The Mars Reconnaissance Orbiter has begun the six months of 鈥渁erobraking鈥� needed to reach its final planet-hugging orbit over the Red Planet. And one of the six scientific instruments onboard has been tested as a way of measuring how dense the atmosphere is before each aerobrake dip into it.
MRO slipped into orbit around Mars on 10 March after a seven-month journey from Earth. But its initial 35-hour orbit is too elongated for the high-resolution observations it is designed to make. So rather than carry fuel from Earth to fire thrusters and slow itself down into a circular orbit, it will employ an aerobraking procedure that has been used by NASA鈥檚 two other spacecraft already in orbit around Mars.
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During the procedure, it will dip into the atmosphere about 500 times over the next six months, using the friction of atmospheric drag to move into a two-hour orbit that skims just 250 kilometres above the planet鈥檚 surface. It made the first such dip on Tuesday, swinging from as far out as 46,000 km down into the uppermost atmosphere, at an altitude of 147 km.
But the aerobraking process can be tricky. To decide which altitude to aim for in a particular dip, engineers must estimate the density of the atmosphere. But fleeting phenomena such as dust storms can increase the atmospheric density at any given altitude by a factor of two or more, says Daniel McCleese, a planetary scientist at NASA鈥檚 Jet Propulsion Laboratory in Pasadena, California, US.
Overcooking it
鈥淚n a real sense, the atmosphere grows,鈥� he told 快猫短视频. 鈥淚f you were aiming at an altitude above the surface and did not know of the density change, you risk overheating the spacecraft 鈥� or in an extreme case, actually causing the spacecraft to crash into the planet.鈥�
So mission members are relying on observations taken by the Thermal Emission Spectrometer onboard the Mars Global Surveyor spacecraft, already in orbit around the planet, to estimate the atmosphere鈥檚 true density. But if that instrument, or another onboard the orbiting Mars Odyssey spacecraft, were to fail, MRO could potentially carry out the observations itself with an instrument called the Mars Climate Sounder (MCS).
When mission managers switched the instrument on at the end of March, they found it could take the necessary data. 鈥淚f something should happen to the instruments on the other spacecraft, we鈥檝e demonstrated we can provide the information needed,鈥� says McCleese, who is the instrument鈥檚 principal investigator. 鈥淚t鈥檚 an engineering back-up.鈥�
Daily weather
Now, the MCS has been switched off and will remain off 鈥� along with the craft鈥檚 other five instruments 鈥� for the next six months, unless it is needed. But when MRO reaches its final, circular orbit, the MCS will begin its science mission. It will use its two telescopes to observe the surface and atmosphere at nine different wavelength ranges, from the ultraviolet to the far infrared.
It will measure the vertical distribution of temperature, atmospheric density, dust and water vapour from the surface up to an altitude of 80 km, McCleese explains. 鈥淲e can put all of this information together to characterise the climate and the daily weather on Mars.鈥�
McCleese is especially anxious to begin these observations because he had led the development of two similar instruments that flew on spacecraft that ultimately failed when they reached Mars.
NASA鈥檚 Mars Observer spacecraft fell silent in 1993 when it approached the planet 鈥� probably because of a leak caused when its propulsion system was pressurised. And the Mars Climate Orbiter is thought to have broken up in the planet鈥檚 atmosphere in 1999 due to the accidental use of both metric and Imperial units by collaborating teams, which scuppered a critical manoeuvre.
鈥淚 feel as if my scientific career has been in suspended animation for 20 years,鈥� says McCleese. 鈥淚鈥檓 elated now 鈥� although all of us are a lot older, we鈥檙e just as excited as we were when we launched the very first time.鈥�
