On 31 August 2019, an interstellar comet was discovered as it passed through the Solar System (2I/Borisov). On the basis of initial imaging observations, 2I/Borisov seemed to be similar to ordinary Solar System comets^1,2—an unexpected characteristic given the multiple peculiarities of the only known previous interstellar visitor, 1I/‘Oumuamua^3,4,5,6. Spectroscopic investigations of 2I/Borisov identified the familiar cometary emissions from CN (refs. ^7,8,9), C₂ (ref. ¹⁰), O i (ref. ¹¹), NH₂ (ref. ¹²), OH (ref. ¹³), HCN (ref. ¹⁴) and CO (refs. ^14,15), revealing a composition similar to that of carbon monoxide-rich Solar System comets. At temperatures greater than 700 kelvin, comets also show metallic vapours that are produced by the sublimation of metal-rich dust grains¹⁶. Observation of gaseous metals had until very recently¹⁷ been limited to bright sunskirting and sungrazing comets^18,19,20 and giant star-plunging exocomets²¹. Here we report spectroscopic observations of atomic nickel vapour in the cold coma of 2I/Borisov at a heliocentric distance of 2.322 astronomical units—equivalent to an equilibrium temperature of 180 kelvin. Nickel in 2I/Borisov seems to originate from a short-lived nickel-containing molecule with a lifetime of \({340}_{-200}^{+260}\) seconds at 1 astronomical unit and is produced at a rate of 0.9 ± 0.3 × 10²² atoms per second, or 0.002 per cent relative to OH and 0.3 per cent relative to CN. The detection of gas-phase nickel in the coma of 2I/Borisov is in line with the recent identification of this atom—as well as iron—in the cold comae of Solar System comets¹⁷.

2019 年 8 月 31 日，一颗星际彗星在穿过太阳系时被发现（2I/Borisov）。根据最初的成像观察，2I/Borisov 似乎与普通的太阳系彗星^1,2相似——考虑到唯一已知的先前星际访客 1I/'Oumuamua ^3,4,5,6的多重特性，这是一个意想不到的特征. 2I/Borisov 的光谱研究确定了来自 CN（参考文献^7、8、9）、C ₂（参考文献¹⁰）、O  i（参考文献¹¹）、NH ₂（参考文献¹²）、OH（参考文献. ¹³ )、HCN (参考¹⁴ ) 和 CO (参考^14,15），揭示了与富含一氧化碳的太阳系彗星相似的成分。在高于 700 开尔文的温度下，彗星还显示出由富含金属的尘埃颗粒升华产生的金属蒸汽¹⁶。直到最近，对气态金属的观测¹⁷仅限于明亮的太阳裙和掠日彗星^18、19、20和巨大的坠星外彗星²¹。在这里，我们报告了在 2.322 个天文单位的日心距离处（相当于 180 开尔文的平衡温度）对 2I/Borisov 冷彗发中原子镍蒸气的光谱观测。2I/Borisov 中的镍似乎来源于一种寿命短的含镍分子，其寿命为\({340}_{-200}^{+260}\)秒，1 天文单位，每秒产生 0.9 ± 0.3 × 10 ²² 个原子，或相对于 OH 的 0.002% 和 0.3%相对于CN。^{在 2I/Borisov 彗发中检测到气相镍与最近在太阳系彗星17}的冷彗发中发现的这种原子以及铁是一致的。