The BepiColombo mission begins its arrival at Mercury after nearly 8 years

An ambitious mission to reveal the secrets of the least explored planet in the inner solar system is about to arrive at its cosmic destination after a lengthy journey.

The BepiColombo mission, a joint effort between the European Space Agency and the Japan Aerospace Exploration Agency, intends to use two complementary orbiters to reveal insights into how Mercury formed and evolved so close to the sun.

The mission launched on an Ariane 5 rocket from Europe’s Spaceport near Kourou, French Guiana, in October 2018. Since then, it has traveled more than 6 billion miles (10 billion kilometers).

BepiColombo is named for Italian engineer and mathematician Giuseppe “Bepi” Colombo, who studied Mercury’s peculiar rotation and proposed trajectories that enabled NASA’s first flybys of the planet in 1974.

Earth is about 10 times closer to Mercury than it is to Jupiter, but BepiColombo has taken longer to reach the tiny planet than previous missions such as Galileo and Juno heading to the largest planet in our outer solar system.

“You can go faster to Mercury if you take a direct way to it,” said Frank Budnik, flight dynamics manager for the BepiColombo mission at the ESA. “But the problem is we want to insert into an orbit with respect to Mercury. You have to bring the spacecraft to the same velocity as Mercury has, and this is a very energy-demanding transfer.”

A combination of electric propulsion and gravitational boosts from a total of nine flybys of Earth, Venus and Mercury have aided BepiColombo in reaching the correct velocity on its long cruise.

But the steep challenges for BepiColombo aren’t over yet. The Mercury Transfer Module, which has powered the multiyear voyage, will separate from BepiColombo’s stack of two science orbiters at 8 a.m. ET Thursday.

The ESA will host a live broadcast beginning at 7:45 a.m. ET Thursday to share milestones of the nail-biting separation, and at 9:53 a.m. ET, the agency will conduct a safety check of the spacecraft to see how BepiColombo has fared so far in its mission.

“The separation of the transfer module will take place in conditions which are exceedingly challenging, and we will have to face a considerable risk that things don’t go as planned,” said Ignacio Tanco, head of Inner Solar System Mission Operations at the ESA.

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“It’s going to be equivalent to launching a new spacecraft, only this spacecraft happens to be around a different planet. We are going to see for the first time systems in operation that we have not been able to fully test.”

And that’s just the beginning of a complex process.

“Unlike in other missions, BepiColombo’s arrival doesn’t mean a single event,” Tanco said. “We have about half a year of intense operations ahead of us.”

Establishing an observational orbit around Mercury is even more difficult than the journey to get there.

ESA’s Mercury Planetary Orbiter, or MPO, and JAXA’s Mercury Magnetospheric Orbiter, or MIO, will be captured by Mercury’s gravity and enter orbit around the planet on November 21.

The two orbiters will separate from one another between December 9 and 10, with the Mercury Planetary Orbiter moving into its orbital position on December 16 and the Mercury Magnetospheric Orbiter following on March 10.

Science operations will officially begin for both orbiters in April.

“It will be the first time that a mission releases two fully independent, fully operational separate spacecrafts around a different planet,” Tanco said.

The MPO will observe the planet itself, while the MIO will focus on the space environment around Mercury, said Santa Martinez, BepiColombo mission manager at the ESA.

Mercury is about 36 million miles (58 million kilometers) from the sun on average, and completes one orbit around our star every 88 days, according to NASA . Such proximity means that the planet’s surface temperatures can reach about 800 degrees Fahrenheit (430 degrees Celsius) during the day.

Mercury’s close distance to the sun has made it a challenging planet to explore — and it’s the main driver of BepiColombo’s complexity. Only two previous missions, NASA’s Mariner 10 in 1974 and its MESSENGER mission in 2011, have been in the vicinity of the rocky world. Like BepiColombo, MESSENGER took nearly seven years to reach Mercury.

“The spacecraft in itself has been designed to operate within conditions which are extremely punishing,” Tanco said. “It’s essentially equivalent to operating with a very hot pizza oven running right on your back.”

