Managers of Europe’s Venus Express satellite likely will have to make do with a lower-than-expected amount of data measuring the planet’s atmospheric and surface temperature because one of the satellite’s seven observing instruments remains inoperable.
Despite months of recovery efforts on the satellite and simulations performed at the European Space Agency’s (ESA) Estec technology center in Noordwijk, Netherlands, the satellite’s Planetary Fourier Spectrometer (PFS) remains locked in a position that will prevent it from collecting any information, according to ESA officials.
Don McCoy, ESA’s Venus Express project manager, said further attempts would be made in the coming months to move the jammed PFS mirror assembly, but that ground teams are running out of ideas on corrective actions.
In a July 13 interview, McCoy said the PFS glitch “remains a mystery to us. We have not been able to reproduce it on the ground and we have not isolated a cause.”
The PFS was designed to measure the temperature and composition of Venus’ atmosphere at altitudes between 55 and 100 kilometers , and from the surface of the planet up to 10 kilometers in altitude. The surface-temperature measurements would have helped isolate areas of volcanic activity.
Losing one of the seven Venus Express instruments will not mean losing one-seventh of the science, McCoy said. Other onboard sensors, especially the Venus Monitoring Camera and the Virtis mapping spectrometer, have near-infrared and infrared observing capabilities and can fill in some of the PFS gaps.
Venus Express was launched in November and entered orbit around Venus in April. Aside from the PFS, the satellite and its payload are healthy and expected to be in operation until October 2007 — and perhaps much longer if ESA approves a mission extension.
The satellite reached its final orbit in May and completed a mission-preparedness review July 4. It is fully operational in a highly elliptical orbit that ranges from 66,000 kilometers to 250 kilometers above the surface of the planet.
The PFS problem lies in its inability to move its pointing mirror assembly into observing position. The pointing device receives incoming radiation from Venus through a small slit at the end of a cylinder. To permit calibration of the instrument, the pointing device is capable of moving its gaze to focus on a black body on the spacecraft, whose temperature is known.
The cylinder has been locked into this calibration angle apparently since Venus Express was launched. Early suspicions that it had moved somewhat gave rise to hopes that it could be further jostled into correct viewing position.
McCoy said it is not clear whether the instrument has moved at all since launch. It may have been stuck in its current position from the beginning, he said, although tests just before launch confirmed that it functioned properly.
ESA officials initially thought the PFS mirror was stuck due to extreme cold. To test that hypothesis and review possible fixes, they placed a PFS model inside a vacuum chamber at Estec and lowered the temperature to simulate the PFS environment.
The same stuck-in-place phenomenon could not be repeated, and thermal conditions were subsequently discarded as a cause when Venus Express moved into a position where the temperature of the spacecraft was much higher.
Hakan Svedhem, Venus Express project scientist, said some data suggests that the mirror assembly might have moved slightly in response to commands in recent months and that “there are still a few things we can try to get it to function,” Svedhem said July 13. “But it is true that things look darker now than they did and that we are now planning for ways to use the other instruments to make up for a loss of the PRS.”
Venus Express uses many of the instruments built initially for ESA’s Mars Express orbiter and the Rosetta comet-chaser satellite, both in orbit. The Venus Express platform also is derived from the Mars Express design to save time and money.
Svedhem said that aside from the PRS problem, the spacecraft’s instruments and platform are performing to specifications, “and in the case of some of the instruments, above specification.”