PARIS — Chicago-based insurance giant Aon Corp.’s $1.6 billion purchase of reinsurance specialist Benfield Group Ltd. of London will bring together two of the top four space-insurance brokerages, Aon and International Space Brokers (ISB), and create a clear market leader in this market niche, according to insurance officials.

Market share among brokerages specializing in assembling launch and satellite insurance packages is not easy to assess and is a subject of dispute. Industry officials say it is even hard to find a consensus on a common set of metrics for such things as brokerage revenue, the value of the premiums paid or of the assets covered. But Marsh, ISB, Willis and Aon have long been considered the top four in the sector.

Aon and Benfield announced Aug. 22 that they had agreed to an all-cash buyout by Aon. The agreement is expected to clear regulatory approval by the end of the year.

Space insurance had nothing to do with the transaction. In what ISB said was a coincidence, ISB announced two days before the Aon-Benfield deal that its president, Roger Bathurst, had left the company and that ISB had assembled a new management team led by Thierry Mangot, who has long been chief executive of Paris-based ISB France.

Mangot said in an Aug. 29 e-mail that the details of how Aon’s space division and ISB will work together have yet to be settled and will await the conclusion of the Benfield acquisition. While Aon is the buyer, it is Benfield’s ISB that is the larger of the two space brokerages.

Tim Wakeman, an ISB executive vice president based in London, said the two space brokerages were unaware when the Aon purchase of Benfield was announced and could not comment on how Aon and ISB will be merged. “Certainly from a space perspective we think it will be a good fit,” Wakeman said Aug. 28.

Bathurst could not be reached for comment the week of Aug. 25, but industry officials said he left ISB for reasons unrelated to the coming merger with Aon and in 12 months will join Willis in a position that Willis has not disclosed.

Officials said Bathurst’s contract with ISB extends until August 2009 and will prevent him from taking any ISB customers to Willis until then, and perhaps for an extended period depending on his contract.

Willis officials declined to comment on Bathurst’s new role.

One official said ISB’s and Aon’s space work forces do not have substantial overlaps and that the combined total of 40 people likely will not be severely slashed as part of the overall synergies that Aon and Benfield expect to effect following the merger.

Satellite fleet operators Intelsat, Arabsat and Inmarsat are among Aon’s customers. ISB customers include satellite fleet operators SES and Telesat, and the mobile satellite constellation Globalstar, which is building a second-generation system for which ISB hopes to be the assigned broker.

Peter B. de Selding was the Paris bureau chief for SpaceNews.