Here’s how engineers plan to save the satellite sent to save NASA’s Swift mission

工程师们计划如何拯救那颗被派去营救 NASA“雨燕”任务的卫星

One week ago, more than 200 miles above the Earth, a refrigerator-size satellite making its way toward an attempted rescue of NASA’s $500 million Swift gamma-ray observatory suddenly spun out of control. It didn’t look good. The spacecraft, named Link, was rotating on multiple axes, rendering it unable to maintain a reliable communications link with the ground and complicating efforts to arrest the spin. Two of the satellite’s three reaction wheels, used for pointing, also stopped working.

一周前，在地球上方 200 多英里处，一颗冰箱大小的卫星在前往营救 NASA 价值 5 亿美元的“雨燕”（Swift）伽马射线天文台途中，突然失控旋转。情况不容乐观。这颗名为“Link”的航天器在多个轴向上旋转，导致其无法与地面保持可靠的通信链路，也增加了停止旋转的难度。卫星用于姿态指向的三个反作用轮中，有两个也停止了工作。

Through sporadic radio contact, engineers discovered a problem with some of the spacecraft’s cold gas thrusters used for finer attitude control. The Link satellite is built, owned, and operated by Katalyst Space Technologies, a satellite servicing startup that won a $30 million contract from NASA to fly up to the Swift observatory, grab onto it, and boost its orbit before succumbing to aerodynamic drag and burning up in the atmosphere.

通过零星的无线电联系，工程师们发现航天器用于精细姿态控制的一些冷气推进器出现了问题。“Link”卫星由卫星服务初创公司 Katalyst Space Technologies 制造、拥有并运营。该公司赢得了 NASA 一份价值 3000 万美元的合同，任务是飞往“雨燕”天文台，将其捕获并提升其轨道，以防其因大气阻力而坠入大气层烧毁。

The clock is ticking. In a few months, Swift will be too low for Katalyst to complete the rescue. This is the first time NASA has contracted with a commercial company to service one of its satellites. The space agency gave Katalyst less than a year to put the mission together. The Link satellite launched July 3 to begin the pursuit of Swift, and the mission proceeded mostly according to plan until last Saturday.

时间紧迫。几个月后，“雨燕”的轨道将降至过低，Katalyst 将无法完成营救。这是 NASA 首次与商业公司签约为其卫星提供服务。NASA 给 Katalyst 的任务筹备时间不到一年。“Link”卫星于 7 月 3 日发射，开始追逐“雨燕”，任务在周六之前基本按计划进行。

“When this happened, it was during one of the passes without comms,” said Ghonhee Lee, CEO of Katalyst, said in an interview with Ars. “We were, immediately prior, in a very stable configuration.” Katalyst’s ground team, working from a control center near Denver, hurried to find a fix and recover the Link satellite. The good news was the spacecraft’s other systems remained healthy, including its power supply, three xenon-fueled electric thrusters, and the rendezvous and robotics hardware the Link satellite needs to capture Swift.

“事故发生时，正处于一次无通信的过境期间，”Katalyst 首席执行官 Ghonhee Lee 在接受 Ars 采访时说。“在此之前，我们一直处于非常稳定的配置中。”Katalyst 的地面团队在丹佛附近的控制中心工作，正争分夺秒地寻找解决方案以恢复“Link”卫星。好消息是航天器的其他系统保持健康，包括电源、三个氙气电力推进器，以及“Link”卫星捕获“雨燕”所需的交会对接和机器人硬件。

Ars spoke with Lee, Katalyst’s chief executive, on Friday afternoon as the company’s ground team worked to stabilize the spacecraft. The first step involves using the satellite’s plasma engines to gradually slow the spin. The engines are primarily designed for orbit-raising, but they’re also suitable for attitude control because they can vector their thrust with a two-axis gimbal. Link’s electric propulsion system is efficient but low in thrust, meaning it takes time to generate enough of an impulse to regain full control of the satellite’s pointing.

周五下午，当 Ars 与 Katalyst 首席执行官 Lee 交谈时，该公司的地面团队正在努力稳定航天器。第一步是利用卫星的等离子发动机逐渐减缓旋转。这些发动机主要用于轨道提升，但也适用于姿态控制，因为它们可以通过双轴万向节改变推力方向。“Link”的电力推进系统效率高但推力小，这意味着需要时间产生足够的冲量来重新完全控制卫星的指向。

“We’ve been able to use the thrusters to point in the opposite direction of the rotation rate and affect the attitude control that way, which has been highly effective,” Lee said. This has worked well so far. As of Friday, the spacecraft had cut its spin rate in half, from about 9 degrees per second to approximately 4 degrees per second. Officials hope to further reduce the spin rate, allowing Katalyst to improve communications with the spacecraft.

