俄火星探测器将于周日坠入地球

The remainder of the crippled Phobos-Grunt spacecraft is set to crash to Earth on Sunday following its botched mission to Mars, space experts have said.

The minibus-sized Russian craft has been in a low orbit around Earth since losing contact with engineers shortly after its launch on November 8.

It had been intended to explore Phobos, one of Mars's two moons, but became stranded while still orbiting Earth and attempts to put it back on its original course failed.

Most of its mass is expected to burn up as the craft re-enters the atmosphere but 20 or 30 pieces of small debris collectively weighing about 200kg could reach Earth.

In a normal re-entry about 20 per cent of the space junk's mass would be likely to reach Earth, but in the case of Phobos-Grunt it could be even less because it contains large quantities of unused fuel which will burn or dissipate in the atmosphere.

In theory the remains could land anywhere south of Watford or north of the Falkland Islands, with a sea landing most likely due to the size of the oceans relative to the continents.

But scientists said observers would be unlikely to even see the debris crash to Earth unless it flew directly overhead in a clear sky, and that the chances of anyone being injured were absolutely minute.

Prof Richard Crowther of the UK Space Agency said: "The chances are so low – it certainly doesn't keep me awake at night worrying about the probability of a piece of space debris coming through my roof."

He added, however, that international space agencies need to discuss the amount of disused equipment being left in Earth's orbit amid fears space junk could reach a "critical mass" where damaging collisions with active satellites and space craft become inevitable.

The craft will explode on its descent meaning any material reaching the ground could be spread over an area measuring 200km long and 20km wide, and many pieces will be so small they would be hard to spot on the ground.

British experts based at RAF Fylingdales in North Yorkshire will monitor the craft as it descends but even as it begins its last orbit, 90 minutes before it arrives, they will only be able to predict its landing spot with an uncertainty of 4,000km.

航天专家日前表示，“福布斯－土壤”火星探测器发射失败后，报废的航天器碎片将于周日坠入地球。

在去年11月8日发射后不久，这个面包车大小的火星探测器就与技术人员失去了联系，进入环绕地球的低轨运行。

这个探测器原本是用来探测火卫一的，火卫一是火星的两颗卫星之一。但是探测器升空后一直处于地球轨道，变轨的努力也失败了，导致计划搁浅。

探测器大多数的碎片都会在再次进入大气层时燃烧殆尽，但其总重约200公斤的20到30个小型残骸将坠入地球。

通常情况下，探测器再次进入大气层时，约有20%的太空垃圾可能会到达地球，但是“福布斯-土壤”带来的太空垃圾可能会更少，因为这个探测器携带了大量未用燃料，燃料会在大气层中燃烧或消散。

理论上看，残骸可能会落在沃特福德以南或福克兰群岛以北的任何地方，但因为这一地区海洋面积比陆地面积大得多，残骸最有可能坠入海中。

但科学家称，观测者可能连残骸坠入地球也无法看到，除非残骸在晴朗的天空从人们头顶飞过，而且人们因残骸坠落而受伤的几率微乎其微。

英国航天局的理查德•克劳塞教授说: “这种几率太小了，我当然不会晚上睡不着，担心航天器的残骸会砸透我的房顶”。

然而，他补充道，各国航空局需要讨论一下遗留在地球轨道中的废弃材料，以免太空垃圾达到“临界质量”，对正在运行中的卫星和航天器造成不可避免的破坏性碰撞。

该航天器将在其下落过程中爆炸，这意味着所有到达地面的残骸都可能分散在长200公里，宽20公里的区域内。一些碎片可能太小，无法在地面上找到。

约克郡北部，费林戴尔斯皇家空军基地的英国专家将监视航天器的坠落。但是尽管该航天器最后一次绕轨运行距其坠地还有90分钟，专家们也只能将其坠落点预测在4000公里内。