Vega At Venus Options

[Paolo]

I have an AWST article that says that the Vega buses were occulted by Venus and performed radioccultation experiments. However, no results were apparently ever published and the occultation is not mentioned anywhere else. Anybody knows anything more?

[bobik]

What did happen with the Vega 1 lander during the descent through the atmosphere of Venus? In his book Infinity Beckoned, Jay Gallentine wrote, based on an interview with Viktor Kerzhanovich who was responsible for the Doppler measurements:

The lander continued its plummet. Forty-five minutes of surf and counting. But with only ninety-five thousand feet remaining, disaster spat once again. Call it a “shock layer,” call it “lightning clouds”: planetary scientists can’t agree on a name but the effect is the same regardless. An incredible surge of energy buffeted the Vega 1 lander, flinging it back upward at seventy miles an hour and fooling the urn into thinking it was already on the ground. With absolutely nothing at its feet the soil drill fred up, probing for scratchy rock that wasn’t there.

This sounds quite incredible. Strangely enough, it seems no scientific paper which details these Doppler measurements was ever published.

[climber]

I guess we have to be patient, this topic is active once every 10 years

[JRehling]

I hadn't heard of this Vega anomaly before, but it makes me curious as to whether or not it is related to the Pioneer Venus anomaly. To standardize units (odd that the Soviet mission has values reported in feet!!!), the Pioneer Venus probes encountered an anomaly at 12.5 km while this Vega anomaly was at 29 km – much higher.

There is a bit of an implicit assumption that the worlds we probe are in a steady state, although we know this not to be true. I like to use hurricanes as an analogue. Only a very tiny fraction of the Earth's surface experiences hurricane conditions at any point in time, but they do exist, and if your probe hit one, you'd witness something remarkable and perhaps essential to the atmosphere's thermal budget, but very atypical.

Maybe Venus has some wild stuff going on in that opaque atmosphere at layers we've never probed with any consistency.

[nprev]

Yep. With an atmosphere of that depth and density along with so much incipient solar energy it is perhaps surprising that Venus is as calm as it seems to be. Small-scale windstorms doubtless occur far more frequently than we've been able to remotely observe.

[Paolo]

this is what I wrote on the subject in my book "Robotic Exploration of the Solar System - part 2"

Meanwhile, the lander, having released its parachute at a height of 47 km, was freely falling. It had begun to report data immediately upon being extracted from the entry shield. About 15 minutes prior to landing, it suffered a major problem. The strong turbulence and wind buffeting led the 8-sensor accelerometer to believe that the landing had occurred. This initiated the surface activity, starting with the drill, which collected air! Remarkably, this occurred near the altitude at which all of the Pioneer Venus atmospheric probes had experienced instrument failures, suggesting that they may have suffered intense vibration.

[rlorenz]

this is what I wrote on the subject in my book "Robotic Exploration of the Solar System - part 2"

The Pioneer Venus sensor failures have been attributed to a materials failure in the external wiring insulation. The NASA workshop report (as discussed in my Space Systems Failures book) suggests adhesive tape decomposition yielded HF which corroded the insulation, although I think there are some who consider the supercritical CO2 to be the culprit.

I don't believe there is any signature of anomalous dynamics in the Pioneer Venus data (either the VLBI/Doppler tracking, or the accelerometers, although the sampling rate was not high). So I think your association of the two event sets (and you're not alone, sometimes the PV failures get wrapped up in the case for lightning on Venus too) is spurious. (I do like your books overall, btw)

VEGA may just have hit an unlucky gust. There's actually a bit of an art to choosing the 'landing detection' threshold for a planetary mission (I participated in that exercise for Huygens - we actually modified the HASI instrument detection algorithm within a couple of years of arrival after re-assessing impact models). You want to make the threshold low enough that there's no danger of missing the impact (even given zero offset drift on the sensor, low sample rate etc.) but not so low that you trigger inadvertently (which you'd ideally have lots of gust data on which to evaluate). It's a do-able problem, but far from trivial.

[JRehling]

You want to make the threshold low enough that there's no danger of missing the impact (even given zero offset drift on the sensor, low sample rate etc.) but not so low that you trigger inadvertently (which you'd ideally have lots of gust data on which to evaluate).

I wish I could find a better citation (perhaps it doesn't exist), but someone told me that a great golfer was once asked, "What is the key to golf?" and the answer went something like, "Air offers less resistance than earth."

[bobik]

A 2012 review article on the VEGA project states the following about the Vega-1 event:

During the descent of the landers in the atmosphere, from the point of the release of the lower hemisphere and until the landing, measurements were made of the atmospheric temperature and pressure as well as the angular velocity of rotation of the vehicle about the three axes. The curves of the atmospheric temperature and pressure during the descent of the Vega 1 and 2 landers are well consistent with, and almost similar to, the calculated curves, which confirms the validity of the computational model of the atmosphere. ... The maximum spin rates around the longitudinal axis during the parachute descent were 64 and 24 deg/s and those around the transverse axis were 60 and 30 deg/s, respectively; during the descent with drag devices, the longitudinal rates did not exceed 25 and 15 deg/s and the transverse rates did not exceed 60 and 35 deg/s. During the descent of the Vega 1 lander, at an altitude of about 17 km above the planet’s surface, the landing signaling device, a shock sensor set for 6 ± 1.5 units overload, was activated. The altitude region from 10 to 20 km is characterized by the maximum gradient of the horizontal wind component, which must be accompanied by a considerable turbulence of the vertical wind component. Calculations suggest that the premature activation of the signaling device of the Vega 1 lander could have been caused by a sudden vortex flow with a velocity of more than 30 m/s.

So in this version of the story, the gust which hit the Vega-1 lander had a velocity of about 30 m/s, but not, as Kerzhanovich indicated, the lander itself was "flung back upward at seventy miles an hour". By the way, wouldn't a gamma-ray densitometer be a reliable instrument for the indication of touchdown?

[JRehling]

So in this version of the story, the gust which hit the Vega-1 lander had a velocity of about 30 m/s, but not, as Kerzhanovich indicated, the lander itself was "flung back upward at seventy miles an hour".

That depends how we parse "more than." And how an object dangling from a parachute would accelerate in an upward gust in gas at that (considerable) density. The two phrasings aren't necessarily contradictory, nor is either very precise regarding what was measured.

[bobik]

At that point, the parachute was already detached from the lander. I agree that all known accounts of the Vega-1 event are very elusive indeed.