# Which deep-space spacecraft flew closest by Earth during a gravitational assist?

The extremely cool NASA JPL video Triumph at Saturn (Part I) is really worth a watch and/or listen. (Don't forget Part II as well!)

At about 26:30 it discusses Cassini's gravitational assists; first two at Venus then one at Earth:

...but the project's first priority was getting safely past the Earth, which occurred in the summer of 1999, giving Cassini another 12,000 miles per hour of speed.

and caption:

Earth Flyby: August 18, 1999, Cassini Speed: 99,000 mph

Several deep space spacecraft have use gravitational assists at earth via close flyby's. I'm wondering:

Question: Which deep-space spacecraft flew closest by Earth during a gravitational assist.

cued at 26:30

• There is a wikipedia page for this, though not fully complete Oct 26, 2021 at 12:45
• @BrendanLuke ya 11 out of 30 distances are absent. But that 301 km certainly seems a likely winner!
– uhoh
Oct 26, 2021 at 12:57
• @uhoh I think that's a false typo in wiki, not sure. Other sources I found said it's NOT 301km. Even the wiki page contradicts itself! "gravity assist en route to Jupiter; minimum distance 960 km" Oct 15, 2022 at 0:06
• @DialFrost good catch! JPL says the first flyby (December 1990) was ~600 km not 301 km; jpl.nasa.gov/news/galileo-earth-flyby Need to look elsewhere for the 2nd Earth flyby in 1992.
– uhoh
Oct 15, 2022 at 0:19
• @uhoh All values accounted and presented! Unknown values are either due to spacecraft malfunction, no information online or me just being blind and unable to find it Oct 15, 2022 at 7:37

Wikipedia - List of earth's flybys states that 301km by Galileo is the shortest flyby. However it has contradicting information, and seems to be false:

gravity assist en route to Jupiter; minimum distance 960 km

The actual distance of this flyby is 960km, not 301km.

So what is the shortest flyby then?

Well, it's also by Galileo, at 303km by the NASA JPL document Galileo End of Mission Press Kit, September 2003 or less accurately at 305km by Wikipedia for Galileo's second flyby of Earth.

After flying past Venus at an altitude of 16,000 kilometers (nearly 10,000 miles) on February 10, 1990, the spacecraft swung past Earth at an altitude of 960 kilometers (597 miles) on December 8, 1990. That flyby increased Galileo's speed enough to send it on a two-year elliptical orbit around the Sun. The spacecraft returned for a second Earth swingby on December 8, 1992, at an altitude of 303 kilometers (188 miles). With this, Galileo left Earth for the third and final time and headed toward Jupiter.

Galileo's flying path from the same NASA document:

Now to prove this, I have to dig up all the closest approaches of the blank spaces in the Wikipedia pages :(

• 1st: 88,997km
• 2nd: 40 earth radius - 255,125 km
• 3rd: 86 earth radius - 548,520 km

StarDust, 1st and 2nd passes:

• 1st: 6,008 km
• 2nd: 9,157 km

Rosseta, 1st, 2nd and 3rd passes:

• 1st: 1,954 km
• 2nd: 5,700 km
• 3rd: 2,481 km

Deep Impact 1st, 2nd, 3rd, 4th, and 5th passes:

• 1st: 15,567 km
• 2nd: 43,450 km
• 3rd: 30,496 km
• 4th: Unknown*
• 5th: Unknown*

Hayabusa2 1st pass:

• 1st: 3,090 km

PROCYON 1st pass:

• 1st: Unknown* (probably due to ion thruster malfunction that occurred)

Shin'en 2 1st pass:

Note that this Wikipedia page is very incomplete, and lots of information can be found online. Note I am also posting all values for anything missing. E.g. if StarDust is missing the value for its 3rd flyby, I'll show all values for all its 1st, 2nd and 3rd flyby.

Supplemental information to @DialFrost's excellent answer:

I ran JPL's Horizons to obtain an interpolated trajectory. It showed a minimum distance "delta" at 1992-Dec-08 15:09:25.000 where it's RA is reported to be 299.49610 (degrees) and declination -33.74661 (degrees). At that time it's distance to Earth's geocenter is calculated to be 6675.02 kilometers.

Note that these are dynamically calculated trajectories in the gravitational field of the solar system which are then fit to range and rate data from round-trip radio links to the spacecraft via its coherent transponder.

To convert the 6675 km geocenter distance to a value above Earth's surface at a latitude $$\lambda$$ (= declination in this approximation) of about -34 degrees, we look to the WGS84 ellipse with a semimajor (equaltorial) axis $$a$$ of 6378137 meters and a semiminor (polar) axis $$b$$ of 6356752 meters, and use a Lissajous (sin/cos) or parametric expression to generate our elliptical cross-section:

$$x = a \cos\lambda$$

$$z = b \sin\lambda$$

$$r = \sqrt{x^2 + z^2}$$

At -34 degrees that works out to be 6371.457 km.

Subtracting that from Horizon's 6675.02 km leaves 303.56 kilometers above the WGS84's -34th parallel which agrees nicely with the other answer :-)

• +1 nice addition to my answer! I like the in-depth info of how it was found :3 Oct 21, 2022 at 5:05