NASA scrambles to launch Dawn asteroid mission

 作者:公西渎铱     |      日期:2019-02-28 01:12:05
By Maggie McKee Managers of NASA’s Dawn mission to two large asteroids are scrambling to solve last-minute problems that have cropped up in the countdown to the spacecraft’s planned launch on 7 July. If they cannot be solved, lift-off will be delayed until September or later – a setback that would add about $25 million to the mission. Dawn is designed to orbit Vesta and Ceres, the two largest asteroids in the asteroid belt between Mars and Jupiter, in 2011 and 2015, respectively. It would be the first mission to orbit an asteroid in the belt, and the first spacecraft ever to orbit two targets. But getting the ambitious mission off the ground is proving to be a challenge. NASA cancelled the mission in March 2006, citing technical problems and a budget overrun of 20%, but then reinstated it a few weeks later after mission managers identified ways of overcoming the problems and reining in costs. More recently, delays in the delivery of rocket parts and problems with a crane at the launch site in Florida, US, pushed the launch date from 20 June to 7 July, and workers had to do a last-minute repair on a solar array that was damaged by a dropped wrench. Now, mission managers are trying to work out other issues that threaten to delay the launch even further. One involves a boat that was supposed to track the spacecraft from launch until it separated from its Delta II launch rocket. The boat, which had been stationed in the Pacific Ocean, suffered mechanical trouble on its way to the launch site and then ran into rough seas on its way to be repaired. So NASA has arranged for an aircraft to track the launch instead, and it will conduct a mission flight readiness review on 3 July to ensure that the plane can make all the necessary observations. During the review, it will also determine whether another problem – involving higher-than-expected forces on parts of the launcher’s solid rocket motors – can be cleared for a 7 July launch. If NASA decides not to attempt a launch in the window from 7 to 11 July, it will pull the spacecraft off the launch pad and put it into storage for a possible launch in early September, a delay that would add about $25 million to the $449 million mission. The agency is hoping to launch Dawn as quickly as possible because it does not want to risk delaying its next mission, the Mars-bound Phoenix lander. That is because Phoenix, which will also launch in Florida, has limited launch opportunities. If for some reason it cannot get off the ground during its first launch window from 3 to 25 August, it will have to wait two years for its next chance to lift off. “It’s a ‘hard’ window constraint to get that thing off by August 25th,” Todd May, deputy associate administrator for science programmes at NASA, told reporters on Tuesday. “What we’re trying to do is to manage Dawn so we’re ready to go by the beginning of [its] launch window, before the Phoenix window opens.” The Dawn spacecraft is set to travel to its launch pad at Cape Canaveral Air Force Station in Florida on Wednesday. Scientists hope the mission will shed light on why its two target asteroids are so different – the more “evolved” Vesta was once covered by liquid magma oceans and separated into layers of different density,