The CHaracterising ExOPlanet Satellite (CHEOPS) was selected on October 19, 2012, as the first small mission (S-mission) in the ESA Science Programme and successfully launched on December 18, 2019, as a secondary passenger on a Soyuz-Fregat rocket from Kourou, French Guiana. CHEOPS is a partnership between ESA and Switzerland with important contributions by ten additional ESA Member States. CHEOPS is the first mission dedicated to search for transits of exoplanets using ultrahigh precision photometry on bright stars already known to host planets. As a follow-up mission, CHEOPS is mainly dedicated to improving, whenever possible, existing radii measurements or provide first accurate measurements for a subset of those planets for which the mass has already been estimated from ground-based spectroscopic surveys. The expected photometric precision will also allow CHEOPS to go beyond measuring only transits and to follow phase curves or to search for exo-moons, for example. Finally, by unveiling transiting exoplanets with high potential for in-depth characterisation, CHEOPS will also provide prime targets for future instruments suited to the spectroscopic characterisation of exoplanetary atmospheres. To reach its science objectives, requirements on the photometric precision and stability have been derived for stars with magnitudes ranging from 6 to 12 in the V band. In particular, CHEOPS shall be able to detect Earth-size planets transiting G5 dwarf stars (stellar radius of 0.9R⊙) in the magnitude range 6 ≤ V ≤ 9 by achieving a photometric precision of 20 ppm in 6 hours of integration time. In the case of K-type stars (stellar radius of 0.7R⊙) of magnitude in the range 9 ≤ V ≤ 12, CHEOPS shall be able to detect transiting Neptune-size planets achieving a photometric precision of 85 ppm in 3 hours of integration time. This precision has to be maintained over continuous periods of observation for up to 48 hours. This precision and stability will be achieved by using a single, frame-transfer, back-illuminated CCD detector at the focal plane assembly of a 33.5 cm diameter, on-axis Ritchey-Chretien telescope. The nearly 275 kg spacecraft is nadir-locked, with a pointing accuracy of about 1 arcsec rms, and will allow for at least 1 Gbit/day downlink. The sun-synchronous dusk-dawn orbit at 700 km altitude enables having the Sun permanently on the backside of the spacecraft thus minimising Earth stray light. A mission duration of 3.5 years in orbit is foreseen to enable the execution of the science programme. During this period, 20% of the observing time is available to the wider community through yearly ESA call for proposals, as well as through discretionary time approved by ESA’s Director of Science. At the time of this writing, CHEOPS commissioning has been completed and CHEOPS has been shown to fulfill all its requirements. The mission has now started the execution of its science programme.

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CHEOPS 的使命

表征 ExOPlanet 卫星（CHEOPS）于 2012 年 10 月 19 日被选为欧空局科学计划中的第一个小任务（S 任务），并于 2019 年 12 月 18 日成功发射，作为联盟号 - 弗雷格特火箭的次要乘客法属圭亚那库鲁。CHEOPS 是欧空局和瑞士之间的合作伙伴关系，另外十个欧空局成员国做出了重要贡献。CHEOPS 是第一个致力于在已知拥有行星的明亮恒星上使用超高精度光度法搜索系外行星凌日的任务。作为后续任务，CHEOPS 主要致力于在可能的情况下改进现有的半径测量或为那些已经通过地面光谱调查估计质量的行星的子集提供第一次准确的测量。例如，预期的光度精度也将使 CHEOPS 超越仅测量凌日和跟踪相位曲线或搜索系外卫星。最后，通过揭示具有深度表征潜力的凌日系外行星，CHEOPS 还将为未来适用于系外行星大气光谱表征的仪器提供主要目标。为实现其科学目标，已经推导出了对 V 波段 6 至 12 星等恒星的光度精度和稳定性要求。特别是，CHEOPS 将能够在 6 ≤ V ≤ 9 的星等范围内通过在 6 小时的积分时间内实现 20 ppm 的光度精度，来探测经过 G5 矮星（恒星半径为 0.9R⊙）的地球大小的行星。在 K 型恒星的情况下（恒星半径为 0. 7R⊙) 的星等在 9 ≤ V ≤ 12 范围内，CHEOPS 应能够探测到过境的海王星大小的行星，在 3 小时的积分时间内达到 85 ppm 的光度精度。这种精度必须在长达 48 小时的连续观察期间保持。这种精度和稳定性将通过在直径为 33.5 厘米的同轴 Ritchey-Chretien 望远镜的焦平面组件上使用单个帧转移背照式 CCD 探测器来实现。近 275 公斤的航天器被锁定在最低点，指向精度约为 1 弧秒均方根，并且将允许至少 1 Gbit/天的下行链路。700 公里高度的太阳同步黄昏-黎明轨道使太阳永久位于航天器的背面，从而最大限度地减少地球杂散光。任务持续时间为 3。预计在轨运行 5 年，以便执行科学计划。在此期间，通过欧空局每年的提案征集以及欧空局科学主任批准的自由裁量时间，更广泛的社区可以使用 20% 的观测时间。在撰写本文时，CHEOPS 调试已经完成，并且已经证明 CHEOPS 能够满足其所有要求。该任务现已开始执行其科学计划。