Schiaparelli's whereabouts. They're all painfully aware that the previous attempt, Beagle 2, crashed without sending back a single blip in 2003. NASA's Mars Reconnaissance Orbiter finally found the wreckage in early 2015. Technically, Beagle 2 was a British craft built by a consortium of organizations, but its loss was a blow to the entire European space-exploration community.
We should know more by early tomorrow, after various orbiters have attempted to contact the lander and relay its communications to Earth..