确保地球安全免受空间核动力源影响

1978年初,当重力将一颗以装有45千克高浓铀燃料的小型反应堆为动力的失控卫星拉向地球时,整个世界都经受了磨练。

最近,核动力源已被用在探测器、着陆器和漫游车上,用于执行离开地球轨道的任务,如图中一位艺术家眼中的卡西尼号任务;卡西尼号是为探索土星及其卫星而发射的。为携载核动力源卫星制定了全球应急准备和响应安排。(照片:美国国家航空航天局)

1978年初,当重力将一颗以装有45千克高浓铀燃料的小型反应堆为动力的失控卫星拉向地球时,整个世界都经受了磨练。由于“宇宙954号”撞击点无法准确预测,因此应急响应人员不得不假设会有有人居住的区域受到污染,并匆忙进行了设备和响应程序的准备。这是世界上首次出现载有放射性物质的空间物体不受控制地重返大气层的情况。

发射到地球轨道或随航天器遨游的放射性物质在发生事故时可能会对人或环境造成危害,因此需要在国际层面进行严格的应急响应规划和有效的信息共享。这是原子能机构上周为应急响应专家举行的一个网络研讨会的主题。

在大多数核和辐射应急情况下,将有足够的信息让人知晓潜在放射性释放的位置,但在空间活动中,并不总是能够预测撞击的确切位置。“原子能机构已经制定了各种安排,以分享有关任何即将返回的核动力卫星的信息。原子能机构事件和紧急情况评定官员Frederic Stephani在网络研讨会上说:“利用这些数据,各国可以迅速做出响应,以保护公众和环境免受事故后可能扩散的放射性影响。”

“宇宙954号”最终于1978年1月24日在加拿大西北地区坠毁,放射性碎片散落在600公里的范围内,而在10万平方公里的范围内都有放射性扩散。这项名为“晨光行动”的清理行动由加拿大和美国联合协调,回收了80件放射性物品。

“宇宙954号”坠毁便成为全球携载核动力源卫星应急准备和响应安排的原型。

利用核动力探索太空

随着空间物体被推进到往往远离太阳的空间,以实现新的科学发现,它们需要核动力源。但在空间核动力源应用的发射、运行和服役终了任务阶段可能发生各种事故。这些事故可能使核动力源暴露在极端的物理条件下,从而导致进入地球大气层的放射性释放。

对于美国国家航空和航天局(国家航空航天局)等空间机构,将根据每次任务的独特特点适用不同的安全要求。美国国家航空航天局核飞行安全保证管理者Don Helton说:“美国采用完善的国家响应框架来应对范围广泛的各种事件,其中包括许多专门针对放射性事件的关键资产,包括监测和评估能力。”

应急情况下的国际合作

在全球范围内,已经规定了明确的义务。根据《及早通报核事故公约》,如果载有核动力源或放射源的卫星或其他空间物体发生事故,发射该空间物体的国家必须通知可能受影响的国家和原子能机构。原子能机构建立了业务安排,以支持各国履行这一义务。

原子能机构全天候受监测的安全网站,即事件和紧急情况信息交流统一系统,为各国在核或辐射应急期间交流紧急通知和后续信息提供了一个平台。国际组织辐射应急联合管理计划规定了国际组织在应急期间协调行动的框架。

根据“国际组织辐射应急联合管理计划”,作为联合国负责外层空间事务的牵头实体,联合国外层空间事务厅(联合国外空厅)负有明确的责任。“在发生事故时,我们将与发射国联络,以收集有关该物体的信息,并在必要时与能够跟踪空间物体的其他国家联络,以确定重返时间框架和可能的撞击点坐标。然后,我们将确保向原子能机构提供最新的轨迹和撞击预测,用于进一步传播,以协助应急响应工作,”联合国外空厅方案干事Natercia Rodrigues在网络研讨会上说。

当前威胁与未来计划

联合国外空厅还维护着《联合国射入外层空间物体登记册》。在发射到地球轨道或地球轨道以外的所有卫星、探测器、着陆器、载人航天器和空间站飞行单元中,迄今已有超过86%在联合国外空厅进行登记。

那么,未来再次发生“宇宙954号”类型坠毁事故的可能性有多大?