“我们已经能够利用推进器指向旋转速率的相反方向，从而影响姿态控制，这非常有效，”Lee 说。到目前为止，效果良好。截至周五，航天器的旋转速率已减半，从每秒约 9 度降至每秒约 4 度。官员们希望进一步降低旋转速率，以便 Katalyst 改善与航天器的通信。

“Once we are in a more stable configuration, with much more high-bandwidth comms, we’ll be able to use that to downlink all the rich data (about the condition of the spacecraft),” Lee said. “We do know the states of the reaction wheels and the thrusters and things like that. So our GNC team—guidance, navigation, and control team—has been very hard at work with NASA, essentially remapping all of the control algorithms to be able to have an updated controller ready to go for when we are able to stabilize the spacecraft.”

“一旦我们进入更稳定的配置，并拥有更高带宽的通信，我们将能够利用它下传所有丰富的数据（关于航天器的状况），”Lee 说。“我们确实了解反作用轮、推进器等的状态。因此，我们的 GNC（制导、导航与控制）团队一直在与 NASA 紧密合作，本质上是在重新映射所有的控制算法，以便在我们能够稳定航天器时，准备好更新后的控制器。”

Katalyst may eventually be able to recover the satellite’s two inoperable reaction wheels, but the team’s priority is restoring control by uplinking a new algorithm to manage its orientation through a combination of its one remaining reaction wheel and thrusters. Engineers aren’t sure yet what caused the emergency with the Link spacecraft last weekend. It could have been an internal problem with the satellite or a collision with space junk. Lee said two cameras on the satellite will look for signs of damage after ground teams stabilize it.

Katalyst 最终或许能够恢复卫星那两个无法工作的反作用轮，但团队目前的优先事项是通过上行链路发送新算法，结合仅存的一个反作用轮和推进器来管理其姿态，从而恢复控制。工程师们尚不确定上周末“Link”航天器紧急情况的原因。可能是卫星内部问题，也可能是与太空垃圾相撞。Lee 表示，在地面团队稳定卫星后，卫星上的两台摄像机将寻找损坏迹象。

Whatever the cause, Katalyst’s mission control team lost contact with Link for more than 24 hours after it spun out of control. The satellite then automatically reset itself, just as it was supposed to do. “This was built-in fault protection logic saying, ‘Hey, if I haven’t heard from anybody in 24 hours, there must be something wrong. I’m going to toggle the power, turn it on and off again,’” Lee said. “It’s just built in. However, that shutdown mode is ungraceful. It basically pulls the plug on everything, and that creates some downstream effects for sensitive systems such as the reaction wheels.”

无论原因如何，在“Link”失控后，Katalyst 的任务控制团队与其失去了超过 24 小时的联系。随后，卫星按照预设程序自动重置。“这是内置的故障保护逻辑在说：‘嘿，如果我 24 小时没收到任何信号，肯定出问题了。我要切换电源，重启一下，’”Lee 说。“这是内置功能。然而，这种关机模式并不优雅。它基本上切断了所有电源，这对反作用轮等敏感系统产生了一些后续影响。”

“Basically, there was a big thermal spike that came about as a result of this that over-temperatured the upstream electronic circuits that control the reaction wheels, which ultimately led them to being inoperable,” Lee said. The problem with the spacecraft’s cold gas thrusters seems to be a “separate issue,” Lee said. But engineers don’t have all the data in hand to make a final determination. “These things all happened at once, so it’s hard to delineate between what contributed to creating the situation and what was the consequence of the situation, and that’s something we’ll have to investigate with all the data,” Lee said.

“基本上，这导致了一个巨大的热峰值，使控制反作用轮的上游电子电路过热，最终导致它们无法工作，”Lee 说。航天器冷气推进器的问题似乎是一个“独立问题”，Lee 说。但工程师们还没有掌握所有数据来做出最终判断。“这些事情同时发生，所以很难区分什么是导致这种情况的原因，什么是这种情况的结果，这是我们必须通过所有数据进行调查的事情，”Lee 说。

Nevertheless, Lee said Katalyst still aims to move Link toward NASA’s Swift observatory, perhaps around the end of August. “We’re very committed to pursuing this mission,” Lee said. “We’re working side by side with NASA. I just came from a technical working meeting with their controls team, where we’re reviewing the algorithms. It seems like there’s nothing that is prohibitive.”

尽管如此，Lee 表示 Katalyst 的目标仍然是推动“Link”前往 NASA 的“雨燕”天文台，时间可能在 8 月底左右。“我们非常致力于完成这项任务，”Lee 说。“我们正与 NASA 并肩工作。我刚参加完与他们控制团队的技术工作会议，我们正在审查算法。看起来没有什么不可逾越的障碍。”