伊人直播app官方正版下载_伊人直播高品质美女在线视频互动社区_伊人直播官方版

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“中國(guó)天眼”將建擴(kuò)展陣 為尋找地外生命“做功課”
時(shí)間:2020-06-05 10:05:36
文章發(fā)布:李陽
原創(chuàng)作者:未知
來源:互聯(lián)網(wǎng)

中國(guó)天眼

“中國(guó)天眼”要邁出探索地外生命的步伐了。

科學(xué)家認(rèn)為,宜居的星球除了應(yīng)當(dāng)有水、適宜的溫度和大氣外,還有一個(gè)必要條件——磁場(chǎng)。

作為世界上最大的單口徑射電望遠(yuǎn)鏡, 500 米口徑球面射電望遠(yuǎn)鏡(FAST)將鎖定太陽系外的太空,在距離地球約 100 光年的范圍內(nèi)尋找與地球一樣擁有磁場(chǎng)的行星,助力尋找地外生命。近期,中法等國(guó)天文學(xué)家在《天文學(xué)及天體物理》雜志上發(fā)表了這一觀測(cè)計(jì)劃。

從零到一: 尋找首顆系外“磁行星”

磁場(chǎng)對(duì)于生命來說至關(guān)重要。

“地球磁場(chǎng)牢牢抓住地球表面的大氣,避免其被太陽風(fēng)吹走,隔絕了絕大多數(shù)強(qiáng)烈的宇宙射線,可保護(hù)生命不受射線的傷害。”中國(guó)科學(xué)院國(guó)家天文臺(tái)研究員、FAST 首席科學(xué)家李菂在接受科技日?qǐng)?bào)記者采訪時(shí)表示。

太陽風(fēng)是從太陽上層大氣射出的超聲速等離子體帶電粒子流,當(dāng)這種等離子體流到達(dá)地球附近時(shí),大部分流體會(huì)被地球磁場(chǎng)干擾,繞過地球繼續(xù)向太陽系外圍前進(jìn)。其余的一小部分則隨著地球磁場(chǎng)線進(jìn)入兩極,并與空氣碰撞,形成絢麗多彩的極光,而非直接照射到動(dòng)植物體身上。

那么,地球磁場(chǎng)是如何形成的?

“地球內(nèi)部的高溫導(dǎo)致物質(zhì)融化,大量電離物質(zhì)特別是液態(tài)的鐵金屬,隨著地球自轉(zhuǎn)長(zhǎng)期處于運(yùn)動(dòng)之中,使地球形成了一個(gè)強(qiáng)度較穩(wěn)定、具有南北磁極的磁場(chǎng)?!崩钋吀嬖V記者,目前關(guān)于地球磁場(chǎng)的細(xì)節(jié)問題也是學(xué)界研究的熱點(diǎn)之一,如經(jīng)常出現(xiàn)磁場(chǎng)反轉(zhuǎn)、磁極加速移動(dòng)等。

在太陽系的八大行星中,除了地球,還有 5 顆擁有行星尺度的磁場(chǎng),分別是水星、木星、土星、天王星和海王星,這些行星磁場(chǎng)的形成機(jī)制與地球大致相同。而對(duì)于太陽系外的空間,目前學(xué)界尚未真正探測(cè)到過擁有磁場(chǎng)的行星,但天文學(xué)家們已經(jīng)發(fā)現(xiàn)了一些擁有磁場(chǎng)的褐矮星。褐矮星又被稱為“失敗的太陽”,因?yàn)樗鼈兿噍^于行星來講太大了,最輕的褐矮星質(zhì)量也有十幾倍的木星大小;而相對(duì)于在核內(nèi)發(fā)生氫聚變的恒星來說又很小。2018 年,研究人員發(fā)現(xiàn)了一顆質(zhì)量介于行星和褐矮星之間的天體(SIMP),磁場(chǎng)比木星還要強(qiáng) 200 倍左右。

“這讓我們有理由相信,太陽系外也會(huì)存在很多有磁場(chǎng)的行星,F(xiàn)AST 將要做的,就是突破這個(gè)零。”李菂說。

提升“視力”: 擴(kuò)展陣助分辨率提升百倍

在星際的巨大尺度中,磁場(chǎng)的探測(cè)永遠(yuǎn)是個(gè)很難的問題。

對(duì)于太陽系內(nèi)的行星結(jié)構(gòu)、物理現(xiàn)象、甚至磁場(chǎng),我們可采用發(fā)射探測(cè)器等方式直接探測(cè)。但系外行星對(duì)于空間探測(cè)器來說,遙不可及。只能通過地球上或附近的空間望遠(yuǎn)鏡等設(shè)備接收信號(hào),測(cè)量恒星的位置和速度是否變化來推測(cè)其周圍是否存在行星;后來進(jìn)一步發(fā)展利用“掩星”或引力透鏡的現(xiàn)象進(jìn)行觀測(cè),而上述常規(guī)的探測(cè)方式皆與行星內(nèi)部的磁場(chǎng)相關(guān)度不高。

近 20 年來,一部分天文學(xué)家開始鎖定射電信號(hào),以期觀測(cè)行星的磁場(chǎng)。為何會(huì)選在射電波段呢?