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Both orbiters are equipped with solar arrays to provide them with power, and they are at risk of overheating so close to the sun.

The MIO will spin 15 times a minute, while the MPO will position its array at an angle almost edge-on to the sun to receive just enough sunlight while also preventing solar radiation damage.

Putting two spacecraft in orbit around Mercury will provide unprecedented views of the planet and open a new chapter in scientists’ understanding of the solar system, Martinez said.

The orbiters will help map the planet in high resolution and answer questions that cropped up during MESSENGER’s detailed survey of Mercury.

Each orbiter is equipped with a range of instruments to explore Mercury’s structure and composition, including craters and evidence of ancient volcanic activity on its surface, as well as the planet’s magnetic field and its exosphere, a thin cloud of gas that surrounds the planet, said Geraint Jones, lead project scientist for BepiColombo at the ESA.

One of the mysteries that scientists are hoping to probe are the so-called hollows spotted on Mercury by Mariner 10 and MESSENGER.

“There are depressions in the surface that look like the surface is being eaten away,” Jones said. “It might be possible that we can see changes in these regions over time.”

Hollows haven’t been found elsewhere on planets in our solar system, and with no wind or water to form the thousands of puzzling depressions, scientists want to know what forces sculpted them.

NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution for Science

The orbiters could also help uncover the truth behind why Mercury is so dense for its small size.

Some 4.5 billion years ago when the planets formed from the debris surrounding the sun, “it’s thought that Mercury started forming, created a modest-sized planet and then it was hit by another large planetesimal, which ripped off its outer crust,” Jones said. “So that means that the overall density of the material that was left behind was much higher, and that’s why it has the high density it has now.”

Learning more about Mercury’s surface and interior could uphold the theory or provide new answers, Jones said.

一项旨在揭开内太阳系探索最少行星秘密的雄心勃勃的任务，经过漫长的旅程，即将抵达其宇宙目的地。

BepiColombo 任务是欧洲航天局和日本宇宙航空研究开发机构的联合项目，旨在利用两个互补的轨道器来揭示水星在如此靠近太阳的地方是如何形成和演化的。

该任务于 2018 年 10 月由阿丽亚娜 5 号火箭从法属圭亚那库鲁附近的欧洲航天发射场发射升空。自那时以来，它已经飞行了超过 60 亿英里（100 亿公里）。

BepiColombo 是以意大利工程师兼数学家朱塞佩·“贝皮”·科伦坡 (Giuseppe “Bepi” Colombo) 的名字命名的，他研究了水星奇特的自转，并提出了轨道，使得 NASA 得以在 1974 年首次飞掠水星。

地球距离水星比距离木星近约 10 倍，但 BepiColombo 到达这颗小行星所花费的时间比之前前往我们外太阳系最大行星的伽利略号和朱诺号等任务要长。

“如果走直线前往水星，速度会更快，”欧洲航天局BepiColombo任务的飞行动力学经理弗兰克·布德尼克说。“但问题是，我们想进入相对于水星的轨道。你必须让航天器达到与水星相同的速度，而这需要消耗大量的能量。”

BepiColombo 凭借电推进和总共九次飞掠地球、金星和水星所产生的引力助推，在漫长的巡航过程中达到了正确的速度。

但贝皮科伦坡号面临的严峻挑战尚未结束。为这趟多年旅程提供动力的“水星转移模块”将于美国东部时间周四上午8点与贝皮科伦坡号的两颗科学轨道器分离。

欧洲航天局将于美国东部时间周四上午 7:45 开始进行现场直播，分享扣人心弦的分离过程中的重要里程碑；美国东部时间上午 9:53，该机构将对航天器进行安全检查，以了解 BepiColombo 迄今为止的任务进展情况。

