Helios 1 Ephemeris, Heliocentric Trajectories, Heliographic, Heliographic Inertial, and Solar Ecliptic Coordinates, HelioWeb, Daily Data

Heliocentric trajectories for Helios 1 in Heliographic, HG, Heliographic Inertial, HGI, and Solar Ecliptic, SE, Coordinates

The original trajectory data are taken from http://ssd.jpl.nasa.gov/horizons.cgi where users can find many more objects. In the case of orbit data for planets, the orbit data can be used as a proxy for spacecraft ephemeris that are in orbit about the planets. On a heliospheric scale, differences between the planet orbital tarjectory and that of the spacecraft are very small. For instance, the heliocentric longitudes differ by only 0.25° for a spacecraft stationed near the L1 Lagrange point at approximately 100 Earth radii upstream of the Earth.

The production of the HG, HGI, and SE trajectory data requires a values for the "Equinox Epoch", which is defined as the epoch time when the direction from the Earth to the sun at the time of the vernal equinox when the sun seems to cross equatorial plane of the Earth from below. This direction is called the First Point of Aries, FPA and it is not a fixed direction but drifts by about 1.4° per century or 50.26" per year. In addition, there are tiny irregularities in FPA drift that are on the order of 1" per year or less. The Equinox Epoch can be determined by using a variety of methods for calculating the instantaneous FPA longitudinal direction and whether the tiny irregularities have been smoothed or averaged out. Four methods for determining the Equinox Epoch are in common usage:

+---------------------------------------------------------------------+
| Method Name | FPA Longitude Definition |
|---------------------------------------------------------------------|
| B1950.0 | the actual FPA at 22:09 UT on December 31, 1949 |
| J2000.0 | the smoothed FPA at 12:00 UT on January 1, 2000 |
| True of Date | the actual FPA at 00:00 UT on the date of interest |
| Mean of Date | the smoothed FPA at 00:00 UT on the date of interest |
+---------------------------------------------------------------------+

The heliocentric trajectory data included in this data product have been calculated by using the Equinox Epoch: defined via the "Mean of Date" method.

More precise coordinates, and some planet-centered coordinates, are found in the "traj" subdirectories of spacecraft specific directories at https://spdf.gsfc.nasa.gov/pub/data/ and http://ssd.jpl.nasa.gov/horizons.cgi.

以日心坐标系统为基础的 Helios 1 号航天器轨迹数据，包括天球坐标系（HG）、天球惯性坐标系（HGI）和太阳黄道坐标系（SE）。原始轨迹数据来源于 http://ssd.jpl.nasa.gov/horizons.cgi，该网站提供更多天体轨迹数据。对于行星轨道数据，其轨迹数据可用作围绕行星运行的航天器摄动数据的代理。在日球尺度上，行星轨道轨迹与航天器轨迹之间的差异极为微小。例如，位于近地平线 L1 拉格朗日点（约在地球半径 100 倍上游）附近的航天器的日心经度差异仅为 0.25°。

生成 HG、HGI 和 SE 轨迹数据需要确定“春分点历元”的值，该历元定义为春分时刻地球至太阳的方向，此时太阳似乎从下方穿越地球赤道面。此方向被称为春分点（FPA），它并非固定方向，而是每世纪约漂移 1.4° 或每年 50.26'。此外，FPA 漂移中存在微小的不规则性，约为每年 1' 或更少。春分点历元可以通过多种方法计算瞬时 FPA 经度方向，并判断微小不规则性是否已被平滑或平均。常见的确定春分点历元的方法有四种：

+---------------------------------------------------------------------+
| 方法名称 | FPA 经度定义 |
|---------------------------------------------------------------------|
| B1950.0 | 1949 年 12 月 31 日 22:09 UTC 实际 FPA |
| J2000.0 | 2000 年 1 月 1 日 12:00 UTC 平滑 FPA |
| 真实历元 | 利益相关日期 00:00 UTC 实际 FPA |
| 平均历元 | 利益相关日期 00:00 UTC 平滑 FPA |
+---------------------------------------------------------------------+

本数据产品中包含的日心轨迹数据是通过“平均历元”方法计算的。

更精确的坐标和一些以行星为中心的坐标可在 https://spdf.gsfc.nasa.gov/pub/data/ 和 http://ssd.jpl.nasa.gov/horizons.cgi 的航天器特定目录的“traj”子目录中找到。