“如果行星有磁場(chǎng),那么在主恒星星風(fēng)的作用下,便會(huì)產(chǎn)生電磁輻射。特別是在射電這一頻率跨度很寬的波段?!崩钋叡硎?。再者,對(duì)于地面的射電望遠(yuǎn)鏡而言,大氣層是“透明”的,即宇宙中的射電信號(hào)能量低、波長(zhǎng)長(zhǎng),穿過大氣層時(shí)不易散射,能夠保留較為完整的信息被地面望遠(yuǎn)鏡這雙“眼睛”接收。以木星為代表的大行星輻射就發(fā)生在射電波段,這也啟發(fā)人們?cè)趯ふ移渌瑯泳哂写艌?chǎng)的行星時(shí)重點(diǎn)關(guān)注射電信號(hào)。

“FAST 最大的優(yōu)勢(shì)在于接收射電信號(hào)的總面積很大,靈敏度高。”在李菂看來,F(xiàn)AST 單位接收面積的性價(jià)比也很高。雖然世界上存在綜合性能更強(qiáng)的射電望遠(yuǎn)鏡,如某些陣列式望遠(yuǎn)鏡,又如 8 臺(tái)望遠(yuǎn)鏡構(gòu)成、觀測(cè)首張黑洞照片的“事件視界望遠(yuǎn)鏡”等,這些望遠(yuǎn)鏡雖然分辨率更高,也能做很多 FAST 目前無法完成的觀測(cè)內(nèi)容,但由于每個(gè)陣列單元都要有獨(dú)立的接收設(shè)備和傳送、處理信號(hào)裝置,因此成本將會(huì)指數(shù)級(jí)增長(zhǎng)。FAST 造價(jià)只有 12 億人民幣,而由 66 個(gè)望遠(yuǎn)鏡單元組成的 ALMA 射電望遠(yuǎn)鏡的造價(jià)為 10 億英鎊,折合成人民幣約 92 億元。

“FAST 已經(jīng)將靈敏度發(fā)揮到了極致,但由于口徑的限制,觀測(cè)系外行星時(shí)的分辨率還達(dá)不到理想要求。我們希望能夠建設(shè) FAST 擴(kuò)展陣,將空間分辨率提升 10—100 倍?!崩钋呁嘎?,目前計(jì)劃采用 5 米口徑的天線和簡(jiǎn)易接收單元,在現(xiàn)有的“球面”周圍鋪設(shè)成密集網(wǎng),將原本 300 米有效基線擴(kuò)大至幾十公里。為了節(jié)省成本,擴(kuò)展陣的每個(gè)單元不設(shè)置數(shù)字信號(hào)處理裝置,而是利用光纖將接收到的電信號(hào)直接傳輸?shù)皆?FAST 的總處理器上。如此一來,“天眼”對(duì)于弱點(diǎn)源的響應(yīng)性能將會(huì)大幅提升,“視力”進(jìn)一步提高。

打開新窗: 或改變對(duì)地外生命現(xiàn)有認(rèn)知

FAST 一旦接收到第一顆系外行星的射電信號(hào),將意味著打開系外宜居行星探索的新窗口。

“當(dāng)這種探測(cè)大范圍展開后,很有可能會(huì)得到一個(gè)結(jié)論,即表明太陽系之外行星磁場(chǎng)是普遍存在的。”李菂認(rèn)為,雖然磁場(chǎng)并非地外生命存在的唯一條件,但作為保護(hù)生命的重要因素,未來愈發(fā)多樣、詳細(xì)的行星磁場(chǎng)的探測(cè)將會(huì)改變?nèi)藗儗?duì)地外生命的現(xiàn)有認(rèn)知,甚至將為搜索智慧生命和地外文明提供助力。

以目前觀測(cè)到的數(shù)據(jù),銀河系之內(nèi)、太陽系之外存在生命的條件是普遍存在的?!叭绻谕耆也坏降赝馍挠白樱炊遣豢伤甲h的。”李菂告訴記者,地球上各種極端條件下皆存在著生命,人們?cè)诘赝庖蔡綔y(cè)到構(gòu)成生命的C、H、O等必要元素和大量復(fù)雜分子。銀河系誕生了 100 多億年。恒星、行星、磁場(chǎng)、水以及足夠的時(shí)間,這些生命演化的必要因素齊備,為何不會(huì)存在地外生命?

很快,F(xiàn)AST 將會(huì)邁出這一步,擴(kuò)展陣也將在3—4 年內(nèi)建成,助力搜索系外行星的射電信號(hào)。

除此之外,F(xiàn)AST 將在 2019 年 9 月結(jié)束長(zhǎng)達(dá) 3 年的測(cè)試期,正式投入使用。超過 10 個(gè)國(guó)家和地區(qū)的天文學(xué)家的科學(xué)團(tuán)隊(duì)正在從事 FAST 相關(guān)研究。李菂透露,以中國(guó)科學(xué)院國(guó)家天文臺(tái)為首的科研團(tuán)隊(duì)已經(jīng)制定了兩個(gè)系統(tǒng)化的巡天計(jì)劃:FAST 多科學(xué)目標(biāo)同時(shí)巡天計(jì)劃(CRAFTS)和銀道面及仙女座星系巡天計(jì)劃(GAS)。目前這兩個(gè)計(jì)劃已經(jīng)確立了相關(guān)模式,進(jìn)行了大量的技術(shù)測(cè)試,預(yù)期今年年底前正式展開,將成為 FAST 系統(tǒng)科學(xué)產(chǎn)出的主要依據(jù),預(yù)期發(fā)現(xiàn)上千顆脈沖星、幾十萬個(gè)新的氣體星系、超過 100 億個(gè)像素點(diǎn)的新一代原子氫氣天圖。

“FAST 多個(gè)觀測(cè)項(xiàng)目,特別是兩個(gè)系統(tǒng)巡天計(jì)劃,將力爭(zhēng)在脈沖星黑洞系統(tǒng)、引力波源、基本物理常數(shù)、星系演化、生命的太空起源等方向催生重大突破?!崩钋呎f。

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