LS: Speaking of Science Column December 15, 2008

Mariner 2 craft cruised by Venus 46 years ago

The first successful mission to another planet in our solar system was being celebrated 46 years ago today.                           

The day before, Dec. 14, 1962, the Mariner 2 spacecraft flew past Venus at a distance of 21,598 miles. The instruments aboard this spacecraft measured the properties of interplanetary space and determined that our closest planetary neighbor had:

•  a surface temperature of 800 degrees Fahrenheit;

•  a thick cloud layer of condensed hydrocarbons from 45 to 60 miles above the surface;

•  200 degree Fahrenheit temperatures at the base of the clouds;

•  much carbon dioxide in the atmosphere below the clouds; 

•  a surface pressure at least 20 times greater than what is experienced on earth;

•  no discernible magnetic field and

•  no detectable radiation belts.

    Since the Sputnik and Explorer missions in 1957 and 1958, the United States was in a space race with Russia. This race entered the interplanetary arena when the first of two redundant Mariner spacecrafts was launched July 22, 1962. (Russia had already launched two unsuccessful spacecraft in February 1961. As it would turn out, the three spacecrafts they launched in the 1962 period also were failures.)

Preparing two spacecrafts for launch during this 19-month opportunity was planned to accommodate the possibility that one spacecraft might not be operative by the time it got to Venus.

We felt fortunate that this plan had been adopted as the Atlas rocket carrying Mariner 1 was aborted by the range safety officer because of erroneous launch vehicle guidance equations.

These problems were corrected, and Mariner 2 was launched Aug. 27. A midcourse maneuver on Sept. 4 corrected trajectory errors remaining after the launch rockets completed their jobs.

This resulted in acceptable estimated errors at the time of closest approach to Venus. We were very hopeful.

It was like the Perils of Pauline en route to Venus, with elements of the spacecraft showing flawed performance that threatened our successful observance of Venus up close 109 days after launch.

Our three-axis-stabilized spacecraft needed to maintain lock on the sun and the earth in order to ensure proper orientation in space. On Sept. 8, we lost attitude control for several minutes for reasons that were not clear, but the spacecraft recovered.

Associated with this event, the earth sensor indicated the brightness of the earth to be well below expected values, which caused great anxiety for what the future may hold.

A little later, the brightness level reading returned to the value expected for that day, determined by the distance from Mariner 2 to the earth. Another temporary loss of attitude was experienced Sept. 29.

On Oct. 31, the output of one of the two solar panels deteriorated abruptly and we turned off the cruise science instruments to save power.

On Nov. 7, the panel was performing normally and we turned on the science instruments.
In another week, the problem reappeared, but by this time, the spacecraft was close enough to the sun that one good solar panel produced enough power to allow the instruments to remain on. Dec. 9 found us losing some temperature telemetry measurements, most likely from a blown fuse.

The spacecraft was overheating quite noticeably by this time. Some temperature sensors reached their maximum readable levels, depriving us of valuable information.

Then a couple of days before closest approach, the on-board computer began showing signs of internal problems. It had been programmed to initiate the encounter sequence at the appropriate time. When it did not do so, a radio command was sent to the spacecraft.

It worked. The rest is history. The scientific discoveries helped re-write science textbooks.

The telemetry continued until Jan. 3, 1963, when the radio signal ceased, probably because of excessive temperatures in the electronic assemblies. That experience taught our team valuable lessons about how to design, build, test and operate interplanetary spacecrafts and the valuable scientific instruments they carry. Many other successful missions followed.

Allan Conrad is a volunteer at the Math & Science Center. He is one of a dwindling number of engineers and scientists who shared in this historical event. He cut his teeth on this project as a spacecraft systems engineer at the Jet Propulsion Laboratory.

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