By Jonathan Amos
Science reporter, BBC News, Farnborough Airshow
The US space agency wants at least one more daylight launch for the shuttle before it will allow a night lift-off.
Recent shuttle launches have been photographed from all angles
Nasa administrator Mike Griffin says flights in darkness will resume once engineers are satisfied with safety changes made to the orbiter system.
The daylight-only rule was set after the Columbia disaster in order that photographers could record a shuttle's climb to space in minute detail.
But the rule limits flights to the International Space Station (ISS).
Dr Griffin told a seminar at the Farnborough International Airshow that the shuttle would need to be able to fly both night and day if the construction programme on the orbiting platform was to be completed by the agreed target date of 2010.
Sixteen missions are necessary to finish the building work - after which the shuttle will be retired.
"The launch of Discovery was one element to resuming night launches," Dr Griffin explained.
"Our plan is to have one more daylight launch before resuming night operations. We do need to resume night operations to complete the space station - we've always known that."
The Nasa administrator said engineers were assessing all the photographic information from the latest launch to confirm that changes to the shuttle's external tank had performed as expected.
Modifications were made to the tank's insulation foam after it was established that damage from flyaway material on lift-off was probably the factor which led to the destruction of the Columbia ship in 2003.
The Atlantis shuttle is scheduled to launch next, in August. If that goes well, Nasa will probably change the launch-time restriction.
That could be good news also for the Hubble Space Telescope. It requires servicing to extend its life - a mission that can only be conducted by a shuttle. Dr Griffin told BBC News a firm decision would come later in the year.
"We have a lot of data to go through before we know whether or not we can do a Hubble flight; and we will probably announce that this Fall.
"We have one more daylight flight we want to do, we have data from [Discovery's] flight that we want to analyse - so it will be this Fall."
Dr Griffin was speaking at a seminar convened to discuss international cooperation in space. He was joined on the panel by the heads of the Russian space agency, Roscosmos, and the European Space Agency (Esa).
Humans in space
Anatoly Perminov and Jean-Jacques Dordain paid tribute to the success of the Discovery mission.
The latter said it was an important milestone for Europe's ambitions in space. It saw the first Esa astronaut, Thomas Reiter, take up a long tour of duty on the space station; and it brought forward the day when Europe's science module, Columbus, could fly to the platform.
Columbus should go to the space station next year (Image: EADS Space)
For the future, Dr Dordain restated his belief that Europe should seek involvement in a next-generation space transportation system capable of taking people into low-Earth orbit and beyond.
The US will produce a Crew Exploration Vehicle once the shuttle is retired; it is a rocket system designed with missions to the Moon and Mars in mind.
And Russia is examining a concept called the Advanced Crew Transportation System (ACTS). Esa's Council has agreed to investigate partnership possibilities in this project and has approved a 15 million euro (£10m; $19m) feasibility study.
"I have a wish that Europe participate in one of the two [next-generation] transportation systems," Dr Dordain told BBC News.
"If we don't, I fear we will always be a second class partner.
"Since now the US is telling me it is not open to cooperation, we are working by default with the Russians. But first we will have 18 months of joint study to make sure that the vehicle we shall design together meets the interests of Europe and Russia."
If Europe and Russia do decide to build a human space vehicle together, it would be launched from Baikonur spaceport in Kazakhstan or Kourou in French Guiana, and would ride into orbit on current or already planned Soyuz and Ariane rocket technology.