联合国外层空间核动力源工作组是在“宇宙954号”之后一年设立的,该工作组主席Sam Harbison解释说,由于太阳能电池板技术的迅速改进,以及为了避免不必要的潜在放射性物质释放,各国已不再在地球轨道上使用核动力源。“目前在地球轨道上的所有核动力源卫星都是在20世纪60年代至80年代发射的,估计其中最早的一颗要在100多年后才能重返地球大气层。”

最近使用核动力源的是离开地球轨道执行任务的探测器、着陆器和漫游车。实例包括探索土星及其卫星的卡西尼号任务,以及机器人漫游车,如最近发射并将于明年初抵达火星的火星2020毅力号任务。人们渴望利用核动力源来支持人类在月球或火星上的殖民地。Harbison说:“太阳能电池板不足以完成这种距离太阳很远的长时间任务”。“它们必须由体积大、重量重、价格昂贵的火箭推进系统来补充。无论是返回旅程还是维持人类在月球或火星表面的活动,都将需要核动力源。”  

Exploring space with nuclear power

As space objects are propelled towards new scientific discoveries in space, often far away from the Sun, they require nuclear power sources. But accidents can occur during the launch, operation and end-of-service mission phases of space nuclear power source applications. These accidents could expose the nuclear power source to extreme physical conditions leading to a radioactive release into the Earth’s atmosphere.

For space agencies such as the United States National Aeronautics and Space Administration (NASA), different safety requirements will apply depending on the unique features of each mission. “The US uses a well-established National Response Framework to respond to a broad range of incidents, and this includes a lot of key assets that are specific to radiological  events, including monitoring and assessment capabilities,”  said Don Helton, Nuclear Flight Safety Assurance Manager at NASA.

International cooperation in emergencies

Globally, clear obligations have been set out. Under the Convention on Early Notification of a Nuclear Accident, in case of an accident with a satellite or other space object with a nuclear power source or with a radioactive source on board, the countries that launched the space object must notify potentially affected States and the IAEA. The IAEA has established operational arrangements to support countries to meet this obligation.

The IAEA’s Unified System for Information Exchange in Incidents and Emergencies (USIE) — a secure 24/7 monitored website — provides a platform for countries to exchange urgent notifications and follow-up information during a nuclear or radiological emergency. The Joint Radiation Emergency Management Plan of the International Organizations (JPLAN) sets out a framework for the coordinated actions of international organizations during an emergency.

The United Nations Office for Outer Space Affairs (UNOOSA), as the lead UN entity for outer space affairs, has clear responsibilities under the JPLAN. “In the event of an accident, we would liaise with the launching state to gather information on the object and, if necessary, with other countries who can track space objects to determine re-entry timeframe and probable impact coordinates. We would then ensure that the most up-to-date trajectory and impact predictions are provided to the IAEA for further dissemination to aid emergency response efforts,” said Natercia Rodrigues, Programme Officer in UNOOSA, during the webinar.

Current threats and future plans

UNOOSA also maintains the UN Register of Objects Launched into Outer Space. To date over 86 per cent of all satellites, probes, landers, crewed spacecraft and space station flight elements launched into Earth orbit or beyond have been registered with UNOOSA.

So, what is the likelihood of another COSMOS 954-type crash happening in the future?

Sam Harbison, Chair of the United Nations Working Group on nuclear power sources in outer space, established in the year after Cosmos 954, explained that countries are no longer using nuclear power sources in Earth orbits because of the rapid improvements in solar panel technology and in order to avoid unnecessary potential releases of radioactive material. “All the nuclear power source satellites presently in Earth orbit were launched during the 1960’s to 1980’s and it is estimated it will be more than a hundred years before the earliest of them will re-enter Earth's atmosphere.”

More recent uses of nuclear power sources have been on probes, landers and rovers on missions that have left Earth orbit. Examples include the Cassini mission to explore Saturn and its moons, and robotic rovers such as the recently launched Mars 2020 Perseverance mission which will reach Mars early next year. There are aspirations to use nuclear power sources to support human colonies on the moon or Mars. “Solar panels are not sufficient for such prolonged missions, at great distances from the Sun,” Harbison said. “They would have to be complemented by rocket propulsion, which is bulky, heavy and expensive. Nuclear power sources will be needed both for the return journey and to sustain human activities on the surface of the Moon or Mars.”