“转移模块的分离将在极其具有挑战性的条件下进行，我们将面临相当大的风险，事情可能不会按计划进行，”欧洲航天局内太阳系任务运营负责人伊格纳西奥·坦科说。

科学家称，在撒哈拉沙漠发现的两颗陨石可能来自太阳系研究最少的岩质行星。

“这相当于发射一艘新的航天器，只不过这艘航天器恰好绕着另一颗行星运行。我们将首次看到一些我们之前无法进行全面测试的系统投入运行。”

而这仅仅是一个复杂过程的开始。

坦科说：“与其他任务不同，贝皮科伦坡的到来并不意味着单一事件的发生。我们接下来将面临大约半年的密集行动。”

建立环绕水星的观测轨道比到达水星的旅程还要困难。

欧洲航天局的水星行星轨道器（MPO）和日本宇宙航空研究开发机构的水星磁层轨道器（MIO）将于 11 月 21 日被水星引力捕获并进入绕水星运行的轨道。

两艘轨道器将于 12 月 9 日至 10 日之间分离，水星行星轨道器将于 12 月 16 日进入其轨道位置，水星磁层轨道器将于 3 月 10 日随后进入其轨道位置。

两颗轨道飞行器的科学运行将于4月正式开始。

“这将是人类首次在不同的行星周围释放两个完全独立、完全运行的独立航天器，”坦科说。

欧洲航天局 BepiColombo 任务经理 Santa Martinez 表示，MPO 将观测水星本身，而 MIO 将专注于水星周围的空间环境。

据美国宇航局称，水星平均距离太阳约3600万英里（5800万公里），绕太阳公转一周需要88天。如此近的距离意味着水星表面温度在白天可以达到约800华氏度（430摄氏度）。

水星距离太阳很近，这使得它成为一颗难以探索的行星——这也是BepiColombo探测器复杂设计的主要原因。此前只有两次探测任务曾接近过这颗岩质行星，分别是1974年NASA的“水手10号”和2011年的“信使号”。与BepiColombo一样，“信使号”也花了近七年时间才抵达水星。

坦科说：“航天器本身的设计就是为了在极其恶劣的条件下运行。这基本上相当于背上顶着一个很热的披萨烤炉。”

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两个轨道飞行器都配备了太阳能电池阵列来提供电力，但由于距离太阳太近，它们有过热的风险。

MIO 每分钟旋转 15 次，而 MPO 将其阵列调整到几乎侧对太阳的角度，以便接收足够的阳光，同时防止太阳辐射损伤。

马丁内斯表示，将两艘航天器送入水星轨道将提供前所未有的水星景象，并开启科学家对太阳系理解的新篇章。

这些轨道器将帮助绘制高分辨率的行星地图，并解答 MESSENGER 探测器对水星进行详细勘测期间出现的问题。

欧洲航天局 BepiColombo 项目首席科学家 Geraint Jones 表示，每个轨道器都配备了一系列仪器，用于探索水星的结构和成分，包括其表面的陨石坑和古代火山活动的证据，以及该行星的磁场和外逸层（环绕该行星的一层薄薄的气体云）。

科学家们希望探究的谜团之一是水手10号和信使号探测器在水星上发现的所谓“空洞”。

琼斯说：“地表有一些凹陷，看起来像是地表正在被侵蚀。随着时间的推移，我们或许可以看到这些区域发生变化。”

在太阳系的其他行星上还没有发现过类似的凹陷，而且由于没有风或水来形成成千上万个令人费解的凹陷，科学家们想知道是什么力量塑造了它们。

美国国家航空航天局/约翰·霍普金斯大学应用物理实验室/卡内基科学研究所

轨道探测器还有助于揭开水星体积虽小但密度却如此之高的真相。

琼斯说，大约45亿年前，当行星由太阳周围的碎片形成时，“人们认为水星开始形成，形成了一颗中等大小的行星，然后它被另一颗大型星子撞击，撕裂了它的外壳。这意味着残留物质的整体密度要高得多，这就是它现在密度如此之高的原因。”

琼斯表示，更多地了解水星的表面和内部结构可能会证实这一理论，或者提供新的答案。